Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2024 • 269 The interplay between electron tunneling and Auger emission in a single quantum emitter weakly coupled to an electron reservoir
    Zöllner, M. and Mannel, H. and Rimek, F. and Maib, B. and Schwarz, N. and Wieck, A.D. and Ludwig, A. and Lorke, A. and Geller, M.
    Applied Physics Letters 124 (2024)
    In quantum dots (QDs), the Auger recombination is a non-radiative scattering process in which the optical transition energy of a charged exciton (trion) is transferred to an additional electron leaving the dot. Electron tunneling from a reservoir is the competing process that replenishes the QD with an electron again. Here, we study the dependence of the tunneling and Auger recombination rate on the applied electric field using high-resolution time-resolved resonance fluorescence (RF) measurements. With the given p-i-n diode structure and a tunnel barrier between the electron reservoir and the QD of 45 nm, we measured a tunneling rate into the QD in the order of ms−1. This rate shows a strong decrease by almost an order of magnitude for decreasing electric field, while the Auger emission rate decreases by a factor of five in the same voltage range. Furthermore, we study in detail the influence of the Auger recombination and the tunneling rate from the charge reservoir into the QD on the intensity and linewidth of the trion transition. In addition to the well-known quenching of the trion transition, we observe in our time-resolved RF measurements a strong influence of the tunneling rate on the observed linewidth. The steady-state RF measurement yields a broadened trion transition of about 1.5 GHz for an Auger emission rate of the same order as the electron tunneling rate. In a non-equilibrium measurement, the Auger recombination can be suppressed, and a more than four times smaller linewidth of 340 MHz (1.4 μeV) is measured. © 2024 Author(s).
    view abstractdoi: 10.1063/5.0183821
  • 2023 • 268 Anomalous Screening Effect of Superlattice-Doped GaAs / (Al,Ga)As Heterostructures under Illumination
    Liu, Xiao-Fei and Spitzer, Nikolai and Kiyama, Haruki and Ludwig, Arne and Wieck, Andreas D. and Oiwa, Akira
    Physical Review Applied 19 (2023)
    The GaAs/(Al,Ga)As heterostructure with short-period superlattice (SPSL) doping possesses ultrahigh mobility of its two-dimensional electron gas by placing donors within the remote GaAs layers. Here, we investigate its magnetotransport property under a heavily doped situation. After long enough illumination at cryogenic temperature, the change of the electron concentration inside the quantum well (QW) is only 5.9%. Meanwhile, the quantum lifetime τq,QW of the electron shows an anomalous behavior. It increases slightly and then exhibits an exponential decay until saturation. This is different from the monotonic increase of τq,QW under illumination for the conventional doping situation. The increase of τq,QW originates from the larger donor filling-fraction-enhanced screening effect. Meanwhile, the decrease of τq,QW may be caused by stronger scattering of ionized d+ states evolved from DX centers. The transfer of excess electrons between the AlAs layers can also cause the decrease of τq,QW. This work provides an insight into the mechanism of DX centers on the quantum transport properties of SPSL-doped heterostructures. © 2023 American Physical Society.
    view abstractdoi: 10.1103/PhysRevApplied.19.024056
  • 2023 • 267 Auger and spin dynamics in a self-assembled quantum dot
    Mannel, H. and Kerski, J. and Lochner, P. and Zöllner, M. and Wieck, A.D. and Ludwig, A. and Lorke, A. and Geller, M.
    Journal of Applied Physics 134 (2023)
    The Zeeman-split spin states of a single quantum dot can be used together with its optical trion transitions to form a spin-photon interface between a stationary (the spin) and a flying (the photon) quantum bit. In addition to long coherence times of the spin state itself, the limiting decoherence mechanisms of the trion states are of central importance. Here, we investigate in time-resolved resonance fluorescence the electron spin and trion dynamics in a single self-assembled quantum dot in an applied magnetic field of up to B = 10 T. The quantum dot is only weakly coupled to an electron reservoir with tunneling rates of about 1 ms − 1 . Using this sample structure, we can measure, in addition to the spin-flip rate of the electron and the spin-flip Raman rate of the trion transition, the Auger recombination process that scatters an Auger electron into the conduction band. The Auger effect destroys the radiative trion transition and leaves the quantum dot empty until an electron tunnels from the reservoir into the dot. The combination of an Auger recombination event with subsequent electron tunneling from the reservoir can flip the electron spin and thus constitutes another mechanism that limits the spin lifetime. © 2023 Author(s).
    view abstractdoi: 10.1063/5.0159775
  • 2023 • 266 Azimuthal ion movement in HiPIMS plasmas—part I: velocity distribution function
    Thiemann-Monjé, S. and Held, J. and Schüttler, S. and von Keudell, A. and Schulz-Von der Gathen, V.
    Plasma Sources Science and Technology 32 (2023)
    Magnetron sputtering discharges feature complex magnetic field configurations to confine the electrons close to the cathode surface. This magnetic field configuration gives rise to a strong electron drift in azimuthal direction, with typical drift velocities on the order of 100 km s−1. In high power impulse magnetron sputtering plasmas, the ions have also been observed to follow the movement of electrons with velocities of a few km s−1, despite being not magnetized. In this work, we report on measurements of the azimuthal ion velocity using spatially resolved optical emission spectroscopy, allowing for a more direct measurement compared to experiments performed using mass spectrometry. The azimuthal ion velocities increase with target distance, peaking at about 1.55 km s−1 for argon ions and 1.25 km s−1 for titanium ions. Titanium neutrals are also found to follow the azimuthal ion movement which is explained with resonant charge exchange collisions. The experiments are then compared to a simple test-particle simulation of the titanium ion movement, yielding good agreement to the experiments when only considering the momentum transfer from electrons to ions via Coulomb collisions as the only source of acceleration in azimuthal direction. Based on these results, we propose this momentum transfer as the primary source for ion acceleration in azimuthal direction. © 2023 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/acfe95
  • 2023 • 265 Coulomb-mediated antibunching of an electron pair surfing on sound
    Wang, Junliang and Edlbauer, Hermann and Richard, Aymeric and Ota, Shunsuke and Park, Wanki and Shim, Jeongmin and Ludwig, Arne and Wieck, Andreas D. and Sim, Heung-Sun and Urdampilleta, Matias and Meunier, Tristan and Kodera, Tet...
    Nature Nanotechnology 18 721 – 726 (2023)
    Electron flying qubits are envisioned as potential information links within a quantum computer, but also promise—like photonic approaches—to serve as self-standing quantum processing units. In contrast to their photonic counterparts, electron-quantum-optics implementations are subject to Coulomb interactions, which provide a direct route to entangle the orbital or spin degree of freedom. However, controlled interaction of flying electrons at the single-particle level has not yet been established experimentally. Here we report antibunching of a pair of single electrons that is synchronously shuttled through a circuit of coupled quantum rails by means of a surface acoustic wave. The in-flight partitioning process exhibits a reciprocal gating effect which allows us to ascribe the observed repulsion predominantly to Coulomb interaction. Our single-shot experiment marks an important milestone on the route to realize a controlled-phase gate for in-flight quantum manipulations. © 2023, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41565-023-01368-5
  • 2023 • 264 Evolution of local misorientations in the γ/γ’-microstructure of single crystal superalloys during creep studied with the rotation vector baseline (RVB) EBSD method
    Gamanov, Stepan and Dlouhy, A. and Bürger, D. and Eggeler, G. and Thome, P.
    Microscopy Research and Technique (2023)
    The present work uses the rotation vector baseline electron back scatter orientation imaging method (RVB-EBSD) to study the evolution of small misorientations between the γ- and γ′-phase in Ni-base single crystal superalloys (SXs) during creep. For this purpose, two material states of the SX ERBO1 (CMSX4 type) were characterized after creep deformation at 850°C and 600 MPa to final strains of 1% and 2%. Obtaining reliable phase boundary misorientation (PBM), kernel average misorientation (KAM) and orientation spread (OS) data represents a challenge for electron backscatter diffraction (EBSD), not only because the method operates at its limits of lateral and angular resolution, but also because it is difficult to differentiate between the two phases merely based on Kikuchi diffraction. The two phases differ in chemical composition which gives rise to different EBSD background intensities. These can be exploited to differentiate between the two phases. In the present work, crystallographic and chemical information are combined to demonstrate that orientation imaging can be used to document the formation of dislocation networks at γ/γ′-interfaces and the filling of γ-channels by dislocations. These findings are in good agreement with reference results from diffraction contrast scanning transmission electron microscopy. It is also shown that misorientations evolve between small groups of equally oriented γ/γ′-neighborhoods, on a size scale above characteristic γ/γ′-dimensions (>0.5 μm) and below distances associated with dendritic mosaicity (<200 μm). The methodological aspects as well as the new material specific results are discussed in the light of previous work published in the literature. Research Highlights: Microstructure evolution during [001] tensile creep of Ni-based single-crystalline alloy. Application of RVB-EBSD technique, focused on angular misorientations between γ/γ′ phases, with accuracy of 0.01°. Separation of γ/γ′ phases using experimental post-processing of raw EBSD data. © 2023 The Authors. Microscopy Research and Technique published by Wiley Periodicals LLC.
    view abstractdoi: 10.1002/jemt.24453
  • 2023 • 263 Homogeneous electron liquid in arbitrary dimensions beyond the random phase approximation
    Duc Pham, L.V. and Sattler, Pascal and Marques, Miguel A L and Benavides-Riveros, Carlos L
    New Journal of Physics 25 (2023)
    The homogeneous electron liquid is a cornerstone in quantum physics and chemistry. It is an archetypal system in the regime of slowly varying densities in which the exchange-correlation energy can be estimated with many methods. For high densities, the behavior of the ground-state energy is well-known for 1, 2, and 3 dimensions. Here, we extend this model to arbitrary integer dimensions and compute its correlation energy beyond the random phase approximation (RPA). We employ the approach developed by Singwi, Tosi, Land, and Sjölander (STLS), whose description of the electronic density response for 2D and 3D for metallic densities is known to be comparable to Quantum Monte-Carlo. For higher dimensions, we compare the results obtained for the correlation energy with the values previously obtained using RPA. We find that in agreement with what is known for 2 and 3 dimensions, the RPA tends to over-correlate the liquid also at higher dimensions. We furthermore provide new analytical formulae for the unconventional-dimensional case both for the real and imaginary parts of the Lindhard polarizability and for the local field correction of the STLS theory, and illustrate the importance of the plasmon contribution at those high dimensions. © 2023 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/acef4c
  • 2023 • 262 Microscopic Origin of Polarization Charges at Ga N / (Al,Ga) N Interfaces
    Yoo, Su-Hyun and Todorova, Mira and Neugebauer, Jörg and Van De Walle, Chris G.
    Physical Review Applied 19 (2023)
    GaN/(Al,Ga)N heterojunctions are at the heart of high-electron-mobility transistors that are being adopted for high-power and high-frequency applications. The strong polarization fields present at this interface significantly enhance the density of the two-dimensional electron gas (2DEG) that is confined on the GaN side of the junction. The microscopic origin of these electrons has been debated over the years: after excluding that they would be contributed by bulk donors, a model that identifies surface states on the (Al,Ga)N surface as the source of electrons has become widely adopted. Recently, it has become clear, however, that the measured density of surface states is insufficient to account for the high electron density in the 2DEG. Here we demonstrate, based on state-of-the-art first-principles calculations, that the source of electrons is intrinsic to the overall structure and that the negative charge in the 2DEG is balanced by fixed charge on the surface. We perform a rigorous study of polarization, using our recently developed methodology for quantifying polarization fields within the finite-sized systems that can be addressed with density-functional calculations. The results show that the electrons that appear in the 2DEG originate locally at the interface, and that the net charge at the interface is predominantly compensated by fixed charge on the surface, rather than surface states. We elucidate the source of this fixed charge and associate it with surface reconstructions or the presence of heterovalent impurities (such as oxygen). Our results force a reassessment of the impact of surface states on the density of the 2DEG: rather than serving as a supply of electrons, the surface states mainly act to pin the Fermi level. Our conclusions allow a fresh interpretation of experimental observations and allow devising guidelines for optimizing carrier densities in the 2DEG. © 2023 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the ""Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.
    view abstractdoi: 10.1103/PhysRevApplied.19.064037
  • 2023 • 261 Nonlocal dynamics of secondary electrons in capacitively coupled radio frequency discharges
    Noesges, K. and Klich, M. and Derzsi, A. and Horváth, B. and Schulze, J. and Brinkmann, R.P. and Mussenbrock, T. and Wilczek, S.
    Plasma Sources Science and Technology 32 (2023)
    In capacitively coupled radio frequency discharges, the interaction of the plasma and the surface boundaries is linked to a variety of highly relevant phenomena for technological processes. One possible plasma-surface interaction is the generation of secondary electrons (SEs), which significantly influence the discharge when accelerated in the sheath electric field. However, SEs, in particular electron-induced SEs (δ-electrons), are frequently neglected in theory and simulations. Due to the relatively high threshold energy for the effective generation of δ-electrons at surfaces, their dynamics are closely connected and entangled with the dynamics of the ion-induced SEs (γ-electrons). Thus, a fundamental understanding of the electron dynamics has to be achieved on a nanosecond timescale, and the effects of the different electron groups have to be segregated. This work utilizes 1 d 3 v particle-in-cell/Monte Carlo collisions simulations of a symmetric discharge in the low-pressure regime (p = 1 Pa) with the inclusion of realistic electron-surface interactions for silicon dioxide. A diagnostic framework is introduced that segregates the electrons into three groups (‘bulk-electrons’, ‘γ-electrons’, and ‘δ-electrons’) in order to analyze and discuss their dynamics. A variation of the electrode gap size L g a p is then presented as a control tool to alter the dynamics of the discharge significantly. It is demonstrated that this control results in two different regimes of low and high plasma density, respectively. The fundamental electron dynamics of both regimes are explained, which requires a complete analysis starting at global parameters (e.g. densities) down to single electron trajectories. © 2023 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ace848
  • 2023 • 260 Secondary electron emission from magnetron targets
    Buschhaus, R. and von Keudell, A.
    Plasma Sources Science and Technology 32 (2023)
    Ion-induced secondary electron emission of surfaces occurs in all gas discharges which have contact to surfaces such as electrodes or chamber walls. These secondary electrons (SEs) play an important role, for instance, in the performance of DC discharges, RF discharges and magnetron sputtering discharges. SE generation can be separated into potential electron emission (PEE) due to the neutralization of the incident ion upon impact and kinetic electron emission (KEE) due to the electronic stopping of the penetrating ion in the solid. SE due to neutralization is usually described by Auger processes and the density of states of the electrons in the solid, whereas KEE scales with the electronic stopping of the ion in the solid, as being calculated by ion collision simulations. The measurement of the energy distribution of the SEs of three metals (Al, Ti, Cu) and their oxides reveals the occurrence of Auger peaks, which are not reflected by standard models such as the Hagstrum model. Instead, in this paper, a model is proposed describing these Auger peaks by Auger neutralization of holes created by the collision cascade of the incident ion. This shows decent agreement. The contribution of Auger peaks in the metals Al and Ti is very significant, whereas it is negligible in the case of Cu. The implication of these energy distributions to the performance of magnetron sputtering discharges is discussed. © 2023 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/acd57e
  • 2022 • 259 Automated and manual classification of metallic nanoparticles with respect to size and shape by analysis of scanning electron micrographs [Automatisierte und manuelle Klassifizierung metallischer Nanopartikel nach Größe und Form aus rasterelektronenmikroskopischen Aufnahmen]
    Bals, J. and Loza, K. and Epple, P. and Kircher, T. and Epple, M.
    Materialwissenschaft und Werkstofftechnik 53 270-283 (2022)
    Automated image analysis has been applied to scanning electron micrographs (transmission mode; STEM) of metallic nanoparticles (silver and gold; about 10 nm to 20 nm). For a reliable particle identification, scanning electron microscopic images must be recorded with distinct contrast and resolution parameters. The particles were separated from the background and classified according to shape and size by machine learning (machine learning). Training images were created with model particles cut out of real electron microscopic images. The automated analysis of the particle size (expressed as area) was well possible, but overlapping particles could not be safely separated. The assignment of particle to six different shape classes (sphere, triangle, square, pentagon, hexagon, rod) by automated analysis was difficult. The fact that real particles never have an ideal geometrical shape but are always distorted or have rough edges or cropped tips is the fundamental reason of this problem. This effect also occurred with human image evaluators and poses a considerable obstacle in the training process for machine learning. Image analysis by machine learning techniques is difficult if different human evaluators disagree on the shape assignment of given particles because a proper training cannot be provided. © 2022 The Authors. Materialwissenschaft und Werkstofftechnik published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/mawe.202100285
  • 2022 • 258 Efficient Nitrate Conversion to Ammonia on f-Block Single-Atom/Metal Oxide Heterostructure via Local Electron-Deficiency Modulation
    Kumar, A. and Lee, J. and Kim, M.G. and Debnath, B. and Liu, X. and Hwang, Y. and Wang, Y. and Shao, X. and Jadhav, A.R. and Liu, Y. and Tüysüz, H. and Lee, H.
    ACS Nano 16 15297-15309 (2022)
    Exploring single-atom catalysts (SACs) for the nitrate reduction reaction (NO3-NitRR) to value-added ammonia (NH3) offers a sustainable alternative to both the Haber-Bosch process and NO3--rich wastewater treatment. However, due to the insufficient electron deficiency and unfavorable electronic structure of SACs, resulting in poor NO3--adsorption, sluggish proton (H*) transfer kinetics, and preferred hydrogen evolution, their NO3--to-NH3selectivity and yield rate are far from satisfactory. Herein, a systematic theoretical prediction reveals that the local electron deficiency of an f-block Gd single atom (GdSA) can be significantly regulated upon coordination with oxygen-defect-rich NiO (GdSA-D-NiO400) support. Thus, facilitating stronger NO3-adsorption via strong Gd5d-O2porbital coupling and further improving the protonation kinetics of adsorption intermediates by rapid H∗ capture from water dissociation catalyzed by the adjacent oxygen vacancy site along with suppressed H∗ dimerization synergistically boosts the NH3selectivity/yield rate. Motivated by DFT prediction, we delicately stabilized electron-deficient (strongly electrophilic) GdSAon D-NiO400(?84% strong electrophilic sites), which exhibited excellent alkaline NitRR activity (NH3Faradaic efficiency ?97% and yield rate ?628 μg/(mgcath)) along with superior structural stability, as revealed by in situ Raman spectroscopy, significantly outperforming weakly electrophilic Gd nanoparticles, defect-free GdSA-P-NiO400, and reported state-of-the-art catalysts. © 2022 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/acsnano.2c06747
  • 2022 • 257 Electron g-factor determined for quantum dot circuit fabricated from (110)-oriented GaAs quantum well
    Nakagawa, T. and Lamoureux, S. and Fujita, T. and Ritzmann, J. and Ludwig, Ar. and Wieck, A.D. and Oiwa, A. and Korkusinski, M. and Sachrajda, A. and Austing, D.G. and Gaudreau, L.
    Journal of Applied Physics 131 (2022)
    The choice of substrate orientation for semiconductor quantum dot circuits offers opportunities for tailoring spintronic properties such as g-factors for specific functionality. Here, we demonstrate the operation of a few-electron double quantum dot circuit fabricated from a (110)-oriented GaAs quantum well. We estimate the in-plane electron g-factor from the profile of the enhanced inter-dot tunneling (leakage) current near-zero magnetic field. Spin blockade due to Pauli exclusion can block inter-dot tunneling. However, this blockade becomes inactive due to hyperfine interaction mediated spin flip-flop processes between electron spin states and the nuclear spin of the host material. The g-factor of absolute value ∼0.1 found for a magnetic field parallel to the direction [1 ¯ 10] is approximately a factor of four lower than that for comparable circuits fabricated from a material grown on widely employed standard (001) GaAs substrates and is in line with reported values determined by purely optical means for quantum well structures grown on (110) GaAs substrates. © 2022 Author(s).
    view abstractdoi: 10.1063/5.0086555
  • 2022 • 256 Exploring stability of a nanoscale complex solid solution thin film by in situ heating transmission electron microscopy
    Manjón, A.G. and Zhang, S. and Völker, B. and Meischein, M. and Ludwig, Al. and Scheu, C.
    MRS Bulletin (2022)
    Abstract: Combining thin film deposition with in situ heating electron microscopy allows to understand the thermal stability of complex solid solution nanomaterials. From a CrMnFeCoNi alloy target a thin film with an average thickness of ~10 nm was directly sputtered onto a heating chip for in situ transmission electron microscopy. We investigate the growth process and the thermal stability of the alloy and compare our results with other investigations on bulk alloys or bulk-like films thicker than 100 nm. For the chosen sputtering condition and SiNx substrate, the sputter process leads to the Stranski–Krastanov growth type (i.e., islands forming on the top of a continuous layer). Directly after sputtering, we detect two different phases, namely CoNi-rich nanoscale islands and a continuous CrMnFe-rich layer. In situ annealing of the thin film up to 700°C leads to Ostwald ripening of the islands, which is enhanced in the areas irradiated by the electron beam during heating. Besides Ostwald ripening, the chemical composition of the continuous layer and the islands changed during the heating process. After annealing, the islands are still CoNi-rich, but lower amounts of Fe and Cr are observed and Mn was completely absent. The continuous layer also changed its composition. Co and Ni were removed, and the amount of Cr lowered. These results confirm that the synthesis of a CrMnFeCoNi thin film with an average thickness of ~10 nm can lead to a different morphology, chemical composition, and stability compared to thicker films (>100 nm). Impact statement: Exploring stability of a complex solid solution thin film by in situ heating transmission electron microscopy is a study of the thermal stability of sputtered complex solid solution thin films with thicknesses of ~10 nm. Complex solid solution materials have a promising electrocatalytic behavior due to the interplay of multi-element active sites. In order to understand their catalytic properties, it is important to identify the different structure-composition-activity correlations. Thus, our investigation helps to clarify and to understand the stability of nanoscale complex solid solution with an average film thickness of ~10 nm. Graphic abstract: Combining sputter deposition with in situ heating transmission electron microscopy allows to understand the thermal stability of nanoscale complex solid solution thin films. [Figure not available: see fulltext.] © 2022, The Author(s).
    view abstractdoi: 10.1557/s43577-021-00217-x
  • 2022 • 255 Influence of Two-Step Heat Treatments on Microstructure and Mechanical Properties of a β-Solidifying Titanium Aluminide Alloy Fabricated via Electron Beam Powder Bed Fusion
    Moritz, J. and Teschke, M. and Marquardt, A. and Heinze, S. and Heckert, M. and Stepien, L. and López, E. and Brueckner, F. and Walther, F. and Leyens, C.
    Advanced Engineering Materials (2022)
    Additive manufacturing technologies, particularly electron beam powder bed fusion (PBF-EB/M), are becoming increasingly important for the processing of intermetallic titanium aluminides. This study presents the effects of hot isostatic pressing (HIP) and subsequent two-step heat treatments on the microstructure and mechanical properties of the TNM-B1 alloy (Ti–43.5Al–4Nb–1Mo–0.1B) fabricated via PBF-EB/M. Adequate solution heat treatment temperatures allow the adjustment of fully lamellar (FL) and nearly lamellar (NL-β) microstructures. The specimens are characterized by optical microscopy and scanning electron microscopy (SEM), X-ray computed tomography (CT), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The mechanical properties at ambient temperatures are evaluated via tensile testing and subsequent fractography. While lack-of-fusion defects are the main causes of failure in the as-built condition, the mechanical properties in the heat-treated conditions are predominantly controlled by the microstructure. The highest ultimate tensile strength is achieved after HIP due to the elimination of lack-of-fusion defects. The results reveal challenges originating from the PBF-EB/M process, for example, local variations in chemical composition due to aluminum evaporation, which in turn affect the microstructures after heat treatment. For designing suitable heat treatment strategies, particular attention should therefore be paid to the microstructural characteristics associated with additive manufacturing. © 2022 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/adem.202200931
  • 2022 • 254 Ion-induced secondary electron emission of oxidized nickel and copper studied in beam experiments
    Buschhaus, R. and Prenzel, M. and Von Keudell, A.
    Plasma Sources Science and Technology 31 (2022)
    Ion-induced secondary electron emission at a target surface is an essential mechanism for laboratory plasmas, i.e. magnetron sputtering discharges. Electron emission, however, is strongly affected by the target condition itself such as oxidation. Data of oxidized targets, however, are very sparse and prone to significant systematic errors, because they were often determined by modeling the complex behavior of the plasma. Thus, it is difficult to isolate the process of ion-induced electron emission from all other plasma-surface-interactions. By utilizing ion beams, the complex plasma environment is avoided and electron yields are determined with higher accuracy. In this study, ion-induced secondary electron emission coefficients (SEECs) of clean, untreated (air-exposed), and intentionally oxidized copper and nickel surfaces were investigated in such a particle beam experiment. Pristine and oxidized metal foils were exposed to beams of singly charged argon ions with energies of 0.2 keV-10 keV. After the ion beam treatment, the surface conditions were analyzed by ex-situ X-ray photoelectron spectroscopy measurements. Further, a model for the electron emission of a partly oxidized surface is presented, which is in agreement with the experimental data. It was found, that oxidized and untreated/air-exposed surfaces do not show the same SEEC: for intentionally oxidized targets, the electron yields were smaller by a factor of 2 than for untreated/air-exposed surfaces. SEECs of oxides were found to be between the values for clean and for untreated metal surfaces. Further, the SEEC was at maximum for untreated/air-exposed surfaces and at minimum for clean surfaces; the electron yields of untreated/air-exposed and clean surfaces were in agreement with values reported in the literature. © 2022 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ac4c4c
  • 2022 • 253 Microstructure analysis and mechanical properties of electron beam powder bed fusion (PBF-EB)-manufactured γ-titanium aluminide (TiAl) at elevated temperatures
    Kotzem, D. and Teschke, M. and Juechter, V. and Körner, C. and Walther, F.
    Materialpruefung/Materials Testing 64 636-646 (2022)
    Additively manufactured γ-titanium aluminide has a high specific strength and temperature resistance. This opens new possibilities for future lightweight constructions for aerospace applications. The objective of this work was to characterize additively manufactured Ti-48Al-2Cr-2Nb alloy specimens, which were successfully manufactured by electron beam powder bed fusion. For microstructural characterization, the as-built state was investigated with light and scanning electron microscopy. In the electron backscatter diffraction analysis, the size and the orientation of the grains were observed. The pore size and distribution were examined in computer tomographic scans, which showed a near fully dense material with a relative density of >99.9%. Furthermore, the hardness curve over the building height was examined in hardness mappings. Thereby, a strong decrease in hardness could be observed with an increase in part height. To evaluate the reliability of the manufactured alloy, quasi-static compression tests were carried out at temperatures up to 650 °C. Within these tests, a high compression strength (σc,p,0.2,650 °C = 684 MPa) was determined, which implicated a potential substitution of nickel-based superalloy components in aerospace applications under compressive loads. © 2022 Walter de Gruyter GmbH, Berlin/Boston.
    view abstractdoi: 10.1515/mt-2021-2137
  • 2022 • 252 Nanoscale subsurface dynamics of solids upon high-intensity femtosecond laser irradiation observed by grazing-incidence x-ray scattering
    Randolph, L. and Banjafar, M. and Preston, T.R. and Yabuuchi, T. and Makita, M. and Dover, N.P. and Rödel, C. and Göde, S. and Inubushi, Y. and Jakob, G. and Kaa, J. and Kon, A. and Koga, J.K. and Ksenzov, D. and Matsuoka, T. an...
    Physical Review Research 4 (2022)
    Observing ultrafast laser-induced structural changes in nanoscale systems is essential for understanding the dynamics of intense light-matter interactions. For laser intensities on the order of 1014W/cm2, highly collisional plasmas are generated at and below the surface. Subsequent transport processes such as heat conduction, electron-ion thermalization, surface ablation, and resolidification occur at picosecond and nanosecond timescales. Imaging methods, e.g., using x-ray free-electron lasers (XFEL), were hitherto unable to measure the depth-resolved subsurface dynamics of laser-solid interactions with appropriate temporal and spatial resolution. Here we demonstrate picosecond grazing-incidence small-angle x-ray scattering (GISAXS) from laser-produced plasmas using XFEL pulses. Using multilayer (ML) samples, both the surface ablation and subsurface density dynamics are measured with nanometer depth resolution. Our experimental data challenges the state-of-the-art modeling of matter under extreme conditions and opens new perspectives for laser material processing and high-energy density science. © 2022 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevResearch.4.033038
  • 2022 • 251 On the Mediated Electron Transfer of Immobilized Galactose Oxidase for Biotechnological Applications
    Zhao, F. and Brix, A.C. and Lielpetere, A. and Schuhmann, W. and Conzuelo, F.
    Chemistry - A European Journal 28 (2022)
    The use of enzymes as catalysts in chemical synthesis offers advantages in terms of clean and highly selective transformations. Galactose oxidase (GalOx) is a remarkable enzyme with several applications in industrial conversions as it catalyzes the oxidation of primary alcohols. We have investigated the wiring of GalOx with a redox polymer; this enables mediated electron transfer with the electrode surface for its potential application in biotechnological conversions. As a result of electrochemical regeneration of the catalytic center, the formation of harmful H2O2 is minimized during enzymatic catalysis. The introduced bioelectrode was applied to the conversion of bio-renewable platform materials, with glycerol as model substrate. The biocatalytic transformations of glycerol and 5-hydroxymethylfurfural (HMF) were investigated in a circular flow-through setup to assess the possibility of substrate over-oxidation, which is observed for glycerol oxidation but not during HMF conversion. © 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/chem.202200868
  • 2022 • 250 Propagation of nanosecond plasmas in liquids - Streamer velocities and streamer lengths
    Jungling, E. and Grosse, K. and Von Keudell, A.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 40 (2022)
    Nanosecond plasmas in liquids are often generated by applying a short high voltage pulse to an electrode immersed in a liquid for biomedical or environmental applications. The plasmas appear as streamers that propagate through the liquid. The understanding of the ignition of these nanosecond plasmas in liquids, however, is an open question. The occurrence of any traditional gas phase ignition mechanism is unlikely, because the formation of a gas bubble prior to ignition is suppressed by the inertia of the liquid. Therefore, either electron multiplication inside nanopores that are induced by an electric field pressure gradient or field effects at the tip and at the ionization front of the liquid streamer may act as electron generation mechanisms. A deeper understanding can be achieved by comparing the velocity and dynamic of the plasma propagation with modeling, where the individual mechanisms and transport coefficients can be analyzed. Here, we are using intensified charge-coupled device imaging to investigate the time dependence of the streamer dynamic and compare this with a 1D fluid code for negative voltages. It is shown that the maximum streamer length scales with the applied electric field, indicating that an electric stability field in the liquid streamer channel is important, as known for gas streamers. The 1D fluid code can reproduce the proper streamer velocities, if transport coefficients for hydrated electrons are chosen. The model suggests that the propagation of liquid streamers is dominated by the local ionization rate at the ionization front rather than by advection or diffusion of electrons as in gases. This also explains the finding that positive and negative streamers exhibit almost identical electron densities. © 2022 Author(s).
    view abstractdoi: 10.1116/6.0001669
  • 2022 • 249 Scaling and Confinement in Ultrathin Chalcogenide Films as Exemplified by GeTe
    Kerres, P. and Zhou, Y. and Vaishnav, H. and Raghuwanshi, M. and Wang, J. and Häser, M. and Pohlmann, M. and Cheng, Y. and Schön, C.-F. and Jansen, T. and Bellin, C. and Bürgler, D.E. and Jalil, A.R. and Ringkamp, C. and Kowalc...
    Small 18 (2022)
    Chalcogenides such as GeTe, PbTe, Sb2Te3, and Bi2Se3 are characterized by an unconventional combination of properties enabling a plethora of applications ranging from thermo-electrics to phase change materials, topological insulators, and photonic switches. Chalcogenides possess pronounced optical absorption, relatively low effective masses, reasonably high electron mobilities, soft bonds, large bond polarizabilities, and low thermal conductivities. These remarkable characteristics are linked to an unconventional bonding mechanism characterized by a competition between electron delocalization and electron localization. Confinement, that is, the reduction of the sample dimension as realized in thin films should alter this competition and modify chemical bonds and the resulting properties. Here, pronounced changes of optical and vibrational properties are demonstrated for crystalline films of GeTe, while amorphous films of GeTe show no similar thickness dependence. For crystalline films, this thickness dependence persists up to remarkably large thicknesses above 15 nm. X-ray diffraction and accompanying simulations employing density functional theory relate these changes to thickness dependent structural (Peierls) distortions, due to an increased electron localization between adjacent atoms upon reducing the film thickness. A thickness dependence and hence potential to modify film properties for all chalcogenide films with a similar bonding mechanism is expected. © 2022 The Authors. Small published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/smll.202201753
  • 2022 • 248 Single-crystal graphene on Ir(110)
    Kraus, S. and Huttmann, F. and Fischer, J. and Knispel, T. and Bischof, K. and Herman, A. and Bianchi, M. and Stan, R.-M. and Holt, A.J. and Caciuc, V. and Tsukamoto, S. and Wende, H. and Hofmann, P. and Atodiresei, N. and Michely, T.
    Physical Review B 105 (2022)
    A single-crystal sheet of graphene is synthesized on the low-symmetry substrate Ir(110) by thermal decomposition of C2H4 at 1500 K. Using scanning tunneling microscopy, low-energy electron diffraction, angle-resolved photoemission spectroscopy, and ab initio density functional theory, the structure and electronic properties of the adsorbed graphene sheet and its moiré with the substrate are uncovered. The adsorbed graphene layer forms a wave pattern of nanometer wavelength with a corresponding modulation of its electronic properties. This wave pattern is demonstrated in density functional theory calculations to enable the templated adsorption of naphthalene molecules, and in experiment to uniaxially align sandwich-molecular wires composed of Eu and cyclooctatetraene molecules. © 2022 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.105.165405
  • 2022 • 247 Single-Electron Oxidation of Carbene-Coordinated Pnictinidenes-Entry into Heteroleptic Radical Cations and Metalloid Clusters
    Krüger, J. and Haak, J. and Wölper, C. and Cutsail, G.E. and Haberhauer, G. and Schulz, S.
    Inorganic Chemistry 61 5878-5884 (2022)
    Stable heavy main group element radicals are challenging synthetic targets. Although several strategies have been developed to stabilize such odd-electron species, the number of heavier pnictogen-centered radicals is limited. We report on a series of two-coordinated pnictogen-centered radical cations [(MecAAC)EGa(Cl)L][B(C6F5)4] (MecAAC = [H2C(CMe2)2NDipp]C; Dipp = 2,6-i-Pr2C6H3; E = As 1, Sb 2, Bi 3; L = HC[C(Me)NDipp]2) synthesized by one-electron oxidation of L(Cl)Ga-substituted pnictinidenes (MecAAC)EGa(Cl)L (E = As I, Sb II, Bi III). 1-3 were characterized by electron paramagnetic resonance (EPR) spectroscopy and single crystal X-ray diffraction (sc-XRD) (1, 2), while quantum chemical calculations support their description as carbene-coordinated pnictogen-centered radical cations. The low thermal stability of 3 enables access to metalloid bismuth clusters as shown by formation of [{LGa(Cl)}3Bi6][B(C6F5)4] (4). © 2022 American Chemical Society.
    view abstractdoi: 10.1021/acs.inorgchem.2c00249
  • 2022 • 246 The effects of the driving frequencies on micro atmospheric pressure He/N2plasma jets driven by tailored voltage waveforms
    Hübner, G. and Bischoff, L. and Korolov, I. and Donkó, Z. and Leimkühler, M. and Liu, Y. and Böke, M. and Schulz-Von Der Gathen, V. and Mussenbrock, T. and Schulze, J.
    Journal of Physics D: Applied Physics 55 (2022)
    Capacitively coupled micro atmospheric pressure plasma jets are important tools for the generation of radicals at room temperature for various applications. Voltage waveform tailoring (VWT), which is based on the simultaneous use of a set of excitation frequencies, has been demonstrated to provide an efficient control of the electron energy probability function (EEPF) in such plasmas and, thus, allows optimizing the electron impact driven excitation and dissociation processes as compared to the classical single-frequency operation mode. In this work, the effects of changing the driving frequencies on the spatio-temporally resolved electron power absorption dynamics, the generation of helium metastables and the dissociation of nitrogen molecules are investigated in He/N2 plasmas based on experiments and simulations. We find that under a single-frequency excitation, the plasma and helium metastable densities are enhanced as a function of the driving frequency at a fixed voltage. When using valleys-type driving voltage waveforms synthesized based on consecutive harmonics of the fundamental driving frequency, the spatial symmetry of the electron power absorption dynamics and of the metastable density profile is broken. Increasing the fundamental frequency at a constant voltage is found to drastically enhance the plasma and metastable densities, which is a consequence of the change of the EEPF. Finally, we compare the energy efficiency of the formation of radicals under single-frequency and VWT operation at different driving frequencies. For a given power dissipated in the plasma, VWT yields a higher helium metastable as well as electron density and a higher dissociation rate of N2. © 2021 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/ac3791
  • 2021 • 245 3d transition-metal high-entropy Invar alloy developed by adjusting the valence-electron concentration
    Rao, Z. and Cąklr, A. and Özgün, Ö. and Ponge, D. and Raabe, D. and Li, Z. and Acet, M.
    Physical Review Materials 5 (2021)
    By considering the valence-electron concentration of 3d transition-metal alloys and compounds, we develop 3d high-entropy alloy Mn12.1Fe34.2Co33.5Ni12.3Cu7.9 with 8.7 electrons per atom, which is identical to that of Fe65Ni35 Invar. We carry out X-ray diffraction, scanning electron microscopy, magnetization, thermal expansion, and elastic modulus measurements, by which we show that the HEA alloy indeed carries Invar properties. This is evidenced particularly by the observed spontaneous volume magnetostriction and the lattice softening covering a broad temperature-range around the ferromagnetic Curie temperature. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.5.044406
  • 2021 • 244 All-electron real-time and imaginary-time time-dependent density functional theory within a numeric atom-centered basis function framework
    Hekele, J. and Yao, Y. and Kanai, Y. and Blum, V. and Kratzer, P.
    Journal of Chemical Physics 155 (2021)
    Real-time time-dependent density functional theory (RT-TDDFT) is an attractive tool to model quantum dynamics by real-time propagation without the linear response approximation. Sharing the same technical framework of RT-TDDFT, imaginary-time time-dependent density functional theory (it-TDDFT) is a recently developed robust-convergence ground state method. Presented here are high-precision all-electron RT-TDDFT and it-TDDFT implementations within a numerical atom-centered orbital (NAO) basis function framework in the FHI-aims code. We discuss the theoretical background and technical choices in our implementation. First, RT-TDDFT results are validated against linear-response TDDFT results. Specifically, we analyze the NAO basis sets’ convergence for Thiel’s test set of small molecules and confirm the importance of the augmentation basis functions for adequate convergence. Adopting a velocity-gauge formalism, we next demonstrate applications for systems with periodic boundary conditions. Taking advantage of the all-electron full-potential implementation, we present applications for core level spectra. For it-TDDFT, we confirm that within the all-electron NAO formalism, it-TDDFT can successfully converge systems that are difficult to converge in the standard self-consistent field method. We finally benchmark our implementation for systems up to ∼500 atoms. The implementation exhibits almost linear weak and strong scaling behavior. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0066753
  • 2021 • 243 Anisotropic expansion of drifting spin helices in GaAs quantum wells
    Anghel, S. and Poshakinskiy, A.V. and Schiller, K. and Passmann, F. and Ruppert, C. and Tarasenko, S.A. and Yusa, G. and Mano, T. and Noda, T. and Betz, M.
    Physical Review B 103 (2021)
    The drift of electron spin helices in an external in-plane electric field in GaAs quantum wells is studied by means of time-resolved magneto-optical Kerr microscopy. The evolution of the spin distribution measured for different excitation powers reveals that, for short delay times and higher excitation powers, the spin helix drift slows down while its envelope becomes anisotropic. The effect is understood as a local decrease of the electron gas mobility due to electron collisions with nonequilibrium holes within the excitation spot and is reproduced well in the kinetic theory framework. For larger delay times, when the electrons constituting the spin helix and nonequilibrium holes are separated by an electric field, the spin helix drift accelerates and the mobility reaches its unperturbed value again. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.035429
  • 2021 • 242 Broad background in electron diffraction of 2D materials as a signature of their superior quality
    Petrović, M. and Meyer Zu Heringdorf, F.J. and Horn-von Hoegen, M. and Thiel, P.A. and Tringides, M.C.
    Nanotechnology 32 (2021)
    An unusually broad bell-shaped component (BSC) has been previously observed in surface electron diffraction on different types of 2D systems. It was suggested to be an indicator of uniformity of epitaxial graphene (Gr) and hexagonal boron nitride (hBN). In the current study we use low-energy electron microscopy and micro-diffraction to directly relate the BSC to the crystal quality of the diffracting 2D material. Specially designed lateral heterostructures were used to map the spatial evolution of the diffraction profile across different 2D materials, namely pure hBN, BCN alloy and pure Gr, where the alloy region exhibits deteriorated structural coherency. The presented results show that the BSC intensity has a minimum in the alloyed region, consequently showing that BSC is sensitive to the lateral domain size and homogeneity of the material under examination. This is further confirmed by the presence of a larger number of sharp moiré spots when the BSC is most pronounced in the pure hBN and Gr regions. Consequently, it is proposed that the BSC can be used as a diagnostic tool for determining the quality of the 2D materials. © 2021 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/ac244f
  • 2021 • 241 Bulk electronic structure of lanthanum hexaboride (La B6) by hard x-ray angle-resolved photoelectron spectroscopy
    Rattanachata, A. and Nicolaï, L.C. and Martins, H.P. and Conti, G. and Verstraete, M.J. and Gehlmann, M. and Ueda, S. and Kobayashi, K. and Vishik, I. and Schneider, C.M. and Fadley, C.S. and Gray, A.X. and Minár, J. and Nemšák, S.
    Physical Review Materials 5 (2021)
    In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB6 has a special place, being a traditional d-band metal without additional f bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of LaB6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true "bulklike"electronic band structure of rare-earth hexaborides and strongly correlated materials. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.5.055002
  • 2021 • 240 Coexistence of Short- And Long-Range Ferromagnetic Proximity Effects in a Fe/(Cd,Mg)Te/CdTe Quantum Well Hybrid Structure
    Kalitukha, I.V. and Ken, O.S. and Korenev, V.L. and Akimov, I.A. and Sapega, V.F. and Yakovlev, D.R. and Dimitriev, G.S. and Langer, L. and Karczewski, G. and Chusnutdinow, S. and Wojtowicz, T. and Bayer, M.
    Nano Letters 21 2370-2375 (2021)
    In a Fe/(Cd,Mg)Te/CdTe quantum well hybrid structure, short-range and long-range ferromagnetic proximity effects are found to coexist. The former is observed for conduction band electrons, while the latter is observed for holes bound to shallow acceptors in the CdTe quantum well. These effects arise from the interaction of charge carriers confined in the quantum well with different ferromagnets, where electrons interact with the Fe film and holes with an interfacial ferromagnet at the Fe/(Cd,Mg)Te interface. The two proximity effects originate from fundamentally different physical mechanisms. The short-range proximity effect for electrons is determined by the overlap of their wave functions with d-electrons of the Fe film. On the contrary, the long-range effect for holes bound to acceptors is not associated with overlapping wave functions and can be mediated by elliptically polarized phonons. The coexistence of the two ferromagnetic proximity effects reveals the presence of a nontrivial spin texture within the same heterostructure. © 2021 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/acs.nanolett.0c04218
  • 2021 • 239 Coherent Beam Splitting of Flying Electrons Driven by a Surface Acoustic Wave
    Ito, R. and Takada, S. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S. and Yamamoto, M.
    Physical Review Letters 126 (2021)
    We develop a coherent beam splitter for single electrons driven through two tunnel-coupled quantum wires by surface acoustic waves (SAWs). The output current through each wire oscillates with gate voltages to tune the tunnel coupling and potential difference between the wires. This oscillation is assigned to coherent electron tunneling motion that can be used to encode a flying qubit and is well reproduced by numerical calculations of time evolution of the SAW-driven single electrons. The oscillation visibility is currently limited to about 3%, but robust against decoherence, indicating that the SAW electron can serve as a novel platform for a solid-state flying qubit. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.126.070501
  • 2021 • 238 Coherent Spin Dynamics of Electrons and Holes in CsPbBr3Colloidal Nanocrystals
    Grigoryev, P.S. and Belykh, V.V. and Yakovlev, D.R. and Lhuillier, E. and Bayer, M.
    Nano Letters 21 8481-8487 (2021)
    The spin dynamics in CsPbBr3 lead halide perovskite nanocrystals are studied by picosecond pump-probe Faraday rotation in an external magnetic field. Coherent Larmor precession of electrons and holes with spin dephasing times of ∼600 ps is detected in a transversal magnetic field. The longitudinal spin relaxation time in weak magnetic fields reaches 80 ns at a temperature of 5 K. In this regime, the carrier spin dynamics is governed by nuclear spin fluctuations characterized by an effective hyperfine field strength of 25 mT. The Landé factors determining the carrier Zeeman splittings are ge = +1.73 for electrons and gh = +0.83 for holes. A comparison with a CsPbBr3 polycrystalline film and bulk single crystals evidences that the spatial confinement of electrons and holes in the nanocrystals only slightly affects their g factors and spin dynamics. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.1c03292
  • 2021 • 237 Control of electron velocity distributions at the wafer by tailored voltage waveforms in capacitively coupled plasmas to compensate surface charging in high-aspect ratio etch features
    Hartmann, P. and Wang, L. and Nösges, K. and Berger, B. and Wilczek, S. and Brinkmann, R.P. and Mussenbrock, T. and Juhasz, Z. and Donkó, Z. and Derzsi, A. and Lee, E. and Schulze, J.
    Journal of Physics D: Applied Physics 54 (2021)
    Low pressure single- or dual-frequency capacitively coupled radio frequency (RF) plasmas are frequently used for high-aspect ratio (HAR) dielectric etching due to their capability to generate vertical ion bombardment of the wafer at high energies. Electrons typically reach the wafer at low energies and with a wide angular distribution during the local sheath collapse. Thus, in contrast to positive ions, electrons cannot propagate deeply into HAR etch features and the bottom as well as the sidewalls of such trenches can charge up positively, while the mask charges negatively. This causes etch stops and distortion of profile shapes. Here, we investigate low pressure, high voltage capacitively coupled RF argon gas discharges by Particle-In-Cell/Monte Carlo collisions simulations and demonstrate that this problem can be solved by Voltage Waveform Tailoring, i.e. the velocity and angular distribution of electrons impacting on the electrodes can be tuned towards high velocities and small angles to the surface-normal, while keeping the energies of the impacting ions high. The applied voltage waveforms consist of a base frequency of 400 kHz with 10 kV amplitude and a series of higher harmonics. A high frequency component at 40 or 60 MHz is used additionally. Square voltage waveforms with different rise-times are examined as well. We show that high fluxes of electrons towards the wafer at normal velocities of up to 2.2 × 107 m s-1 (corresponding to 1.4 keV energy) can be realized. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/abf229
  • 2021 • 236 Distant spin entanglement via fast and coherent electron shuttling
    Jadot, B. and Mortemousque, P.-A. and Chanrion, E. and Thiney, V. and Ludwig, Ar. and Wieck, A.D. and Urdampilleta, M. and Bäuerle, C. and Meunier, T.
    Nature Nanotechnology 16 570-575 (2021)
    In the quest for large-scale quantum computing, networked quantum computers offer a natural path towards scalability. While recent experiments have demonstrated nearest neighbour entanglement for electron spin qubits in semiconductors, on-chip long-distance entanglement could bring more versatility to connect quantum core units. Here, we employ the moving trapping potential of a surface acoustic wave to realize the controlled and coherent transfer of a pair of entangled electron spins between two distant quantum dots. The subsequent electron displacement induces coherent spin rotations, which drives spin quantum interferences. We observe high-contrast interference as a signature of the preservation of the entanglement all along the displacement procedure, which includes a separation of the two spins by a distance of 6 μm. This work opens the route towards fast on-chip deterministic interconnection of remote quantum bits in semiconductor quantum circuits. © 2021, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41565-021-00846-y
  • 2021 • 235 Dynamic polarization of electron spins in indirect band gap (In,Al)As/AlAs quantum dots in a weak magnetic field: Experiment and theory
    Shamirzaev, T.S. and Shumilin, A.V. and Smirnov, D.S. and Rautert, J. and Yakovlev, D.R. and Bayer, M.
    Physical Review B 104 (2021)
    A novel spin orientation mechanism - dynamic electron spin polarization - has been recently suggested in Phys. Rev. Lett. 125, 156801 (2020)PRLTAO0031-900710.1103/PhysRevLett.125.156801. It takes place for unpolarized optical excitation in weak magnetic fields of the order of a few millitesla. In this paper we demonstrate experimentally and theoretically that the dynamic electron spin polarization degree changes sign as a function of time, strength of the applied magnetic field, and its direction. The studies are performed on indirect band-gap (In,Al)As/AlAs quantum dots and their results are explained in the framework of a theoretical model developed for our experimental setting. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.104.115405
  • 2021 • 234 Electron heating mode transitions in radio-frequency driven micro atmospheric pressure plasma jets in He/O2: A fluid dynamics approach
    Liu, Y. and Korolov, I. and Hemke, T. and Bischoff, L. and Hübner, G. and Schulze, J. and Mussenbrock, T.
    Journal of Physics D: Applied Physics 54 (2021)
    A two-dimensional fluid model is used to investigate the electron heating dynamics and the production of neutral species in a capacitively coupled radio-frequency micro atmospheric pressure helium plasma jet - specifically the COST jet - with a small oxygen admixture. Electron heating mode transitions are found to be induced by varying the driving voltage amplitude and the O2 concentration numerically and experimentally. The helium metastable density, and the charged species densities are highly relevant to the electron heating dynamics. By analyzing the creation and destruction mechanisms of the negative ions, we find that the generation of negative ions strongly depends on the O2 concentration. The increase of the electronegativity with the increasing O2 concentration leads to an enhancement of the bulk drift electric field. The distributions of the different neutral species densities along the direction of the gas flow inside the jet, as well as in the effluent differ a lot due to the relevant chemical reaction rates and the effect of the gas flow. The simulated results show that a fluid model can be an effective tool for qualitative investigations of micro atmospheric pressure plasma jets. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/abf370
  • 2021 • 233 Enhanced spin coherence while displacing electron in a two-dimensional array of quantum dots
    Mortemousque, P.-A. and Jadot, B. and Chanrion, E. and Thiney, V. and Bäuerle, C. and Ludwig, Ar. and Wieck, A.D. and Urdampilleta, M. and Meunier, T.
    PRX Quantum 2 (2021)
    The ability to shuttle coherently individual electron spins in arrays of quantum dots is a key procedure for the development of scalable quantum information platforms. It allows the use of sparsely populated electron spin arrays, envisioned to efficiently tackle the one- and two-qubit gate challenges. When the electrons are displaced in an array, they are exposed to site-dependent environment interactions such as hyperfine coupling with substrate nuclear spins. Here, we demonstrate that the electron multidirectional displacement in a 3×3 array of tunnel-coupled quantum dots enhances the spin-coherence time via the motional narrowing phenomenon. More specifically, up to ten charge configurations are explored by the electrons to study the impact of the displacement on spin dynamics. An increase of the coherence time by a factor up to 10 is observed in the case of fast and repetitive displacement. A simple model quantitatively captures the physical mechanism underlying this enhancement of the spin-coherence time induced by displacement. The implications on spin-coherence properties during the electron displacement are discussed in the context of large-scale quantum circuits. © 2021 authors.
    view abstractdoi: 10.1103/PRXQuantum.2.030331
  • 2021 • 232 Ignition and propagation of nanosecond pulsed plasmas in distilled water - Negative vs positive polarity applied to a pin electrode
    Grosse, K. and Falke, M. and Von Keudell, A.
    Journal of Applied Physics 129 (2021)
    Nanosecond plasmas in liquids are being used for water treatment, electrolysis, or biomedical applications. The exact nature of these very dynamic plasmas and, most importantly, their ignition physics are strongly debated. The ignition itself may be explained by two competing hypotheses: ignition in water may occur (i) via field effects at the tip of the electrode followed by tunneling of electrons in between water molecules causing field ionization or (ii) via gaseous processes of electron multiplication in nanovoids that are created from liquid ruptures due to the strong electric field gradients. Both hypotheses are supported by theory, but experimental data are very sparse due to the difficulty in monitoring the very fast processes in space and time. In this paper, we analyze nanosecond plasmas in water that are created by applying a positive and a negative polarity to a sharp tungsten electrode. The main diagnostics are fast camera measurements and fast emission spectroscopy. It is shown that plasma ignition is dominated by field effects at the electrode-liquid interface either as field ionization for positive polarity or as field emission for negative polarity. This leads to a hot tungsten surface at a temperature of 7000 K for positive polarity, whereas the surface temperature is much lower for negative polarity. At ignition, the electron density reaches 4 × 10 25 m - 3 for the positive and 2 × 10 25 m - 3 for the negative polarity. At the same time, the emission of the H α light for the positive polarity is four times higher than that for the negative polarity. During plasma propagation, the electron densities are almost identical of the order of 1- 2 × 10 25 m - 3 followed by a decay after the end of the pulse over 15 ns. It is concluded that plasma propagation is governed by field effects in a low density region that is created either by nanovoids or by density fluctuations in supercritical water surrounding the electrode that is created by the pressure and temperature at the moment of plasma ignition. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0045697
  • 2021 • 231 In situ investigation of nanometric cutting of 3C-SiC using scanning electron microscope
    Tian, D. and Xu, Z. and Liu, L. and Zhou, Z. and Zhang, J. and Zhao, X. and Hartmaier, A. and Liu, B. and Song, L. and Luo, X.
    International Journal of Advanced Manufacturing Technology (2021)
    Experimentally revealing the nanometric deformation behavior of 3C-SiC is challenging due to its ultra-small feature size for brittle-to-ductile transition. In the present work, we elucidated the nanometric cutting mechanisms of 3C-SiC by performing in situ nanometric cutting experiments under scanning electron microscope (SEM), as well as post-characterization by electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). In particular, a new method based on the combination of image processing technology and SEM online observation was proposed to achieve in situ measurement of cutting force with an uncertainty less than 1 mN. Furthermore, the cutting cross-section was characterized by atomic force microscope (AFM) to access the specific cutting energy. The results revealed that the specific cutting energy increase non-linearly with the decrease of cutting depth due to the size effect of cutting tool in nanometric cutting. The high-pressure phase transformation (HPPT) may play the major role in 3C-SiC ductile machining under the parameters of this experiment. © 2021, The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature.
    view abstractdoi: 10.1007/s00170-021-07278-x
  • 2021 • 230 Investigations of electron-electron and interlayer electron-phonon coupling in van der Waals hBN/WSe2/hBN heterostructures by photoluminescence excitation experiments
    Jadczak, J. and Kutrowska-Girzycka, J. and Schindler, J.J. and Debus, J. and Watanabe, K. and Taniguchi, T. and Ho, C.-H. and Bryja, L.
    Materials 14 1-12 (2021)
    Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′ 1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma14020399
  • 2021 • 229 Kinetic simulation of electron cyclotron resonance assisted gas breakdown in split-biased waveguides for ITER collective Thomson scattering diagnostic
    Trieschmann, J. and Larsen, A.W. and Mussenbrock, T. and Korsholm, Sø.B.
    Physics of Plasmas 28 (2021)
    For the measurement of the dynamics of fusion-born alpha particles E α ≤ 3.5 MeV in ITER using collective Thomson scattering (CTS), safe transmission of a gyrotron beam at mm-wavelength (1 MW, 60 GHz) passing the electron cyclotron resonance (ECR) in the in-vessel tokamak "port plug"vacuum is a prerequisite. Depending on neutral gas pressure and composition, ECR-assisted gas breakdown may occur at the location of the resonance, which must be mitigated for diagnostic performance and safety reasons. The concept of a split electrically biased waveguide (SBWG) has been previously demonstrated in C.P. Moeller, U.S. patent 4,687,616 (1987). The waveguide is longitudinally split and a kV bias voltage is applied between the two halves. Electrons are rapidly removed from the central region of high radio frequency electric field strength, mitigating breakdown. As a full scale experimental investigation of gas and electromagnetic field conditions inside the ITER equatorial port plugs is currently unattainable, a corresponding Monte Carlo simulation study is presented. Validity of the Monte Carlo electron model is demonstrated with a prediction of ECR breakdown and the mitigation pressure limits for the above-quoted reference case with 1H2 (and pollutant high Z elements). For the proposed ITER CTS design with a 88.9 mm inner diameter SBWG, ECR breakdown is predicted to occur down to a pure 1H2 pressure of 0.3 Pa, while mitigation is shown to be effective at least up to 10 Pa using a bias voltage of 1 kV. The analysis is complemented by results for relevant electric/magnetic field arrangements and limitations of the SBWG mitigation concept are addressed. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0055461
  • 2021 • 228 Micro atmospheric pressure plasma jets excited in He/O2by voltage waveform tailoring: A study based on a numerical hybrid model and experiments
    Liu, Y. and Korolov, I. and Trieschmann, J. and Steuer, D. and Schulz-Von Der Gathen, V. and Böke, M. and Bischoff, L. and Hübner, G. and Schulze, J. and Mussenbrock, T.
    Plasma Sources Science and Technology 30 (2021)
    A hybrid simulation code is developed to treat electrons fully kinetically by the particle-in-cell/Monte Carlo collision (PIC/MCC) algorithm, while ions and neutral species are handled by a fluid model, including a time slicing technique to reduce the computational expenses caused by the responses of various species on different time scales. The code is used to investigate a capacitively coupled COST reference micro atmospheric pressure helium plasma jet with 0.1% oxygen admixture excited by a valley-type tailored voltage waveform with a fixed peak-to-peak voltage of 400 V, and a fundamental frequency of 13.56 MHz. The computational results are compared to experiments based on several sophisticated diagnostics, showing good agreement in the electron impact helium excitation rate, the helium metastable density, and the atomic oxygen density. The spatio-temporal electron heating dynamics, are found to be asymmetrical due to the specific shape of the driving voltage waveform. Tailoring the voltage waveform is shown to enable to control the electron energy probability function (EEPF) in distinct spatio-temporal regions of interest. As a consequence, the generation of reactive neutral species can be enhanced by increasing the number of consecutive harmonics. Based on a simplified two dimensional neutral transport model in the hybrid code, it is demonstrated that the transport between the electrodes, as well as the gas flow have different effects on various neutral species distributions due to the relevant chemical reaction rates for the generation and destruction of species. © 2021 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/abd0e0
  • 2021 • 227 Momentum and energy dissipation of hot electrons in a Pb/Ag(111) quantum well system
    Haag, F. and Eul, T. and Grad, L. and Haag, N. and Knippertz, J. and Mathias, S. and Cinchetti, M. and Aeschlimann, M. and Stadtmüller, B.
    Physical Review B 104 (2021)
    The band structure of multilayer systems plays a crucial role for the ultrafast hot carrier dynamics at interfaces. Here, we study the energy- and momentum-dependent quasiparticle lifetimes of excited electrons in a highly ordered Pb monolayer film on Ag(111) prior and after the adsorption of a monolayer of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA). Using time-resolved two-photon momentum microscopy with femtosecond visible light pulses, we show that the electron dynamics of the Pb/Ag(111) quantum well system is largely dominated by two types of scattering processes: (i) isotropic intraband scattering processes within the quantum well state (QWS) and (ii) isotropic interband scattering processes from the -like QWS into the Pb band. In the latter case, the Pb QWS acts as an electron source for the momentum space refilling process of the Pb band. This conclusion is confirmed by the modification of the band structure and the quasiparticle dynamics of the Pb/Ag(111) bilayer film after the adsorption of PTCDA. We find both an adsorption-induced suppression of the QWS itself as well as of the refilling process into the Pb band. Our study hence demonstrates the isotropic nature of the momentum-dependent scattering processes of metallic bilayer systems and uncovers a new possibility to selectively tune and control scattering processes occurring in quantum (well) materials by the adsorption of organic molecules. ©2021 American Physical Society
    view abstractdoi: 10.1103/PhysRevB.104.104308
  • 2021 • 226 Non-linear effects and electron heating dynamics in radio-frequency capacitively coupled plasmas with a non-uniform transverse magnetic field
    Liu, Y. and Trieschmann, J. and Berger, B. and Schulze, J. and Mussenbrock, T.
    Physics of Plasmas 28 (2021)
    A non-uniform transverse magnetic field is used to increase the plasma density and create an asymmetry in radio frequency capacitively coupled plasmas for plasma sputtering and plasma vapor deposition. Based on one-dimensional particle-in-cell/Monte Carlo collision simulations, the effect of the magnetic field magnitude on the non-linear behavior and the electron heating dynamics is studied for a pure helium plasma at a pressure of 30 mTorr. The results show that increasing the magnetic field magnitude can generate a more positive DC self-bias. As a result, non-linear oscillations of the electron current density and the electric field close to the grounded electrode are enhanced. An electric field reversal is induced when the powered electrode sheath collapses to balance electron and ion fluxes toward this boundary due to the strong confinement of electrons. Anomalous energetic electron beams are observed propagating from the collapsed sheath toward the plasma bulk. It is shown that such beams are reflections of the beams originating from the opposite expanding sheath based on the analysis of single particle motions. We show that energetic electron beams can be reflected by the transverse magnetic field. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0045947
  • 2021 • 225 On the possible influence of the Fermi–Dirac statistics on the potential and entropy of galvanic cells
    Mertin, G.K. and Richter, E. and Oldenburger, M. and Hofmann, M.H. and Wycisk, D. and Wieck, A.D. and Birke, K.P.
    Journal of Power Sources 498 (2021)
    The open circuit voltage of galvanic cells is temperature dependent and the effect responsible for this dependency is its entropy. While it is well known that the Nernst equation plays an important role in describing this temperature dependency of the open-circuit voltage, this paper displays another effect. Measurements of the entropy for lithium-ion batteries show a significant temperature dependency, which cannot be explained by the linear Nernst equation. But this temperature dependency can be described by the free electron potential adapting via Fermi–Dirac statistics. This approach results in a quadratic temperature dependence of the measured potentials, which in the here shown cases for commercial lithium ion cells, could explain the measured effect. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2021.229870
  • 2021 • 224 Optically detected magnetic resonance of indirect excitons in an ensemble of (In,Al,Ga)As/(Al,Ga)As quantum dots
    Ivanov, V.Yu. and Tolmachev, D.O. and Shamirzaev, T.S. and Słupinski, T. and Yakovlev, D.R. and Bayer, M.
    Physical Review B 104 (2021)
    The energy level structure as well as the exciton recombination and spin dynamics are studied in a dense ensemble of (In,Al,Ga)As/(Al,Ga)As quantum dots (QDs). The band alignment in the QDs is shown to have type-I, indirect character with the lowest electron state at the X valleys of the conduction band and the top hole state in the Γ point of the valence band, so that indirect excitons are formed in the QDs. Time-resolved photoluminescence and magnetic-field-induced circular polarization allow us to distinguish electron states belonging to the QDs and the wetting layer. Suppression of the exciton migration within the QD ensemble and along the wetting layer in the magnetic field is found. A pronounced effect of applied microwave radiation on the recombination and spin polarization of the indirect excitons is observed in longitudinal magnetic fields. Optically detected magnetic resonance (ODMR) is detected in both the intensity and the circular polarization degree of the QD emission. The ODMR resonance corresponds to the g factor of 1.97, associated with X-valley electrons. The spin relaxation time of the X-valley electrons is measured to be 600±25 ns. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.104.195306
  • 2021 • 223 Plasmon-to-exciton spin conversion in semiconductor-metal hybrid nanostructures
    Akimov, I.A. and Poddubny, A.N. and Vondran, J. and Vorobyov, Yu.V. and Litvin, L.V. and Jede, R. and Karczewski, G. and Chusnutdinow, S. and Wojtowicz, T. and Bayer, M.
    Physical Review B 103 (2021)
    Optical spin control is the basis for ultrafast spintronics: circularly polarized light in combination with spin-orbit coupling enables spin manipulation of electronic states in condensed matter. However, the conventional approach is limited to longitudinal spin initialization along one particular axis that is dictated by the direction of light propagation. Here, plasmonics opens new possibilities, allowing one to tailor light polarization at the nanoscale. We demonstrate ultrafast optical excitation of electron spin on femtosecond timescales via plasmon-to-exciton spin conversion. By time resolving the THz spin dynamics in a hybrid (Cd,Mn)Te quantum-well structure covered with a metallic grating, we unambiguously determine the orientation of the photoexcited electron spins which is locked to the propagation direction of the optically excited surface plasmon polaritons. Using the spin of the incident photons as an additional degree of freedom, one can adjust not only the longitudinal, but also the transverse electron spin components normal to the light propagation at will. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.085425
  • 2021 • 222 Segregation-Enhanced Epitaxy of Borophene on Ir(111) by Thermal Decomposition of Borazine
    Omambac, K.M. and Petrović, M. and Bampoulis, P. and Brand, C. and Kriegel, M.A. and Dreher, P. and Janoschka, D. and Hagemann, U. and Hartmann, N. and Valerius, P. and Michely, T. and Meyer Zu Heringdorf, F.J. and Horn-von Hoegen, M.
    ACS Nano 15 7421-7429 (2021)
    Like other 2D materials, the boron-based borophene exhibits interesting structural and electronic properties. While borophene is typically prepared by molecular beam epitaxy, we report here on an alternative way of synthesizing large single-phase borophene domains by segregation-enhanced epitaxy. X-ray photoelectron spectroscopy shows that borazine dosing at 1100 °C onto Ir(111) yields a boron-rich surface without traces of nitrogen. At high temperatures, the borazine thermally decomposes, nitrogen desorbs, and boron diffuses into the substrate. Using time-of-flight secondary ion mass spectrometry, we show that during cooldown the subsurface boron segregates back to the surface where it forms borophene. In this case, electron diffraction reveals a (6 × 2) reconstructed borophene χ6-polymorph, and scanning tunneling spectroscopy suggests a Dirac-like behavior. Studying the kinetics of borophene formation in low energy electron microscopy shows that surface steps are bunched during the borophene formation, resulting in elongated and extended borophene domains with exceptional structural order. ©
    view abstractdoi: 10.1021/acsnano.1c00819
  • 2021 • 221 Shielding of external magnetic field by dynamic nuclear polarization in (In,Ga)As quantum dots
    Evers, E. and Kopteva, N.E. and Yugova, I.A. and Yakovlev, D.R. and Bayer, M. and Greilich, A.
    Physical Review B 104 (2021)
    The dynamics of the coupled electron-nuclear spin system is studied in an ensemble of singly charged (In,Ga)As/GaAs quantum dots (QDs) using periodic optical excitation at 1 GHz repetition rate. In combination with the electron-nuclei interaction, the highly repetitive excitation allows us to lock the electron spins into magnetic resonance in a transverse external magnetic field. Sweeping the field to higher values, the locking leads to an effective "diamagnetic"response of significant strength due to dynamic nuclear polarization, which shields the QD electrons at least partly from the external field and can even keep the internal magnetic field constant up to 1.3 T field variation. We model the effect through a magnetic field-dependent polarization rate of the nuclei, from which we suggest a strategy for adjusting the nuclear polarization through the detuning between optical excitation and electronic transition, in addition to tuning the magnetic field. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.104.075302
  • 2021 • 220 Spin-polarized quantized electronic structure of Fe(001) with symmetry breaking due to the magnetization direction
    Młyńczak, E. and Aguilera, I. and Gospodarič, P. and Heider, T. and Jugovac, M. and Zamborlini, G. and Tusche, C. and Suga, S. and Feyer, V. and Blügel, S. and Plucinski, L. and Schneider, C.M.
    Physical Review B 103 (2021)
    Quantum well states formed by d electrons in metallic thin films are responsible for many fundamental phenomena that oscillate with layer thickness, such as magnetic anisotropy or magnetoresistance. Using momentum microscopy and angle-resolved photoemission, we mapped in unprecedented detail the quantized electronic states of Fe(001) in a broad photon energy range starting from soft x-ray (160 eV) down to vacuum ultraviolet (8.4 eV). We show that it is possible to simulate the experimentally observed photoemission spectra with high accuracy by using the ab initio electronic bulk band structure as the initial state, taking into account that free electron final electronic states are intrinsically broadened along the wave vector direction perpendicular to the sample surface. To simulate the thin-film case, we take into account a subset of the initial electronic states, which results in the reproduction of the quantized electronic structure observed in the experiment. In addition, we present results of the spin-sensitive measurements, which are confronted with the photoemission simulation that takes into account the spin degree of freedom. We demonstrate electronic states that can be responsible for the oscillations of the magnetic anisotropy in Fe(001) thin films with periods of about 5 and 9 monolayers. We show that these quantum well states change position in reciprocal space depending on the magnetization direction. Our photoemission simulation reproduces this effect, which highlights its origin in the relativistic bulk electronic band structure of bcc Fe. We also observed magnetization-dependent spin-orbit gaps with the symmetry lower than the bulk symmetry. We believe that the same method of simulating photoemission spectra might facilitate interpretation of the photoemission intensities measured for other three-dimensional materials, especially when the spin-polarized quantized electronic states are considered. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.035134
  • 2021 • 219 Structural, electronic paramagnetic resonance and magnetic properties of praseodymium-doped rare earth CeO2 semiconductors
    Oliveira, L.L. and Cortés, J.A. and Caldeira, B.S. and Strusch, T. and Wiedwald, U. and Simoes, A.Z.
    Ceramics International (2021)
    In this work, praseodymium (Pr) doped cerium oxide (CeO2) was prepared using the microwave-assisted hydrothermal method (MAH) and the properties were investigated by X-ray diffraction analysis (XRD), Raman spectroscopy, Field Emission Gun Scanning Electron Microscope (FEG-SEM), BET method, Photoluminescence spectroscopy (PL), Fourier-transform infrared spectroscopy (FTIR), Ultraviolet–visible spectroscopy (UV–Vis), Electron paramagnetic resonance spectroscopy (EPR) and Magnetometry. The results showed that increasing the Pr-doping promotes a structural disorder due to increased oxygen vacancies. XRD confirmed a cubic structure without deleterious phases with modifications in the structure caused by alteration in the cerium oxidation state as well as changes in the crystallite size and strain obtained by Wellinson-Hall method. Raman spectroscopy shows that changing the Pr content results in samples with different defect densities at short range. FEG-SEM showed that the nanocrystals are agglomerated with small particles tend to aggregate spontaneously to decrease the surface energy. BET method showed that the Pr doping results in a gain of specific surface area. PL indicated that Pr3+ leads to distinct emissions; red emission associated to oxygen vacancies located near the conduction band (shallow defects), green emission associated to electron-hole recombination and orange emission associated to shallow defects and electron-hole recombination. FTIR indicated the complete process of nucleation with no other phase. UV–Vis showed the transitions between oxygen 2p, cerium 4f and praseodymium 4f states. The EPR signal shows events occurring around 344 mT. These events can be related due the presence of paramagnetic elements containing unpaired electrons, such as Ce (III), which is indicative of cerium reduction caused by Pr ions, as evidenced by Rietveld data. Regardless of the Pr concentration used in this research, the magnetic measurements show a superparamagnetic system below the blocking temperature of ~20 K and a paramagnetic system above this temperature, which indicates no significant changes in the average size of the nanoparticles. Surface area, crystallite size and the temperature are important parameters, which control the magnetic properties of such N-type semiconductors. © 2021 Elsevier Ltd and Techna Group S.r.l.
    view abstractdoi: 10.1016/j.ceramint.2021.04.133
  • 2021 • 218 Suppression of nuclear spin fluctuations in an InGaAs quantum dot ensemble by GHz-pulsed optical excitation
    Evers, E. and Kopteva, N.E. and Yugova, I.A. and Yakovlev, D.R. and Reuter, D. and Wieck, A.D. and Bayer, M. and Greilich, A.
    npj Quantum Information 7 (2021)
    The coherent electron spin dynamics of an ensemble of singly charged (In,Ga)As/GaAs quantum dots in a transverse magnetic field is driven by periodic optical excitation at 1 GHz repetition frequency. Despite the strong inhomogeneity of the electron g factor, the spectral spread of optical transitions, and the broad distribution of nuclear spin fluctuations, we are able to push the whole ensemble of excited spins into a single Larmor precession mode that is commensurate with the laser repetition frequency. Furthermore, we demonstrate that an optical detuning of the pump pulses from the probed optical transitions induces a directed dynamic nuclear polarization and leads to a discretization of the total magnetic field acting on the electron ensemble. Finally, we show that the highly periodic optical excitation can be used as universal tool for strongly reducing the nuclear spin fluctuations and preparation of a robust nuclear environment for subsequent manipulation of the electron spins, also at varying operation frequencies. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41534-021-00395-1
  • 2021 • 217 Synchronising optical emission spectroscopy to spokes in magnetron sputtering discharges
    Maaß, P.A. and Schulz-Von Der Gathen, V. and Von Keudell, A. and Held, J.
    Plasma Sources Science and Technology 30 (2021)
    Spokes are patterns of increased light emission, observed to rotate in front of the targets of magnetron sputtering discharges. They move through the plasma at velocities of several km s-1 in or against the E × B direction of the discharge. The high velocity and their initial creation at arbitrary positions render measurements of spokes challenging. For more demanding plasma diagnostic techniques that require data acquisition over multiple discharge pulses, synchronisation to the spoke movement is necessary. In this publication, we present optical emission spectroscopy of spokes in both high power impulse magnetron sputtering (HiPIMS) as well as direct current magnetron sputtering (DCMS) discharges, performed by triggering a camera on the spoke movement. Optical filters between plasma and camera allow us to isolate emission lines of metal and working gas neutrals and ions. Based on these optical measurements and previous probe studies, the dynamics of electrons drifting through spokes in both DCMS and HiPIMS is discussed. In HiPIMS, the much shorter mean free path for inelastic electron collisions enables strong ionisation inside the spoke, causing a sudden variation in electron density which leads to the distinct spoke shape. In contrast, the spoke shape for DCMS discharges seems to rather be indicative of electron energy variations. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ac3210
  • 2021 • 216 There is Plenty of Room for THz Tunneling Electron Devices beyond the Transit Time Limit
    Villani, M. and Clochiatti, S. and Prost, W. and Weimann, N. and Oriols, X.
    IEEE Electron Device Letters 42 224-227 (2021)
    The traditional transmission coefficient present in the original Landauer formulation, which is valid for quasi-static scenarios with working frequencies below the inverse of the electron transit time, is substituted by a novel time-dependent displacement current coefficient valid for frequencies above this limit. Our model captures in a simple way the displacement current component of the total current, which at frequencies larger than the inverse of the electron transit time can be more relevant than the particle component. The proposed model is applied to compute the response of a resonant tunneling diode from 10 GHz up to 5 THz. We show that tunneling electron devices are intrinsically nonlinear at such high frequencies, even under small-signal conditions, due to memory effects related to the displacement current. We show that these intrinsic nonlinearities (anharmonicities) represent an advantage, rather than a drawback, as they open the path for tunneling devices in many THz applications, and avoid further device downscaling. © 1980-2012 IEEE.
    view abstractdoi: 10.1109/LED.2021.3049229
  • 2021 • 215 Ultra-Shallow All-Epitaxial Aluminum Gate GaAs/AlxGa1−xAs Transistors with High Electron Mobility
    Ashlea Alava, Y. and Wang, D.Q. and Chen, C. and Ritchie, D.A. and Ludwig, A. and Ritzmann, J. and Wieck, A.D. and Klochan, O. and Hamilton, A.R.
    Advanced Functional Materials (2021)
    The electron mobility in shallow GaAs/AlxGa1−xAs heterostructures is strongly suppressed by charge wafer surface, which arises from native surface oxide layers formed when the wafer is removed from the crystal growth system. Here an in situ epitaxial aluminum gate, grown as part of the wafer, is used to eliminate surface charge scattering. Transmission electron microscope characterization shows that the in situ epitaxial aluminum is crystalline, and the wafer surface is free of native oxide. The influence of Al thickness and the use of different semiconductor wetting layers at the semiconductor-aluminum interface are examined and correlated with electron mobility. The electron mobility is found to strongly depend on aluminum thickness. For 8 nm thick aluminum, the electron mobility is also influenced by the wetting layer, with aluminum grown on GaAs producing higher mobility compared to AlAs or Al0.33Ga0.67As wetting layers. The suppression of surface charge scattering in these all-epitaxial devices allows for high mobilities across a wide density range despite the shallow conduction channel (35 nm below the gate). These measurements also provide a uniquely sensitive method of determining the electrical quality of the semiconductor–metal interface, relevant to the formation of hybrid semiconductor–superconductor devices. © 2021 Wiley-VCH GmbH
    view abstractdoi: 10.1002/adfm.202104213
  • 2021 • 214 Upconversion of Light into Bright Intravalley Excitons via Dark Intervalley Excitons in hBN-Encapsulated WSe2Monolayers
    Jadczak, J. and Glazov, M. and Kutrowska-Girzycka, J. and Schindler, J.J. and Debus, J. and Ho, C.-H. and Watanabe, K. and Taniguchi, T. and Bayer, M. and Bryja, L.
    ACS Nano (2021)
    Semiconducting monolayers of transition-metal dichalcogenides are outstanding platforms to study both electronic and phononic interactions as well as intra- and intervalley excitons and trions. These excitonic complexes are optically either active (bright) or inactive (dark) due to selection rules from spin or momentum conservation. Exploring ways of brightening dark excitons and trions has strongly been pursued in semiconductor physics. Here, we report on a mechanism in which a dark intervalley exciton upconverts light into a bright intravalley exciton in hBN-encapsulated WSe2 monolayers. Excitation spectra of upconverted photoluminescence reveals resonances at energies 34.5 and 46.0 meV below the neutral exciton in the nominal WSe2 transparency range. The required energy gains are theoretically explained by cooling of resident electrons or by exciton scattering with D- or K-valley phonons. Accordingly, an elevated temperature and a moderate concentration of resident electrons are necessary for observing the upconversion resonances. The interaction process observed between the inter- and intravalley excitons elucidates the importance of dark excitons for the optics of two-dimensional materials. ©
    view abstractdoi: 10.1021/acsnano.1c08286
  • 2021 • 213 X-ray-Based Techniques to Study the Nano-Bio Interface
    Sanchez-Cano, C. and Alvarez-Puebla, R.A. and Abendroth, J.M. and Beck, T. and Blick, R. and Cao, Y. and Caruso, F. and Chakraborty, I. and Chapman, H.N. and Chen, C. and Cohen, B.E. and Conceição, A.L.C. and Cormode, D.P. and C...
    ACS Nano 15 3754-3807 (2021)
    X-ray-based analytics are routinely applied in many fields, including physics, chemistry, materials science, and engineering. The full potential of such techniques in the life sciences and medicine, however, has not yet been fully exploited. We highlight current and upcoming advances in this direction. We describe different X-ray-based methodologies (including those performed at synchrotron light sources and X-ray free-electron lasers) and their potentials for application to investigate the nano-bio interface. The discussion is predominantly guided by asking how such methods could better help to understand and to improve nanoparticle-based drug delivery, though the concepts also apply to nano-bio interactions in general. We discuss current limitations and how they might be overcome, particularly for future use in vivo. ©
    view abstractdoi: 10.1021/acsnano.0c09563
  • 2020 • 212 Adsorbate induced manipulation of 1D atomic wires: Degradation of long-range order in the Si(553)-Au system
    Hafke, B. and Witte, T. and Brand, C. and Horn-von Hoegen, M.
    Surface Science 700 (2020)
    Deposition of Au on vicinal Si(553) surfaces results in the self-assembly of one-dimensional (1D) Au atomic wires. Charge transfer from the Au wire to the Si step edge leads to a chain of Si dangling-bond orbitals with a long range ordered threefold periodicity along the steps and finite interchain interaction perpendicular to the steps. Employing spot-profile analysis low-energy electron diffraction (SPA-LEED) we observed a broadening of spot width with time, indicative for the degradation of Si dangling-bond chain and Au wire length. We introduce a new mechanism how adsorbates act as origin for this degradation beyond the intuitive picture of bond saturation. Here, zero-dimensional anti-phase translational domain boundaries are generated which immediately destroy the long-range order along and perpendicular to the steps, respectively. From the temporal evolution of the decreasing coherence length, we conclude that the Au wires are less reactive to adsorption than the Si dangling bond chains. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2020.121673
  • 2020 • 211 Anomalous magnetic suppression of spin relaxation in a two-dimensional electron gas in a GaAs/AlGaAs quantum well
    Belykh, V.V. and Belykh, V.V. and Kochiev, M.V. and Sob'Yanin, D.N. and Yakovlev, D.R. and Yakovlev, D.R. and Bayer, M. and Bayer, M.
    Physical Review B 101 (2020)
    We study the spin dynamics in a high-mobility two-dimensional electron gas confined in a GaAs/AlGaAs quantum well. An unusual magnetic field dependence of the spin relaxation is found: As the magnetic field becomes stronger, the spin relaxation time first increases quadratically but then changes to a linear dependence before it eventually becomes oscillatory, whereby the longitudinal and transverse times reach maximal values at even and odd filling Landau level factors, respectively. We show that the suppression of spin relaxation is due to the effect of electron gyration on the spin-orbit field, while the oscillations correspond to oscillations of the density of states appearing at low temperatures and high magnetic fields. The transition from quadratic to linear dependence can be related to a transition from classical to Bohm diffusion and reflects an anomalous behavior of the two-dimensional electron gas analogous to that observed in magnetized plasmas. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.235307
  • 2020 • 210 Characterization of a transient spark micro-discharge in nitrogen using simultaneous two-wavelength diagnostics
    Gröger, S. and Fiebrandt, M. and Hamme, M. and Bibinov, N. and Awakowicz, P.
    Measurement Science and Technology 31 (2020)
    A transient spark micro-discharge in nitrogen is investigated between two sharpened electrodes at a pressure of 0.5 bar. The plasma parameters (gas temperature, electron density and reduced electric field) are determined using optical emission spectroscopy (OES) and numerical simulations. The gas temperature of 3500 ± 100 K is determined by the comparison of the measured and simulated rotational distributions of the photoemission spectra of neutral molecular nitrogen N2(C-B,0-0). Both direct and stepwise electron impact excitation are considered in the collision-radiative model. The rate constants for electron impact excitation processes are calculated for different electric field values using the electron velocity distribution function, which is simulated by solving the Boltzmann equation. The applied broadband echelle spectrometer is absolutely calibrated in a spectral range of 200 nm to 800 nm, using two standard light sources, a deuterium lamp and a tungsten ribbon lamp, which are certificated by the Physikalisch-Technische Bundesanstalt (PTB), Germany. With the aid of this absolutely calibrated echelle spectrometer and a microwave atmospheric plasma source operated in a nitrogen flow, the intensified charge-coupled device (ICCD) camera, provided with an in-house made optical arrangement for simultaneous two-wavelength diagnostic is calibrated. The spatial resolution of this diagnostic system under the studied plasma conditions amounts to 13 m. The accurate examination of the experimental results allows determining the dominant process of electron impact excitation of molecular nitrogen ion from ionic ground state. Applying the chosen excitation model of the nitrogen photoemission, the spatially resolved reduced electric field and the electron density are determined. This is done by using the inverse Abel transformation of the absolute intensities of molecular nitrogen bands N2(C-B,0-0) and N2 + (B-X,0-0), which were measured with the calibrated ICCD camera. The measured electric current of the micro-discharge is compared with the calculated one using the measured plasma parameters and a good coincidence is established. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6501/ab7e69
  • 2020 • 209 Charge Separation Dynamics in CdSe/CdS Core/Shell Nanoplatelets Addressed by Coherent Electron Spin Precession
    Feng, D. and Yakovlev, D.R. and Dubertret, B. and Bayer, M.
    ACS Nano 14 7237-7244 (2020)
    We investigate the charge separation dynamics provided by carrier surface trapping in CdSe/CdS core/shell nanoplatelets by means of a three-laser-beam pump-orientation-probe technique, detecting the electron spin coherence at room temperature. Signals with two Larmor precession frequencies are found, which strongly differ in their dynamical characteristics and dependencies on pump power and shell thickness. The electron trapping process occurs on a time scale of about 10 ns, and the charge separation induced thereby has a long lifetime of up to hundreds of microseconds. On the other hand, the hole trapping requires times from subpicoseconds to hundreds of picoseconds, and the induced charge separation has a lifetime of a few nanoseconds. With increasing CdS shell thickness the hole trapping vanishes, while the electron trapping is still detectable. These findings have important implications for understanding the photophysical processes of nanoplatelets and other colloidal nanostructures. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.0c02402
  • 2020 • 208 Charged particle dynamics and distribution functions in low pressure dual-frequency capacitively coupled plasmas operated at low frequencies and high voltages
    Hartmann, P. and Wang, L. and Nösges, K. and Berger, B. and Wilczek, S. and Brinkmann, R.P. and Mussenbrock, T. and Juhasz, Z. and Donkó, Z. and Derzsi, A. and Lee, E. and Schulze, J.
    Plasma Sources Science and Technology 29 (2020)
    In high aspect ratio (HAR) dielectric plasma etching, dual-frequency capacitively coupled radio-frequency plasmas operated at low pressures of 1 Pa or less are used. Such plasma sources are often driven by a voltage waveform that includes a low-frequency component in the range of hundreds of kHz with a voltage amplitude of 10 kV and more to generate highly energetic vertical ion bombardment at the wafer. In such discharges, the energetic positive ions can overcome the repelling potential created by positive wall charges inside the etch features, which allows high aspect ratios to be reached. In order to increase the plasma density a high-frequency driving component at several 10 MHz is typically applied simultaneously. Under such discharge conditions, the boundary surfaces are bombarded by extremely energetic particles, of which the consequences are poorly understood. We investigate the charged particle dynamics and distribution functions in this strongly non-local regime in argon discharges by particle-in-cell simulations. By including a complex implementation of plasma-surface interactions, electron induced secondary electron emission (δ-electrons) is found to have a strong effect on the ionization dynamics and the plasma density. Due to the high ion energies at the electrodes, very high yields of the ion induced secondary electron emission (γ-electrons) are found. However, unlike in classical capacitive plasmas, these γ-electrons do not cause significant ionization directly, since upon acceleration in the high voltage sheaths, these electrons are too energetic to ionize the neutral gas efficiently. These γ- and δ-electrons as well as electrons created in the plasma bulk and accelerated towards the electrodes to high energies by reversed electric fields during the local sheath collapse are found to induce the emission of a high number of δ-electrons, when they hit boundary surfaces. This regime is understood fundamentally based on the following approach: first, dual-frequency discharges with identical electrode materials are studied at different pressures and high-frequency driving voltages. Second, the effects of using electrodes made of different materials and characterized by different secondary electron emission coefficients are studied. The electron dynamics and charged particle distribution functions at boundary surfaces are determined including discharge asymmetries generated by using different materials at the powered and grounded electrodes. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab9374
  • 2020 • 207 Coherence of a Driven Electron Spin Qubit Actively Decoupled from Quasistatic Noise
    Nakajima, T. and Noiri, A. and Kawasaki, K. and Yoneda, J. and Stano, P. and Amaha, S. and Otsuka, T. and Takeda, K. and Delbecq, M.R. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Tarucha, S.
    Physical Review X 10 (2020)
    The coherence of electron spin qubits in semiconductor quantum dots suffers mostly from low-frequency noise. During the past decade, efforts have been devoted to mitigate such noise by material engineering, leading to substantial enhancement of the spin dephasing time for an idling qubit. However, the role of the environmental noise during spin manipulation, which determines the control fidelity, is less understood. We demonstrate an electron spin qubit whose coherence in the driven evolution is limited by high-frequency charge noise rather than the quasistatic noise inherent to any semiconductor device. We employ a feedback-control technique to actively suppress the latter, demonstrating a π-flip gate fidelity as high as 99.04±0.23% in a gallium arsenide quantum dot. We show that the driven-evolution coherence is limited by the longitudinal noise at the Rabi frequency, whose spectrum resembles the 1/f noise observed in isotopically purified silicon qubits. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevX.10.011060
  • 2020 • 206 Coherent control of individual electron spins in a two-dimensional quantum dot array
    Mortemousque, P.-A. and Chanrion, E. and Jadot, B. and Flentje, H. and Ludwig, Ar. and Wieck, A.D. and Urdampilleta, M. and Bäuerle, C. and Meunier, T.
    Nature Nanotechnology (2020)
    The coherent manipulation of individual quantum objects organized in arrays is a prerequisite to any scalable quantum information platform. The cumulated efforts to control electron spins in quantum dot arrays have permitted the recent realization of quantum simulators and multielectron spin-coherent manipulations. Although a natural path to resolve complex quantum-matter problems and to process quantum information, two-dimensional (2D) scaling with a high connectivity of such implementations remains undemonstrated. Here we demonstrate the 2D coherent control of individual electron spins in a 3 × 3 array of tunnel-coupled quantum dots. We focus on several key quantum functionalities: charge-deterministic loading and displacement, local spin readout and local coherent exchange manipulation between two electron spins trapped in adjacent dots. This work lays some of the foundations to exploit a 2D array of electron spins for quantum simulation and information processing. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41565-020-00816-w
  • 2020 • 205 Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?
    Chang, Y. and Zhang, S. and Liebscher, C.H. and Dye, D. and Ponge, D. and Scheu, C. and Dehm, G. and Raabe, D. and Gault, B. and Lu, W.
    Scripta Materialia 178 39-43 (2020)
    A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x ≥ 1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2019.11.010
  • 2020 • 204 Effect of electric current on the optical orientation of interface electrons in AlGaAs/GaAs heterostructures
    Ken, O.S. and Zhukov, E.A. and Akimov, I.A. and Korenev, V.L. and Kopteva, N.E. and Kalitukha, I.V. and Sapega, V.F. and Wieck, A.D. and Ludwig, Ar. and Schott, R. and Kusrayev, Y.G. and Yakovlev, D.R. and Bayer, M.
    Physical Review B 102 (2020)
    The effect of a lateral electric current on the photoluminescence H band of an AlGaAs/GaAs heterostructure is investigated. The photoluminescence intensity and optical orientation of electrons contributing to the H band are studied by means of continuous-wave and time-resolved photoluminescence spectroscopy and time-resolved Kerr rotation. It is shown that the H band is due to recombination of the heavy holes localized at the heterointerface with photoexcited electrons attracted to the heterointerface from the GaAs layer. Two lines with significantly different decay times constitute the H band: a short-lived high-energy one and a long-lived low-energy one. The high-energy line originates from recombination of electrons freely moving along the structure plane, while the low-energy one is due to recombination of donor-bound electrons near the interface. Application of a lateral electric field of ∼100-200 V/cm results in a quenching of both lines. This quenching is due to a decrease of electron concentration near the heterointerface as a result of a photocurrent-induced heating of electrons in the GaAs layer. On the contrary, electrons near the heterointerface are effectively cooled, so the donors near the interface are not completely empty up to ∼100 V/cm, which is in stark contrast with the case of bulk materials. The optical spin polarization of the donor-bound electrons near the heterointerface weakly depends on the electric field. Their polarization kinetics is determined by the spin dephasing in the hyperfine fields of the lattice nuclei. The long spin memory time (>40 ns) can be associated with suppression of the Bir-Aronov-Pikus mechanism of spin relaxation for electrons. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.045302
  • 2020 • 203 Electron correlation and spin transition
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology 35-43 (2020)
    Theoretical treatment of functional metalorganics is non-trivial for the metal centers with narrow bands (3d, 4d of transition metals or 4f bands of rare-earth metals), featuring a sizeable Coulomb interaction. An interplay between crystal field, spin-orbit coupling and Coulomb interaction expresses the properties of the molecule. Correlated metal centers, immersed in the electron bath of organic ring makes it ideal to treat with Anderson’s impurity model. In this chapter, we will focus on the description of electron correlation in functional metalorganics with the aid of density functional theory, combined with a many body approach. For most of the illustrative purposes, we will consider iron porphyrin (FeP) molecule. The chapter will reveal the importance of the treatment of explicit electron correlation in order to accurately identify the spin transition, magnetic anisotropy, Kondo effect etc., which are key ingredients for molecular spintronics and electronics. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstractdoi: 10.1007/978-981-15-3719-6_5
  • 2020 • 202 Electron density, temperature and the potential structure of spokes in HiPIMS
    Held, J. and Maaß, P.A. and Gathen, V.S.-V.D. and Keudell, A.V.
    Plasma Sources Science and Technology 29 (2020)
    In high power impulse magnetron sputtering (HiPIMS) bright plasma spots are observed during the discharge pulses that rotate with velocities in the order of 10 km s-1 in front of the target surface. It has proven very difficult to perform any quantitative measurements on these so-called spokes, which emerge stochastically during the build-up of each plasma pulse. In this paper, we propose a new time shift averaging method to perform measurements integrating over many discharge pulses, but without phase averaging of the spoke location, thus preserving the information of the spoke structure. This method is then applied to perform Langmuir probe measurements, employing magnetized probe theory to determine the plasma parameters inside the magnetic trap region of the discharge. Spokes are found to have a higher plasma density, electron temperature and plasma potential than the surrounding plasma. The electron density slowly rises at the leading edge of the spoke to a maximum value of about 1 1020 m-3 and then drops sharply at the trailing edge to 4 1019 m-3. The electron temperature rises from 2.1 eV outside the spoke to 3.4 eV at the trailing end of the spoke. A reversal of the plasma potential from about -7 V outside the spoke to values just above 0 V in a spoke is observed, as has been proposed in the literature. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab5e46
  • 2020 • 201 Electron dynamics in low pressure capacitively coupled radio frequency discharges
    Wilczek, S. and Schulze, J. and Brinkmann, R.P. and Donkó, Z. and Trieschmann, J. and Mussenbrock, T.
    Journal of Applied Physics 127 (2020)
    In low temperature plasmas, the interaction of the electrons with the electric field is an important current research topic that is relevant for many applications. Particularly, in the low pressure regime (≤ 10 Pa), electrons can traverse a distance that may be comparable to the reactor dimensions without any collisions. This causes "nonlocal,"dynamics which results in a complicated space- and time-dependence and a strong anisotropy of the distribution function. Capacitively coupled radio frequency (CCRF) discharges, which operate in this regime, exhibit extremely complex electron dynamics. This is because the electrons interact with the space- and time-dependent electric field, which arises in the plasma boundary sheaths and oscillates at the applied radio frequency. In this tutorial paper, the fundamental physics of electron dynamics in a low pressure electropositive argon discharge is investigated by means of particle-in-cell/Monte Carlo collisions simulations. The interplay between the fundamental plasma parameters (densities, fields, currents, and temperatures) is explained by analysis (aided by animations) with respect to the spatial and temporal dynamics. Finally, the rendered picture provides an overview of how electrons gain and lose their energy in CCRF discharges. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0003114
  • 2020 • 200 Electron Irradiation of Metal Contacts in Monolayer MoS2Field-Effect Transistors
    Pelella, A. and Kharsah, O. and Grillo, A. and Urban, F. and Passacantando, M. and Giubileo, F. and Iemmo, L. and Sleziona, S. and Pollmann, E. and Madauß, L. and Schleberger, M. and Di Bartolomeo, A.
    ACS Applied Materials and Interfaces 12 40532-40540 (2020)
    Metal contacts play a fundamental role in nanoscale devices. In this work, Schottky metal contacts in monolayer molybdenum disulfide (MoS2) field-effect transistors are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is demonstrated that irradiation lowers the Schottky barrier at the contacts because of thermally induced atom diffusion and interfacial reactions. The simulation of electron paths in the device reveals that most of the beam energy is absorbed in the metal contacts. The study demonstrates that electron beam irradiation can be effectively used for contact improvement through local annealing. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c11933
  • 2020 • 199 Electron-nuclei interaction in the X valley of (In,Al)As/AlAs quantum dots
    Kuznetsova, M.S. and Rautert, J. and Kavokin, K.V. and Smirnov, D.S. and Yakovlev, D.R. and Bakarov, A.K. and Gutakovskii, A.K. and Shamirzaev, T.S. and Bayer, M.
    Physical Review B 101 (2020)
    The electron-nuclei hyperfine interaction of electrons in indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment has been experimentally studied by measuring the polarization degree of the photoluminescence in a transverse magnetic field (Hanle effect) and the polarization recovery in a longitudinal magnetic field. The different symmetries of the X valley electron Bloch amplitudes at the As, In, and Al nuclei strongly affect the hyperfine interaction. The hyperfine constants corresponding to these nuclei have been determined. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.075412
  • 2020 • 198 Highly charged ion impact on graphene leading to the emission of low energy electrons
    Schwestka, J. and Niggas, A. and Creutzburg, S. and Kozubek, R. and Madauß, L. and Heller, R. and Schleberger, M. and Facsko, S. and Wilhelm, R.A. and Aumayr, F.
    Journal of Physics: Conference Series 1412 (2020)
    Recent experiments found that the neutralisation of highly charged ions interacting with a freestanding single layer of graphene proceeds on a femtosecond time scale. This ultra-fast deexcitation was attributed to Interatomic Coulombic Decay (ICD), a process in which core holes in the projectile are filled by previously captured outer electrons and the energy is transferred to electrons of the surrounding carbon atoms. ICD therefore predicts the emission of many low energy electrons. We now present experimental evidence that e.g. Xe40+ indeed emits up to 85 electrons with energies below 20 eV. © 2019 Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1412/20/202012
  • 2020 • 197 Impact of electron solvation on ice structures at the molecular scale
    Bertram, C. and Auburger, P. and Bockstedte, M. and Stähler, J. and Bovensiepen, U. and Morgenstern, K.
    Journal of Physical Chemistry Letters 11 1310-1316 (2020)
    Electron attachment and solvation at ice structures are well-known phenomena. The energy liberated in such events is commonly understood to cause temporary changes at such ice structures, but it may also trigger permanent modifications to a yet unknown extent. We determine the impact of electron solvation on D2O structures adsorbed on Cu(111) with low-Temperature scanning tunneling microscopy, two-photon photoemission, and ab initio theory. Solvated electrons, generated by ultraviolet photons, lead not only to transient but also to permanent structural changes through the rearrangement of individual molecules. The persistent changes occur near sites with a high density of dangling OD groups that facilitate electron solvation. We conclude that energy dissipation during solvation triggers permanent molecular rearrangement via vibrational excitation. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.9b03723
  • 2020 • 196 Inelastic electron tunneling spectroscopy for probing strongly correlated many-body systems by scanning tunneling microscopy
    Eickhoff, F. and Kolodzeiski, E. and Esat, T. and Fournier, N. and Wagner, C. and Deilmann, T. and Temirov, R. and Rohlfing, M. and Tautz, F.S. and Anders, F.B.
    Physical Review B 101 (2020)
    We present an extension of the tunneling theory for scanning tunneling microscopy (STM) to include different types of electron-vibrational couplings responsible for inelastic contributions to the tunnel current in the strong-coupling limit. It allows for a better understanding of more complex scanning tunneling spectra of molecules on a metallic substrate in separating elastic and inelastic contributions. The starting point is the exact solution of the spectral functions for the electronically active local orbitals in the absence of the STM tip. This includes electron-phonon coupling in the coupled system comprising the molecule and the substrate to arbitrary order including the antiadiabatic strong-coupling regime as well as the Kondo effect on a free-electron spin of the molecule. The tunneling current is derived in second order of the tunneling matrix element which is expanded in powers of the relevant vibrational displacements. We use the results of an ab initio calculation for the single-particle electronic properties as an adapted material-specific input for a numerical renormalization group approach for accurately determining the electronic properties of a 1,4,5,8-naphthalene-Tetracarboxylic acid dianhydride molecule on Ag(111) as a challenging sample system for our theory. Our analysis shows that the mismatch between the ab initio many-body calculation of the tunnel current in the absence of any electron-phonon coupling to the experimental scanning tunneling spectra can be resolved by including two mechanisms: (i) a strong unconventional Holstein term on the local substrate orbital leads to the reduction of the Kondo temperature and (ii) a further electron-vibrational coupling to the tunneling matrix element is responsible for inelastic steps in the dI/dV curve at finite frequencies. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.125405
  • 2020 • 195 Irreversible Structural Changes of Copper Hexacyanoferrate Used as a Cathode in Zn-Ion Batteries
    Lim, J. and Kasiri, G. and Sahu, R. and Schweinar, K. and Hengge, K. and Raabe, D. and La Mantia, F. and Scheu, C.
    Chemistry - A European Journal 26 4917-4922 (2020)
    The structural changes of copper hexacyanoferrate (CuHCF), a Prussian blue analogue, which occur when used as a cathode in an aqueous Zn-ion battery, are investigated using electron microscopy techniques. The evolution of ZnxCu1−xHCF phases possessing wire and cubic morphologies from initial CuHCF nanoparticles are monitored after hundreds of cycles. Irreversible introduction of Zn ions to CuHCF is revealed locally using scanning transmission electron microscopy. A substitution mechanism is proposed to explain the increasing Zn content within the cathode material while simultaneously the Cu content is lowered during Zn-ion battery cycling. The present study demonstrates that the irreversible introduction of Zn ions is responsible for the decreasing Zn ion capacity of the CuHCF cathode in high electrolyte concentration. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.201905384
  • 2020 • 194 Kinetic approach to nuclear-spin polaron formation
    Fischer, A. and Kleinjohann, I. and Anders, F.B. and Glazov, M.M.
    Physical Review B 102 (2020)
    Under optical cooling of nuclei, a strongly correlated nuclear-spin polaron state can form in semiconductor nanostructures with localized charge carriers due to the strong hyperfine interaction of the localized electron spin with the surrounding nuclear spins. Here we develop a kinetic-equation formalism describing the nuclear-spin polaron formation. We present a derivation of the kinetic equations for an electron-nuclear spin system coupled to reservoirs of different electron and nuclear spin temperatures which generate the exact thermodynamic steady state for equal temperatures independent of the system size. We illustrate our approach using the analytical solution of the central spin model in the limit of an Ising form of the hyperfine coupling. For homogeneous hyperfine coupling constants, i.e., the box model, the model is reduced to an analytically solvable form. Based on the analysis of the nuclear-spin distribution function and the electron-nuclear spin correlators, we derive a relation between the electron and nuclear spin temperatures, where the correlated nuclear-spin polaron state is formed. In the limit of large nuclear baths, this temperature line coincides with the critical temperature of the mean-field theory for polaron formation. The criteria of the polaron formation in a finite-size system are discussed. We demonstrate that the system's behavior at the transition temperature does not depend on details of the hyperfine-coupling distribution function but only on the effective number of coupled bath spins. In addition, the kinetic equations enable the analysis of the temporal formation of the nuclear-polaron state, where we find the build-up process predominated by the nuclear spin-flip dynamics. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.165309
  • 2020 • 193 Mapping the mechanical properties in nitride coatings at the nanometer scale
    Zhang, Z. and Chen, Z. and Holec, D. and Liebscher, C.H. and Koutná, N. and Bartosik, M. and Zheng, Y. and Dehm, G. and Mayrhofer, P.H.
    Acta Materialia 194 343-353 (2020)
    We report on a multilayered structure comprising of rock-salt (rs) structured CrN layers of constant thickness and AlN layers of varying thicknesses, which surprisingly enables the growth of metastable zinc-blende (zb) AlN layers for certain layer-thickness combinations. The multilayer exhibits an atomic and electronic structure gradient as revealed using advanced electron microscopy and electron spectroscopy. Gradient structures are also accompanied by a modulation of the chemical compositions. A combined experimental analysis based on valence electrons and inner shell electrons allowed mapping the mechanical properties of the multilayer at the nanometer scale and further unveiled the effect of oxygen impurities on the bulk modulus. We found that the presence of oxygen impurities causes a remarkable reduction of the bulk modulus of rs-CrN while having no significant effect on the bulk modulus of the stable wurtzite structure wz-AlN layers. The findings are unambiguously validated by theoretical calculations using density functional theory. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.04.024
  • 2020 • 192 Measurement of Backaction from Electron Spins in a Gate-Defined GaAs Double Quantum dot Coupled to a Mesoscopic Nuclear Spin Bath
    Bethke, P. and McNeil, R.P.G. and Ritzmann, J. and Botzem, T. and Ludwig, Ar. and Wieck, A.D. and Bluhm, H.
    Physical Review Letters 125 (2020)
    Decoherence of a quantum system arising from its interaction with an environment is a key concept for understanding the transition between the quantum and classical world as well as performance limitations in quantum technology applications. The effects of large, weakly coupled environments are often described as a classical, fluctuating field whose dynamics is unaffected by the qubit, whereas a fully quantum description still implies some backaction from the qubit on the environment. Here we show direct experimental evidence for such a backaction for an electron-spin qubit in a GaAs quantum dot coupled to a mesoscopic environment of order 106 nuclear spins. By means of a correlation measurement technique, we detect the backaction of a single qubit-environment interaction whose duration is comparable to the qubit's coherence time, even in such a large system. We repeatedly let the qubit interact with the spin bath and measure its state. Between such cycles, the qubit is reinitialized to different states. The correlations of the measurement outcomes are strongly affected by the intermediate qubit state, which reveals the action of a single electron spin on the nuclear spins. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.125.047701
  • 2020 • 191 On the assessment of creep damage evolution in nickel-based superalloys through correlative HR-EBSD and cECCI studies
    Sulzer, S. and Li, Z. and Zaefferer, S. and Hafez Haghighat, S.M. and Wilkinson, A. and Raabe, D. and Reed, R.
    Acta Materialia 185 13-27 (2020)
    The evolution of dislocation density with creep strain in single-crystal superalloys is studied quantitatively using high-resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging under controlled diffraction conditions (cECCI). Data regarding dislocation density/structure is measured for deformation at 900 °C and 450 MPa up to ≈ 1% plastic strain. Effects of chemical composition are elucidated via three purpose-designed superalloys of differing rhenium and ruthenium contents. The evidence indicates that dislocation avalanching is already prevalent at plastic strains of ≈ 0.1%; thereafter, an exponential decay in the dislocation multiplication rate is indicative of self-hardening due to dislocation constriction within the matrix channels, as confirmed by the imaging. The results are rationalised using discrete dislocation dynamics modelling: a universal dislocation evolution law emerges, which will be useful for alloy design efforts. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.07.018
  • 2020 • 190 Radiative Auger process in the single-photon limit
    Löbl, M.C. and Spinnler, C. and Javadi, A. and Zhai, L. and Nguyen, G.N. and Ritzmann, J. and Midolo, L. and Lodahl, P. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Nature Nanotechnology 15 558-562 (2020)
    In a multi-electron atom, an excited electron can decay by emitting a photon. Typically, the leftover electrons are in their ground state. In a radiative Auger process, the leftover electrons are in an excited state and a redshifted photon is created1–4. In a semiconductor quantum dot, radiative Auger is predicted for charged excitons5. Here we report the observation of radiative Auger on trions in single quantum dots. For a trion, a photon is created on electron–hole recombination, leaving behind a single electron. The radiative Auger process promotes this additional (Auger) electron to a higher shell of the quantum dot. We show that the radiative Auger effect is a powerful probe of this single electron: the energy separations between the resonance fluorescence and the radiative Auger emission directly measure the single-particle splittings of the electronic states in the quantum dot with high precision. In semiconductors, these single-particle splittings are otherwise hard to access by optical means as particles are excited typically in pairs, as excitons. After the radiative Auger emission, the Auger carrier relaxes back to the lowest shell. Going beyond the original theoretical proposals, we show how applying quantum optics techniques to the radiative Auger photons gives access to the single-electron dynamics, notably relaxation and tunnelling. This is also hard to access by optical means: even for quasi-resonant p-shell excitation, electron relaxation takes place in the presence of a hole, complicating the relaxation dynamics. The radiative Auger effect can be exploited in other semiconductor nanostructures and quantum emitters in the solid state to determine the energy levels and the dynamics of a single carrier. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41565-020-0697-2
  • 2020 • 189 Renormalization of the electron g factor in the degenerate two-dimensional electron gas of ZnSe- And CdTe-based quantum wells
    Zhukov, E.A. and Mantsevich, V.N. and Yakovlev, D.R. and Kopteva, N.E. and Kirstein, E. and Waag, A. and Karczewski, G. and Wojtowicz, T. and Bayer, M.
    Physical Review B 102 (2020)
    The effective electron g factor, geff, is measured in a two-dimensional electron gas (2DEG) in modulation-doped ZnSe- and CdTe-based quantum wells by means of time-resolved pump-probe Kerr rotation. The measurements are performed in magnetic fields applied in the Voigt geometry, i.e., normal to the optical axis parallel to the quantum well plane, in the field range 0.05-6 T at temperatures 1.8-50K. The geff absolute value considerably increases with increasing electron density ne. geff changes in the ZnSe-based QWs from +1.1 to +1.9 in the ne range 3×1010-1.4×1012cm-2 and in the CdTe-based QWs from -1.55 down to -1.76 in the ne range 5×109-3×1011cm-2. The modification of geff reduces with increasing magnetic field, increasing temperature of lattice and 2DEG, the latter achieved by a higher photoexcitation density. A theoretical model is developed that considers the renormalization of the spin-orbit coupling constant of the two-dimensional electrons by the electron-electron interaction and takes into account corrections to the electron-electron interaction in the Hubbard form. The model results are in good agreement with experimental data. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.125306
  • 2020 • 188 Reversal of uniaxial magnetic anisotropy in Fe/GaAs (110) films driven by surface relaxation: An in situ ferromagnetic resonance study
    Bhagat, B. and Semisalova, A. and Meckenstock, R. and Farle, M.
    AIP Advances 10 (2020)
    We report an in situ study of the time evolution of magnetic anisotropy constants of an uncapped 4 nm [∼27 monolayers (ML)] Fe film epitaxially grown on a GaAs (110) substrate at room temperature under ultra-high vacuum (UHV) conditions. The structural and chemical properties are monitored by low energy electron diffraction and Auger spectroscopy with a sensitivity of 0.01 ML. The in situ UHV ferromagnetic resonance (FMR) study over a period of 6 days in <10-9 Pa reveals that there is a slow magneto-morphological transition of the Fe film surface at room temperature. The resonance field measured in situ in the [11-0] direction initially changes at a rate of 0.3 mT/h within 30 h after deposition and later at 0.1 mT/h over 80 h. We determine the time-dependent changes in the in-plane and out-of-plane anisotropy constants and find a sign change in the uniaxial in-plane anisotropy in the first 24 h due to morphological changes at the surface. The in situ FMR measurements and the Auger analysis allow us to exclude changes in the magnetization and anisotropy due to the contamination and oxidation of the Fe film. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0004261
  • 2020 • 187 Short range proximity effect induced by exchange interaction in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells
    Kirstein, E. and Kozyrev, N.V. and Afanasiev, M.M. and Mantsevich, V.N. and Krivenko, I.S. and Kalevich, V.K. and Salewski, M. and Chusnutdinow, S. and Wojtowicz, T. and Karczewski, G. and Kusrayev, Y.G. and Zhukov, E.A. and Yakov...
    Physical Review B 101 (2020)
    The coherent spin dynamics of electrons in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells (QWs) is studied by time-resolved pump-probe Kerr rotation. The coupled QWs have different thicknesses; the narrow one is doped by Mn2+ magnetic ions. A short range proximity effect between them is observed: the Zeeman splitting of electrons in the wide QW is given in addition to the intrinsic electron g factor by the exchange interaction with the Mn2+ ions mediated by electron tunneling into the narrow QW. The exchange interaction strength scales with the Cd0.88Mg0.12Te barrier thickness separating the QWs. The Kerr rotation signal measured on the wide QW shows two close frequencies of electron spin Larmor precession in a transverse magnetic field. These components have very different spin dephasing times, 50 ps and 1 ns. The two frequencies originate from electrons in the wide QW being either part of an exciton or being resident. The proximity effect of the exciton electron is smaller due to the binding by Coulomb interaction, which decreases the tunneling to the narrow well. The experimental data are in good agreement with model calculations. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.035301
  • 2020 • 186 Single and Double Electron Spin-Flip Raman Scattering in CdSe Colloidal Nanoplatelets
    Kudlacik, D. and Sapega, V.F. and Yakovlev, D.R. and Kalitukha, I.V. and Shornikova, E.V. and Rodina, A.V. and Ivchenko, E.L. and Dimitriev, G.S. and Nasilowski, M. and Dubertret, B. and Bayer, M.
    Nano Letters 20 517-525 (2020)
    CdSe colloidal nanoplatelets are studied by spin-flip Raman scattering in magnetic fields up to 5 T. We find pronounced Raman lines shifted from the excitation laser energy by an electron Zeeman splitting. Their polarization selection rules correspond to those expected for scattering mediated by excitons interacting with resident electrons. Surprisingly, Raman signals shifted by twice the electron Zeeman splitting are also observed. The theoretical analysis and experimental dependences show that the mechanism responsible for the double flip involves two resident electrons interacting with a photoexcited exciton. Effects related to various orientations of the nanoplatelets in the ensemble and different orientations of the magnetic field are analyzed. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b04262
  • 2020 • 185 Soft X-ray diffraction patterns measured by a LiF detector with sub-micrometre resolution and an ultimate dynamic range
    Makarov, S. and Pikuz, S. and Ryazantsev, S. and Pikuz, T. and Buzmakov, A. and Rose, M. and Lazarev, S. and Senkbeil, T. and Von Gundlach, A. and Stuhr, S. and Rumancev, C. and Dzhigaev, D. and Skopintsev, P. and Zaluzhnyy, I. an...
    Journal of Synchrotron Radiation 27 625-632 (2020)
    The unique diagnostic possibilities of X-ray diffraction, small X-ray scattering and phase-contrast imaging techniques applied with high-intensity coherent X-ray synchrotron and X-ray free-electron laser radiation can only be fully realized if a sufficient dynamic range and/or spatial resolution of the detector is available. In this work, it is demonstrated that the use of lithium fluoride (LiF) as a photoluminescence (PL) imaging detector allows measuring of an X-ray diffraction image with a dynamic range of ∼107 within the sub-micrometre spatial resolution. At the PETRA III facility, the diffraction pattern created behind a circular aperture with a diameter of 5μm irradiated by a beam with a photon energy of 500eV was recorded on a LiF crystal. In the diffraction pattern, the accumulated dose was varied from 1.7 × 105Jcm-3 in the central maximum to 2 × 10-2Jcm-3 in the 16th maximum of diffraction fringes. The period of the last fringe was measured with 0.8μm width. The PL response of the LiF crystal being used as a detector on the irradiation dose of 500eV photons was evaluated. For the particular model of laser-scanning confocal microscope Carl Zeiss LSM700, used for the readout of the PL signal, the calibration dependencies on the intensity of photopumping (excitation) radiation (λ = 488nm) and the gain have been obtained. © 2020. J. Synchrotron Rad.
    view abstractdoi: 10.1107/S1600577520002192
  • 2020 • 184 Tracking the ultrafast nonequilibrium energy flow between electronic and lattice degrees of freedom in crystalline nickel
    Maldonado, P. and Chase, T. and Reid, A.H. and Shen, X. and Li, R.K. and Carva, K. and Payer, T. and Horn-von Hoegen, M. and Sokolowski-Tinten, K. and Wang, X.J. and Oppeneer, P.M. and Dürr, H.A.
    Physical Review B 101 (2020)
    Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. By combining ab initio calculations with ultrafast diffuse electron scattering, we gain a detailed understanding of the complex nonequilibrium energy transfer between electrons and phonons in laser-excited Ni metal. Our experimental results show that the wave-vector-resolved population dynamics of phonon modes is distinctly different throughout the Brillouin zone and are in remarkable agreement with our theoretical results. We find that zone-boundary phonon modes become occupied first. As soon as the energy in these modes becomes larger than the average electron energy, a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent excitation of lower-energy phonon modes drives the thermalization of the whole system on the picosecond time scale. We determine the evolving nonequilibrium phonon occupations, which we find to deviate markedly from thermal occupations. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "" Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by "" Bibsam.
    view abstractdoi: 10.1103/PhysRevB.101.100302
  • 2019 • 183 A photosystem i monolayer with anisotropic electron flow enables Z-scheme like photosynthetic water splitting
    Zhao, F. and Wang, P. and Ruff, A. and Hartmann, V. and Zacarias, S. and Pereira, I.A.C. and Nowaczyk, M.M. and Rögner, M. and Conzuelo, F. and Schuhmann, W.
    Energy and Environmental Science 12 3133-3143 (2019)
    Photosynthetic protein complexes are attractive building blocks for the fabrication of semi-artificial energy conversion devices. However, limitations in the efficiency of the implemented biophotovoltaic systems prevent their use in practical applications. In particular, the effective minimization of recombination processes in photosystem I (PSI) modified bioelectrodes is crucial for enabling a unidirectional electron flow allowing the true potential of the large charge separation at PSI being exploited. Here, we present controlled immobilization of PSI monolayers with a predefined preferential orientation that translates into anisotropic electron flow upon irradiation. Further interface of the oriented PSI monolayer with redox polymers allows an efficient electron transfer and minimization of possible short-circuiting pathways. To complete the functional biophotocathode, the PSI monolayer is coupled to a hydrogenase (H2ase) to realize light-induced H2 evolution. The PSI/H2ase biocathode is then combined with a redox polymer/photosystem II-based bioanode demonstrating a fully light-driven Z-scheme mimic biophotovoltaic cell for bias-free water splitting. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ee01901d
  • 2019 • 182 Advances in in situ nanomechanical testing
    Minor, A.M. and Dehm, G.
    MRS Bulletin 44 438-442 (2019)
    In situ nanomechanical testing provides critical insight into the fundamental processes that lead to deformation phenomena in materials. Often, in situ tests are performed in relevant conditions such as high or low temperatures, tribological contact, gas environments, or under radiation exposure. Modern diffraction and imaging methods of materials under load provide high spatial resolution and enable extraction of quantitative mechanical data from local microstructure components or nano-sized objects. The articles in this issue cover recent advances in different types of in situ nanomechanical testing methods, spanning from dedicated nanomechanical testing platforms and microelectromechanical systems devices to deformation analyses via in situ diffraction and imaging methods. This includes scanning electron microscopy, advanced scanning transmission electron microscopy, electron diffraction, x-ray diffraction, and synchrotron techniques. Emerging areas such as in situ tribology enable novel insights into the origin of deformation mechanisms, while the evolution of microelectromechanical systems for controlled in situ testing provide opportunities for advanced control and loading strategies. Discussion on the current state of the art for in situ nanomechanical testing and future opportunities in imaging, strain sensing, and testing environments are also addressed. Copyright © Materials Research Society 2019.
    view abstractdoi: 10.1557/mrs.2019.127
  • 2019 • 181 All for one and one for all
    Bayer, M.
    Science 364 30-31 (2019)
    doi: 10.1126/science.aaw8823
  • 2019 • 180 Characterization of a nitrogen gliding arc plasmatron using optical emission spectroscopy and high-speed camera
    Gröger, S. and Ramakers, M. and Hamme, M. and Medrano, J.A. and Bibinov, N. and Gallucci, F. and Bogaerts, A. and Awakowicz, P.
    Journal of Physics D: Applied Physics 52 (2019)
    A gliding arc plasmatron (GAP), which is very promising for purification and gas conversion, is characterized in nitrogen using optical emission spectroscopy and high-speed photography, because the cross sections of electron impact excitation of N2 are well known. The gas temperature (of about 5500 K), the electron density (up to 1.5 × 1015 cm-3) and the reduced electric field (of about 37 Td) are determined using an absolutely calibrated intensified charge-coupled device (ICCD) camera, equipped with an in-house made optical arrangement for simultaneous two-wavelength diagnostics, adapted to the transient behavior of a GA channel in turbulent gas flow. The intensities of nitrogen molecular emission bands, N2(C-B,0-0) as well as (B-X,0-0), are measured simultaneously. The electron density and the reduced electric field are determined at a spatial resolution of 30 μm, using numerical simulation and measured emission intensities, applying the Abel inversion of the ICCD images. The temporal behavior of the GA plasma channel and the formation of plasma plumes are studied using a high-speed camera. Based on the determined plasma parameters, we suggest that the plasma plume formation is due to the magnetization of electrons in the plasma channel of the GAP by an axial magnetic field in the plasma vortex. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aaefe4
  • 2019 • 179 Control of electron dynamics, radical and metastable species generation in atmospheric pressure RF plasma jets by Voltage Waveform Tailoring
    Korolov, I. and Donkó, Z. and Hübner, G. and Bischoff, L. and Hartmann, P. and Gans, T. and Liu, Y. and Mussenbrock, T. and Schulze, J.
    Plasma Sources Science and Technology 28 (2019)
    Atmospheric pressure capacitively coupled radio frequency discharges operated in He/N2 mixtures and driven by tailored voltage waveforms are investigated experimentally using a COST microplasma reference jet and by means of kinetic simulations as a function of the reactive gas admixture and the number of consecutive harmonics used to drive the plasma. Pulse-type 'peaks'-waveforms, that consist of up to four consecutive harmonics of the fundamental frequency (f = 13.56 MHz), are used at a fixed peak-to-peak voltage of 400 V. Based on an excellent agreement between experimental and simulation results with respect to the DC self-bias and the spatio-temporal electron impact excitation dynamics, we demonstrate that Voltage Waveform Tailoring allows for the control of the dynamics of energetic electrons, the electron energy distribution function in distinct spatio-temporal regions of interest, and, thus, the generation of atomic nitrogen as well as helium metastables, which are highly relevant for a variety of technological and biomedical applications. By tuning the number of driving frequencies and the reactive gas admixture, the generation of these important species can be optimised. The behaviour of the DC self-bias, which is different compared to that in low pressure capacitive radio frequency plasmas, is understood based on an analytical model. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab38ea
  • 2019 • 178 Correlated mode analysis of a microwave driven ICP source
    Porteanu, H.-E. and Stefanović, I. and Bibinov, N. and Klute, M. and Awakowicz, P. and Brinkmann, R.P. and Heinrich, W.
    Plasma Sources Science and Technology 28 (2019)
    Microwave and optical measurements are correlated to identify the mode evolution in a miniature, microwave driven, inductively coupled plasma (ICP) source. The very compact design of the source is derived from previous work (Porteanu et al 2013 Plasma Sources Sci. Technol. 22 035016). Microwave spectroscopy of the system resonances during the simultaneous microwave excitation of the plasma (‘Hot-S-Parameter’ spectroscopy) is a novel method to determine the electron density and to identify the type of coupling mode. The method corresponds directly to the kind of numerical simulations employed. The purpose of this analysis is finally to find the minimum power necessary to drive the source into the ICP mode. The efficiency of microwave energy transfer to the plasma is also discussed. Nitrogen at pressures between 50 and 1000 Pa and a gas flow of 150 sccm is used as test plasma, for which the electron density is determined. Analysis of the microwave resonance frequency shows that the electron density exceeds 1019 m−3 at 50 Pa for 11 W and at 1000 Pa for 26 W absorbed power. 3D theoretical analysis of this source confirms that at this electron density an ICP mode is present. © 2019 IOP Publishing Ltd
    view abstractdoi: 10.1088/1361-6595/ab06a7
  • 2019 • 177 Diffraction paradox: An unusually broad diffraction background marks high quality graphene
    Chen, S. and Horn-von Hoegen, M. and Thiel, P.A. and Tringides, M.C.
    Physical Review B 100 (2019)
    The realization of the unusual properties of two-dimensional (2D) materials requires the formation of large domains of single-layer thickness, extending over the mesoscale. It is found that the formation of uniform graphene on SiC, contrary to textbook diffraction, is signaled by a strong bell-shaped component (BSC) around the (00) and G(10) spots (but not around the substrate spots). The BCS is also seen on graphene grown on metals, because a single uniform graphene layer can be also grown with large lateral size. It is only seen by electron diffraction but not with X-ray or He scattering. Although the origin of such an intriguing result is unclear, its presence in the earlier literature (but never mentioned) points to its robustness and significance. A likely mechanism relates to the the spatial confinement of the graphene electrons, within a single layer. This leads to large spread in their wave vector which is transferred by electron-electron interactions to the elastically scattered electrons to generate the BSC. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.100.155307
  • 2019 • 176 Disrupting the spatio-temporal symmetry of the electron dynamics in atmospheric pressure plasmas by voltage waveform tailoring
    Gibson, A.R. and Donkó, Z. and Alelyani, L. and Bischoff, L. and Hübner, G. and Bredin, J. and Doyle, S. and Korolov, I. and Niemi, K. and Mussenbrock, T. and Hartmann, P. and Dedrick, J.P. and Schulze, J. and Gans, T. and O'Connell, D.
    Plasma Sources Science and Technology 28 (2019)
    Single frequency, geometrically symmetric Radio-Frequency (RF) driven atmospheric pressure plasmas exhibit temporally and spatially symmetric patterns of electron heating, and consequently, charged particle densities and fluxes. Using a combination of phase-resolved optical emission spectroscopy and kinetic plasma simulations, we demonstrate that tailored voltage waveforms consisting of multiple RF harmonics induce targeted disruption of these symmetries. This confines the electron heating to small regions of time and space and enables the electron energy distribution function to be tailored. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aaf535
  • 2019 • 175 Dissipated electrical power and electron density in an RF atmospheric pressure helium plasma jet
    Golda, J. and Kogelheide, F. and Awakowicz, P. and Gathen, V.S.-V.D.
    Plasma Sources Science and Technology 28 (2019)
    Atmospheric pressure plasmas have great potential, especially for biomedical applications, due to the large number of reactive species produced. In particular with regard to these applications, the comparability of processes through appropriate control of plasma parameters is essential for treatment safety. Here we present a method for the operando determination of absolute absorbed power in an RF atmospheric pressure helium plasma discharge using miniaturized probes. A detailed error analysis demonstrates the reliability of the measured power values. With the help of a global model, the sheath width and electron density (4 1016-11 1016 m-3) are derived from these power measurements and compared to literature. The results and thus the validity of the electrical model are confirmed by a second, independent characterization method using optical emission spectroscopy and time-averaged imaging. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab393d
  • 2019 • 174 Electron g-factor in coupled quantum wells CdTe and CdMnTe
    Afanasiev, M.M. and Kozyrev, N.V. and Kirstein, E. and Kalevich, V.K. and Zhukov, E.A. and Mantsevich, V.N. and Krivenko, I.S. and Karczewski, G. and Yakovlev, D.R. and Kusraev, Yu.G. and Bayer, M.
    Journal of Physics: Conference Series 1400 (2019)
    In tunnel-coupled quantum wells (QWs) CdTe (20 nm wide) and CdMnTe (8 nm wide) separated by Cd0.88Mg0.12Te barrier with a thickness of L B = 5, 7, 9, and 11 monolayers (MLs), the dependence of electron g-factor on the barrier thickness and temperature is investi-gated by means of the pump-probe Kerr rotation technique. The renormalization of the electron g-factor occurs due to the s-d exchange interaction of electrons with manganese ions in a magnetic QW. The most change of the electron g-factor value Δg = 0.25 is registered at the least barrier width of 5 MLs and temperature T = 5 K. In this case, the penetration of the electron wave function from the nonmagnetic QW into the magnetic one is estimated to be 0.6%. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1400/6/066023
  • 2019 • 173 Electron spin dynamics in mesoscopic GaN nanowires
    Buß, J.H. and Fernández-Garrido, S. and Brandt, O. and Hägele, D. and Rudolph, J.
    Applied Physics Letters 114 (2019)
    The electron spin dynamics in spontaneously formed GaN nanowires (NWs) on Si(111) is investigated by time-resolved magneto-optical Kerr-rotation spectroscopy for temperatures from 15 to 260 K. A strong increase in the electron spin relaxation time by more than an order of magnitude is found as compared to bulk GaN. The temperature dependence of spin relaxation is characterized by two regimes, which are explained by a model taking into account the coexistence of two different mechanisms. As a result, the spin lifetime is limited by hyperfine interaction of localized electron spins with nuclear spins at low temperatures. The mesoscopic electron confinement in the NWs leads to a dominance of Dyakonov-Perel spin relaxation driven by interface-induced contributions at high temperatures, resulting in a slow-down, but not complete suppression of spin relaxation as compared to bulk GaN. These findings underline the important role of the high surface-to-volume ratio in NWs. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5080508
  • 2019 • 172 Electron-Beam-Induced Current Measurements of Thin-Film Solar Cells
    Abou-Ras, D. and Kirchartz, T.
    ACS Applied Energy Materials 2 6127-6139 (2019)
    The present tutorial review provides a practical guide to the analysis of semiconductor devices using electron-beam-induced currents (EBICs). The authors focus on cross-sectional EBIC measurements that provide an experimental assay of the efficiency of charge carrier collection in a semiconductor diode. The tutorial covers the fundamental physics of the technique, specimen preparation, data acquisition, and numerical simulation and analysis of the experimental data. A key focus is put on application cases from the field of thin-film photovoltaics as well as specific pitfalls that may occur, such as effects occurring under high-level injection and at grain boundaries of polycrystalline materials. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b01172
  • 2019 • 171 Evalution of Drude parameters for liquid Germanium nanoparticles through aerosol-based line-of-sight attenuation measurements
    Daun, K.J. and Menser, J. and Asif, M. and Musikhin, S. and Dreier, T. and Schulz, C.
    Journal of Quantitative Spectroscopy and Radiative Transfer 226 146-156 (2019)
    The objective of this study is to infer Drude model parameters for liquid germanium nanoparticles from extinction measurements made across an aerosol within a microwave plasma reactor using a halogen lamp (410–700 nm) and a laser-driven light source (205–585 nm). The plasma frequency and relaxation time are inferred using Rayleigh theory, Mie theory, and a fourth-order Mie approximation. These parameters are compared with those found using the ellipsometry-derived complex dielectric function as well as the bulk density and electrical resistivity of liquid germanium. The analysis is carried out in a probabilistic context using Bayesian inference, which accounts for both the measurement noise and model error. While all the candidate models can reproduce the shape of the experimentally-derived extinction spectra, the Bayesian inference showed that extinction-derived parameters differed from those obtained from the density and electrical resistivity in a statistically-significant way. This highlights the limitations of the free-electron model that underpins Drude theory, and suggests potential opportunities for model refinement. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jqsrt.2019.01.021
  • 2019 • 170 Experimental realization of a quantum dot energy harvester
    Jaliel, G. and Puddy, R.K. and Sánchez, R. and Jordan, A.N. and Sothmann, B. and Farrer, I. and Griffiths, J.P. and Ritchie, D.A. and Smith, C.G.
    Physical Review Letters 123 (2019)
    We demonstrate experimentally an autonomous nanoscale energy harvester that utilizes the physics of resonant tunneling quantum dots. Gate-defined quantum dots on GaAs/AlGaAs high-electron-mobility transistors are placed on either side of a hot-electron reservoir. The discrete energy levels of the quantum dots are tuned to be aligned with low energy electrons on one side and high energy electrons on the other side of the hot reservoir. The quantum dots thus act as energy filters and allow for the conversion of heat from the cavity into electrical power. Our energy harvester, measured at an estimated base temperature of 75 mK in a He3/He4 dilution refrigerator, can generate a thermal power of 0.13 fW for a temperature difference across each dot of about 67 mK. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.123.117701
  • 2019 • 169 Harmonic off-axis seeding at the Delta short-pulse source
    Meyer auf der Heide, A. and Büsing, B. and Khan, S. and Krieg, D. and Mai, C.
    Proceedings of the 39th International Free-Electron Laser Conference, FEL 2019 246-249 (2019)
    At the 1.5-GeV synchrotron light source DELTA operated by the TU Dortmund University, a short-pulse source employs the coherent harmonic generation (CHG) scheme. Here, a laser pulse interacts with a stored electron bunch forming a microbunching structure to generate ultrashort synchrotron light pulses at harmonics of the laser wavelength. As an upgrade of the short-pulse facility, the echo-enabled harmonic generation (EEHG) scheme will be implemented, which requires a second laser-electron interaction to yield much higher harmonics compared to CHG. In a study towards twofold laser seeding, the possibility of seeding at undulator harmonics with a crossing angle between laser and electron beam was investigated. Content from this work may be used under the terms of the CC BY 3.0 licence (© 2019). Any distribution of this work must maintain attribution to the author(s).
    view abstractdoi: 10.18429/JACoW-FEL2019-TUP080
  • 2019 • 168 Mitigation of EC breakdown in the gyrotron transmission line of the ITER Collective Thomson Scattering diagnostic via a Split Biased Waveguide
    Larsen, A.W. and Korsholm, S.B. and Gonçalves, B. and Gutierrez, H.E. and Henriques, E. and Infante, V. and Jensen, T. and Jessen, M. and Klinkby, E.B. and NonbØl, E. and Luis, R. and Vale, A. and Lopes, A. and Naulin, V. and Ni...
    Journal of Instrumentation 14 (2019)
    In this paper we present the results of the R&D work that has been performed on avoiding electron cyclotron (EC) gas breakdown inside the launcher transmission line (TL) of the ITER collective Thomson scattering (CTS) diagnostic, due to encountering the fundamental EC resonance, which is located inside the port plug vacuum for the baseline ITER magnetic field scenario. If an EC breakdown occurs, this can lead to strong local absorption of the CTS gyrotron beam, as well as arcing inside the ITER vacuum vessel, which must be avoided. Due to the hostile, restrictive, and nuclear environment in ITER, it is not possible to implement the standard method for avoiding EC breakdown - a controlled atmosphere at the EC resonance. Instead, the CTS diagnostic will include a longitudinally-split electrically-biased corrugated waveguide (SBWG) in the launcher transmission line. The SBWG works by applying a transverse DC bias voltage across the two electrically-isolated waveguide halves, causing free electrons to diffuse out of the EC resonant region before they can cause an electron-impact ionisation-avalanche, and thus an EC breakdown. Due to insufficient experimental facilities, the functionality of the SBWG is validated through Monte Carlo electron modelling. © 2019 IOP Publishing Ltd and Sissa Medialab.
    view abstractdoi: 10.1088/1748-0221/14/11/C11009
  • 2019 • 167 Nanoscale Physical and Chemical Structure of Iron Oxide Nanoparticles for Magnetic Particle Imaging
    Hufschmid, R. and Landers, J. and Shasha, C. and Salamon, S. and Wende, H. and Krishnan, K.M.
    Physica Status Solidi (A) Applications and Materials Science 216 (2019)
    In this work, the role of the nanoscale chemical and magnetic structure on relaxation dynamics of iron oxide nanoparticles in the context of magnetic particle imaging (MPI) is investigated with Mössbauer spectroscopy (MS) and electron energy loss spectroscopy (EELS). Two samples of 27 nm monodisperse iron oxide nanoparticles are compared, with and without an additional oxidation optimization step, with corresponding differences in structure and properties. Iron oxide nanoparticles synthesized in the presence of sufficient oxygen form single crystalline, inverse-spinel magnetite (Fe 3 O 4 ) and display magnetic properties suitable for MPI. A secondary wüstite (FeO) phase is observed in the diffraction pattern of unoptimized nanoparticles, which is antiferromagnetic and therefore unsuitable for MPI. Mössbauer spectra confirm the composition of the optimized nanoparticles to be ≈70% magnetite, with the remaining 30% oxidized to maghemite; in contrast, the as-synthesized particles (without the oxidation step) contained about 40% wüstite and 60% magnetite. The authors use scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to probe iron 2p-3d electronic transitions and correlate their intensities with the oxidation state with sub-nanometer spatial resolution. The optimally oxidized nanoparticles are uniform in crystallography and phase, while the mixed phase nanoparticles are core-shell wüstite/magnetite. Further confirming the core-shell structure of the mixed phase nanoparticles, considerable spin canting in the in-field Mössbauer spectrum, likely caused by interface coupling, is observed. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/pssa.201800544
  • 2019 • 166 Nanoscopic Porous Iridium/Iridium Dioxide Superstructures (15 nm): Synthesis and Thermal Conversion by In Situ Transmission Electron Microscopy
    Pappert, K. and Loza, K. and Shviro, M. and Hagemann, U. and Heggen, M. and Dunin-Borkowski, R.E. and Schierholz, R. and Maeda, T. and Kaneko, K. and Epple, M.
    Chemistry - A European Journal 25 11048-11057 (2019)
    Porous particle superstructures of about 15 nm diameter, consisting of ultrasmall nanoparticles of iridium and iridium dioxide, are prepared through the reduction of sodium hexachloridoiridate(+IV) with sodium citrate/sodium borohydride in water. The water-dispersible porous particles contain about 20 wt % poly(N-vinylpyrrolidone) (PVP), which was added for colloidal stabilization. High-resolution transmission electron microscopy confirms the presence of both iridium and iridium dioxide primary particles (1–2 nm) in each porous superstructure. The internal porosity (≈58 vol%) is demonstrated by electron tomography. In situ transmission electron microscopy up to 1000 °C under oxygen, nitrogen, argon/hydrogen (all at 1 bar), and vacuum shows that the porous particles undergo sintering and subsequent compaction upon heating, a process that starts at around 250 °C and is completed at around 800 °C. Finally, well-crystalline iridium dioxide is obtained under all four environments. The catalytic activity of the as-prepared porous superstructures in electrochemical water splitting (oxygen evolution reaction; OER) is reduced considerably upon heating owing to sintering of the pores and loss of internal surface area. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201901623
  • 2019 • 165 Nanosecond plasmas in water: Ignition, cavitation and plasma parameters
    Grosse, K. and Held, J. and Kai, M. and Von Keudell, A.
    Plasma Sources Science and Technology 28 (2019)
    Nanosecond plasmas in liquids play an important role in the field of decontamination, electrolysis or plasma medicine. The understanding of these very dynamic plasmas requires information about the temporal variation of species densities and temperatures. This is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages (HVs) between 14 and 26 kV and pulse lengths of 10 ns applied to a tungsten tip with 50 μm diameter immersed in water. Ignition of the plasma causes the formation of a cavitation bubble that is monitored by shadowgraphy to measure the dynamic of the created bubble and the sound speed of the emitted acoustic waves surrounding this tungsten tip. The temporal evolution of the bubble size is compared with cavitation theory yielding good agreement for an initial bubble radius of 25 μm with an initial pressure of 5 ×108 Pa at a temperature of 1200 K for a HV of 20 kV. This yields an initial energy in the range of a few 10-5 J that varies with the applied HV. The dissipated energy by the plasma drives the adiabatic expansion of water vapor inside the bubble from its initial supercritical state to a low pressure, low temperature state at maximum bubble expansion reaching values of 103 Pa and 50 K, respectively. These predictions from cavitation theory are corroborated by optical emission spectroscopy. After igniting the nanosecond plasma, the electrical power oscillates in the feed line between HV pulser and plasma chamber with a ring down time of the order of 60 ns. These reflected pulses re-ignite a plasma inside the expanding bubble periodically. Broadband emission due to recombination and Bremsstrahlung becomes visible within the first 30 ns. At later times, line emission dominates. Stark broadening of the spectral lines of Hα (656 nm) and OI (777 nm) is evaluated to determine both the electron density and the electron temperature in these re-ignited plasmas. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab26fc
  • 2019 • 164 Nonequilibrium carrier dynamics in self-assembled quantum dots
    Geller, M.
    Applied Physics Reviews 6 (2019)
    Self-assembled quantum dots are still one of the best model systems for artificial atoms in a solid-state environment, where the electronic states can be accessed by electrical and optical means. This article focuses on nonequilibrium carrier dynamics in these quantum dots, using the ability of recent developments in electrical and optical spectroscopy techniques. All-electrical transconductance spectroscopy is introduced, where a two-dimensional electron gas serves as a fast and sensitive detector for the electron/hole dynamics and charge/spin state preparation and relaxation in an ensemble of dots. Latest results on single electron tunneling and nonequilibrium Auger recombination in a single quantum dot using a high-resolution optical experiment (the time-resolved resonance fluorescence) are summarized. This article concludes with a perspective view on a future combination of both techniques toward an electro-optical measurement toolbox to link the coherent control of quantum states by optical means with an electrical preparation of electron charge and spin states. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5091742
  • 2019 • 163 Origin of Two Larmor Frequencies in the Coherent Spin Dynamics of Colloidal CdSe Quantum Dots Revealed by Controlled Charging
    Hu, R. and Yakovlev, D.R. and Liang, P. and Qiang, G. and Chen, C. and Jia, T. and Sun, Z. and Bayer, M. and Feng, D.
    Journal of Physical Chemistry Letters 10 3681-3687 (2019)
    Coherent spin dynamics in colloidal CdSe quantum dots (QDs) typically show two spin components with different Larmor frequencies, whose origin is an open question. We exploit the photocharging approach to identify their origin and find that surface states play a key role in the appearance of the spin signals. By controlling the photocharging with electron or hole acceptors, we show that the specific spin component can be enhanced by the choice of acceptor type. In core/shell CdSe/ZnS QDs, the spin signals are significantly weaker. Our results exclude the neutral exciton as the spin origin and suggest that both Larmor frequencies are related to the coherent spin precession of electrons in photocharged QDs. The lower frequency is due to the electron confined in the middle of the QD, and the higher frequency to the electron additionally localized in the vicinity of the surface. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.9b01534
  • 2019 • 162 Photogeneration of a single electron from a single Zeeman-resolved light-hole exciton with preserved angular momentum
    Kuroyama, K. and Larsson, M. and Chang, C.Y. and Muramoto, J. and Heya, K. and Fujita, T. and Allison, G. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Matsuo, S. and Oiwa, A. and Tarucha, S.
    Physical Review B 99 (2019)
    Quantum state transfer from a single photon to a single electron following selection rules can only occur for a spin-resolved light-hole excitation in GaAs quantum dots; however, these phenomena have yet to be experimentally realized. Here, we report on single-shot readout of a single electron spin via the Zeeman-resolved light-hole excitation using an optical spin blockade method in a GaAs quantum dot and a Pauli spin blockade method in a double GaAs quantum dot. The observed photoexcitation probability strongly depends on the photon polarization, an indication of angular momentum transfer from a single photon to an electron. Our demonstration will open a pathway to further investigation of fundamental quantum physics and applications of quantum networking technology. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.085203
  • 2019 • 161 Precision Plasmonics with Monomers and Dimers of Spherical Gold Nanoparticles: Nonequilibrium Dynamics at the Time and Space Limits
    Schumacher, L. and Jose, J. and Janoschka, D. and Dreher, P. and Davis, T.J. and Ligges, M. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X. and Meyer Zu Heringdorf, F. and Sokolowski-Ti...
    Journal of Physical Chemistry C 123 13181-13191 (2019)
    Monomers and dimers of spherical gold nanoparticles (NPs) exhibit highly uniform plasmonic properties at the single-particle level due to their high structural homogeneity (precision plasmonics). Recent investigations in precision plasmonics have largely focused on static properties using conventional techniques such as transmission electron microscopy and optical dark-field microscopy. In this Feature Article, we first highlight the application of femtosecond time-resolved electron diffraction for monitoring the nonequilibrium dynamics of spherical gold NPs after ultrafast optical excitation. The analysis of the transient diffraction patterns allows us to directly obtain quantitative information on the incoherent excitation of the lattice, that is, heating upon electron-lattice equilibration, as well as on the development of strain due to lattice expansion on picosecond time scales. The controlled assembly of two spherical gold NPs into a dimer with a few nanometers gap leads to unique optical properties. Specifically, extremely high electric fields (hot spot) in the gap are generated upon resonant optical excitation. Conventional optical microscopy cannot spatially resolve this unique hot spot due to the optical diffraction limit. We therefore employed nonlinear photoemission electron microscopy to visualize hot spots in single dimers of spherical gold NPs. A quantitative comparison of different single dimers confirms the homogeneity of the hot spots on the single-particle level. Overall, these initial results are highly encouraging because they pave the way to investigate nonequilibrium dynamics in highly uniform plasmonic nanostructures at the time and space limits. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.9b01007
  • 2019 • 160 Quantum non-demolition measurement of an electron spin qubit
    Nakajima, T. and Noiri, A. and Yoneda, J. and Delbecq, M.R. and Stano, P. and Otsuka, T. and Takeda, K. and Amaha, S. and Allison, G. and Kawasaki, K. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Tarucha, S.
    Nature Nanotechnology 14 555-560 (2019)
    Measurements of quantum systems inevitably involve disturbance in various forms. Within the limits imposed by quantum mechanics, there exists an ideal projective measurement that does not introduce a back action on the measured observable, known as a quantum non-demolition (QND) measurement1,2. Here we demonstrate an all-electrical QND measurement of a single electron spin in a gate-defined quantum dot. We entangle the single spin with a two-electron, singlet–triplet ancilla qubit via the exchange interaction3,4 and then read out the ancilla in a single shot. This procedure realizes a disturbance-free projective measurement of the single spin at a rate two orders of magnitude faster than its relaxation. The QND nature of the measurement protocol5,6 enables enhancement of the overall measurement fidelity by repeating the protocol. We demonstrate a monotonic increase of the fidelity over 100 repetitions against arbitrary input states. Our analysis based on statistical inference is tolerant to the presence of the relaxation and dephasing. We further exemplify the QND character of the measurement by observing spontaneous flips (quantum jumps)7 of a single electron spin. Combined with the high-fidelity control of spin qubits8–13, these results will allow for various measurement-based quantum state manipulations including quantum error correction protocols14. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41565-019-0426-x
  • 2019 • 159 Relaxation of electrons in quantum-confined states in Pb/Si(111) thin films from master equation with first-principles-derived rates
    Kratzer, P. and Zahedifar, M.
    New Journal of Physics 21 (2019)
    Atomically thin films of Pb on Si(111) provide an experimentally tunable system comprising a highly structured electronic density of states. The lifetime of excited electrons in these states is limited by both electron-electron (e-e) and electron-phonon (e-ph) scattering. We employ the description by a master equation for the electronic occupation numbers to analyze the relative importance of both scattering mechanisms. The electronic and phononic band structures, as well as the matrix elements for electron-phonon coupling within deformation potential theory were obtained from density functional calculations, thus taking into account quantum confinement effects. For the relaxation dynamics, the contribution of impact ionization processes to the lifetime is estimated from the imaginary part of the electronic self-energy calculated in the GW approximation. By numerically solving rate equations for the occupations of the Pb-derived electronic states coupled to a phononic heat bath, we are able to follow the distribution of the electronic excitation energy to the various modes of Pb lattice vibrations. While e-e scattering is the dominant relaxation mechanism, we demonstrate that the e-ph scattering is highly phonon-mode-specific, with a large contribution from surface phonons. At electron energies of about 0.3 eV above the Fermi surface, a 'phonon bottleneck' characteristic of relaxation in nanostructures with well-separated electronic states is observed. The time scales extracted from the simulations are compared to data from pump-probe experiments using time-resolved two-photon photoemission. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ab5c76
  • 2019 • 158 Selective Delamination upon Femtosecond Laser Ablation of Ceramic Surfaces
    Kiel, F. and Bulgakova, N.M. and Ostendorf, A. and Gurevich, E.L.
    Physical Review Applied 11 (2019)
    We report on the experimental observation of selective delamination of semitransparent materials on the example of yttria-stabilized zirconia ceramics upon femtosecond laser processing of its surface with a low numerical aperture lens. The delamination of a ceramic layer of dozens of micrometers takes place as a side effect of surface processing and is observed above the surface ablation threshold. The onset of delamination (delamination threshold) depends on the degree of overlap of the irradiation spots from consecutive laser pulses upon beam scanning over material surface. Analysis of the delaminated layer indicates that the material undergoes melting on both of its surfaces. The mechanism of delamination is identified as a complex interplay between the optical response of laser-generated free-electron plasma and nonlinear effects upon laser beam propagation in semitransparent ceramics. The discovered effect enables controllable laser microslicing of brittle ceramic materials. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevApplied.11.024038
  • 2019 • 157 Temperature and bias anomalies in the photoluminescence of InAs quantum dots coupled to a Fermi reservoir
    Korsch, A.R. and Nguyen, G.N. and Schmidt, M. and Ebler, C. and Valentin, S.R. and Lochner, P. and Rothfuchs, C. and Wieck, A.D. and Ludwig, Ar.
    Physical Review B 99 (2019)
    We present anomalous behavior of temperature-dependent photoluminescence (PL) measurements on InAs quantum dot ensembles coupled to an electron reservoir in an n-i-p diode structure. When negative gate voltages are applied to the sample, an anomalous initial increase of the integrated PL signal with rising temperature is observed for the ground-state and first-excited-state emission peaks. In contrast, measurements at positive gate voltages show no such anomaly and are well described by the commonly used Arrhenius model. Unlike previous studies on uncoupled quantum dot ensembles, we show that in quantum dot diode structures the anomalous temperature dependence and its dependence on the applied bias voltage is dominated by electrons tunneling from the electron reservoir to the quantum dots. Tunneling electrons enhance the PL signal by recombining with holes stored in the quantum dots and the tunneling rate depends on temperature via the Fermi distribution in the electron reservoir. With the implementation of a rate-based tunnel coupling, we develop a modified Arrhenius model that takes the observed anomalies excellently into account. Gate voltage dependent PL measurements at 77 K are further compared to capacitance-voltage spectroscopy measurements on the same sample, supporting the proposed interpretation. The PL peak width shows a characteristic evolution as a function of temperature, which is discussed qualitatively in terms of our model. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.165303
  • 2019 • 156 Voltage waveform tailoring in radio frequency plasmas for surface charge neutralization inside etch trenches
    Krüger, F. and Wilczek, S. and Mussenbrock, T. and Schulze, J.
    Plasma Sources Science and Technology 28 (2019)
    The etching of sub micrometer high-aspect-ratio (HAR) features into dielectric materials in low pressure radio frequency technological plasmas is limited by the accumulation of positive surface charges inside etch trenches. These are, at least partially, caused by highly energetic positive ions that are accelerated by the sheath electric field to high velocities perpendicular to the wafer. In contrast to these anisotropic ions, thermal electrons typically reach the electrode only during the sheath collapse and cannot penetrate deeply into HAR features to compensate the positive surface charges. This problem causes significant reductions of the etch rate and leads to deformations of the features due to ion deflection, i.e. the aspect ratio is limited. Here, we demonstrate that voltage waveform tailoring can be used to generate electric field reversals adjacent to the wafer during sheath collapse to accelerate electrons towards the electrode to allow them to penetrate deeply into HAR etch features to compensate positive surface charges and to overcome this process limitation. Based on 1D3V particle-in-cell/Monte Carlo collision simulations of a capacitively coupled plasma operated in argon at 1 Pa, we study the effects of changing the shape, peak-to-peak voltage, and harmonics' frequencies of the driving voltage waveform on this electric field reversal as well as on the electron velocity and angular distribution function at the wafer. We find that the angle of incidence of electrons relative to the surface normal at the wafer can be strongly reduced and the electron velocity perpendicular to the wafer can be significantly increased by choosing the driving voltage waveform in a way that ensures a fast and short sheath collapse. This is caused by the requirement of flux compensation of electrons and ions at the electrode on time average in the presence of a short and steep sheath collapse. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab2c72
  • 2018 • 155 An Option to Generate Seeded FEL Radiation for FLASH1
    Grattoni, V. and Assmann, R.W. and Bödewadt, J. and Lechner, C. and Kazemi, M.M. and Manschwetus, B. and Hartl, I. and Plath, T. and Khan, S. and Hillert, W. and Miltchev, V. and Rossbach, J.
    Journal of Physics: Conference Series 1067 (2018)
    The FLASH free-electron laser (FEL) at DESY is currently operated in self-amplified spontaneous emission (SASE) mode in both beamlines FLASH1 and FLASH2. Seeding offers unique properties for the FEL pulse, such as full coherence, spectral and temporal stability. In this contribution, possible ways to carry the seeded FEL radiation to the user hall are presented with analytical considerations and simulations. For this, components of the sFLASH seeding experiment are used. © 2018 Institute of Physics Publishing. All rights reserved.
    view abstractdoi: 10.1088/1742-6596/1067/3/032013
  • 2018 • 154 Analysis of Energy Dissipation Channels in a Benchmark System of Activated Dissociation: N2 on Ru(0001)
    Shakouri, K. and Behler, J. and Meyer, J. and Kroes, G.-J.
    Journal of Physical Chemistry C 122 23470-23480 (2018)
    The excitation of electron-hole pairs in reactive scattering of molecules at metal surfaces often affects the physical and dynamical observables of interest, including the reaction probability. Here, we study the influence of electron-hole pair excitation on the dissociative chemisorption of N2 on Ru(0001) using the local density friction approximation method. The effect of surface atom motion has also been taken into account by a high-dimensional neural network potential. Our nonadiabatic molecular dynamics simulations with electronic friction show that the reaction of N2 is more strongly affected by the energy transfer to surface phonons than by the energy loss to electron-hole pairs. The discrepancy between the computed reaction probabilities and experimental results is within the experimental error both with and without friction; however, the incorporation of electron-hole pairs yields somewhat better agreement with experiments, especially at high collision energies. We also calculate the vibrational efficacy for the N2 + Ru(0001) reaction and demonstrate that the N2 reaction is more enhanced by exciting the molecular vibrations than by adding an equivalent amount of energy into translation. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b06729
  • 2018 • 153 Bioelectrocatalytic and electrochemical cascade for phosphate sensing with up to 6 electrons per analyte molecule
    Kopiec, G. and Starzec, K. and Kochana, J. and Kinnunen-Skidmore, T.P. and Schuhmann, W. and Campbell, W.H. and Ruff, A. and Plumeré, N.
    Biosensors and Bioelectronics 117 501-507 (2018)
    Despite the availability of numerous electroanalytical methods for phosphate quantification, practical implementation in point-of-use sensing remains virtually nonexistent because of interferences from sample matrices or from atmospheric O2. In this work, phosphate determination is achieved by the purine nucleoside phosphorylase (PNP) catalyzed reaction of inosine and phosphate to produce hypoxanthine which is subsequently oxidized by xanthine oxidase (XOx), first to xanthine and then to uric acid. Both PNP and XOx are integrated in a redox active Os-complex modified polymer, which not only acts as supporting matrix for the bienzymatic system but also shuttles electrons from the hypoxanthine oxidation reaction to the electrode. The bienzymatic cascade in this second generation phosphate biosensor selectively delivers four electrons for each phosphate molecule present. We introduced an additional electrochemical process involving uric acid oxidation at the underlying electrode. This further enhances the anodic current (signal amplification) by two additional electrons per analyte molecule which mitigates the influence of electrochemical interferences from the sample matrix. Moreover, while the XOx catalyzed reaction is sensitive to O2, the uric acid production and therefore the delivery of electrons through the subsequent electrochemical process are independent of the presence of O2. Consequently, the electrochemical process counterbalances the O2 interferences, especially at low phosphate concentrations. Importantly, the electrochemical uric acid oxidation specifically reports on phosphate concentration since it originates from the product of the bienzymatic reactions. These advantageous properties make this bioelectrochemical-electrochemical cascade particularly promising for point-of-use phosphate measurements. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2018.06.047
  • 2018 • 152 Coherent optical spectroscopy of charged exciton complexes in semiconductor nanostructures
    Akimov, I.A. and Poltavtsev, S.V. and Salewski, M. and Yugova, I.A. and Karczewski, G. and Wiater, M. and Wojtowicz, T. and Reichelt, M. and Meier, T. and Yakovlev, D.R. and Bayer, M.
    Proceedings of SPIE - The International Society for Optical Engineering 10530 (2018)
    We present results on photon echo spectroscopy for resonant excitation of localized charged exciton complexes (trions) in CdTe/CdMgTe semiconductor quantum wells. We demonstrate that the Zeeman splitting of resident electron spin levels in transverse magnetic field leads to quantum beats in the photon echoes with the Larmor precession frequency. This allows us to perform a coherent transfer of optical excitation into a spin ensemble and to observe long-lived photon echoes. Our approach can be used as a tool for remarkably high resolution spectroscopy of the ground state levels: We are able to resolve splittings between the spin levels with sub-μeV precision and to distinguish between different types of electrons in the ensemble, namely electrons either bound to donors or localized on quantum well potential fluctuations. To that end we show that stimulated step-like Raman processes in the two-pulse excitation scheme allow us to probe the electron spin ensemble with high selectivity and precision even for systems with broad optical transitions. Next, Rabi oscillations for exciton complexes with different degree of localization are detected by photon echo spectroscopy. We observe that an increase of the area of either the first or the second pulse leads to a significant decrease of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex. © 2018 COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstractdoi: 10.1117/12.2288788
  • 2018 • 151 Combined experimental and theoretical description of direct current magnetron sputtering of Al by Ar and Ar/N2 plasma
    Trieschmann, J. and Ries, S. and Bibinov, N. and Awakowicz, P. and Mráz, S. and Schneider, J.M. and Mussenbrock, T.
    Plasma Sources Science and Technology 27 (2018)
    Direct current magnetron sputtering of Al by Ar and Ar/N2 low pressure plasmas was characterized by experimental and theoretical means in a unified consideration. Experimentally, the plasmas were analyzed by optical emission spectroscopy, while the film deposition rate was determined by weight measurements and laser optical microscopy, and the film composition by energy dispersive x-ray spectroscopy. Theoretically, a global particle and power balance model was used to estimate the electron temperature T e and the electron density n e of the plasma at constant discharge power. In addition, the sputtering process and the transport of the sputtered atoms were described using Monte Carlo models - TRIDYN and dsmcFoam, respectively. Initially, the non-reactive situation is characterized based on deposition experiment results, which are in agreement with predictions from simulations. Subsequently, a similar study is presented for the reactive case. The influence of the N2 addition is found to be twofold, in terms of (i) the target and substrate surface conditions (e.g., sputtering, secondary electron emission, particle sticking) and (ii) the volumetric changes of the plasma density n e governing the ion flux to the surfaces (e.g., due to additional energy conversion channels). It is shown that a combined experimental/simulation approach reveals a physically coherent and, in particular, quantitative understanding of the properties (e.g., electron density and temperature, target surface nitrogen content, sputtered Al density, deposited mass) involved in the deposition process. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aac23e
  • 2018 • 150 Control of FEL radiation properties by tailoring the seed pulses
    Grattoni, V. and Assmann, R.W. and Bödewadt, J. and Lechner, C. and Kazemi, M.M. and Manschwetus, B. and Hartl, I. and Plath, T. and Khan, S. and Azima, A. and Hillert, W. and Miltchev, V. and Rossbach, J.
    Journal of Physics: Conference Series 1067 (2018)
    Seeded free-electron lasers (FELs) produce intense, ultrashort and fully coherent X-ray pulses. These seeded FEL pulses depend on the initial seed properties. Therefore, controlling the seed laser allows tailoring the FEL radiation for phase-sensitive experiments. In this contribution, we present detailed simulation studies to characterize the FEL process and to predict the operation performance of seeded pulses. In addition, we show experimental data on the temporal characterization of the seeded FEL pulses performed at the sFLASH experiment in Hamburg. © 2018 Institute of Physics Publishing. All rights reserved.
    view abstractdoi: 10.1088/1742-6596/1067/3/032012
  • 2018 • 149 Dynamics of Electron-Phonon Coupling in Bicontinuous Nanoporous Gold
    Zheng, Q. and Shen, X. and Sokolowski-Tinten, K. and Li, R.K. and Chen, Z. and Mo, M.Z. and Wang, Z.L. and Weathersby, S.P. and Yang, J. and Chen, M.W. and Wang, X.J.
    Journal of Physical Chemistry C 122 16368-16373 (2018)
    The nanosize effect on electron-phonon interactions in free-electron noble metals has been a subject of intense discussion because of their important applications in physics, chemistry, and biomedicine. However, the interference from supports of dispersive nanoparticulate metals has led to controversial observations. We utilize freestanding, bicontinuous nanoporous gold (NPG) films to investigate electron-phonon interaction dynamics using ultrafast MeV electron diffraction. Compared to solid gold films, NPG shows faster electron-phonon interaction and thus noticeably higher electron-phonon coupling constant. The results demonstrate that surface states of electrons and phonons play an important role in electron-phonon coupling of nanostructured materials. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b03299
  • 2018 • 148 Experimental and computational investigations of electron dynamics in micro atmospheric pressure radio-frequency plasma jets operated in He/N 2 mixtures
    Bischoff, L. and Hübner, G. and Korolov, I. and Donkó, Z. and Hartmann, P. and Gans, T. and Held, J. and Schulz-Von Der Gathen, V. and Liu, Y. and Mussenbrock, T. and Schulze, J.
    Plasma Sources Science and Technology 27 (2018)
    The electron power absorption dynamics in radio frequency driven micro atmospheric pressure capacitive plasma jets are studied based on experimental phase resolved optical emission spectroscopy and the computational particle in cell simulations with Monte Carlo treatment of collisions. The jet is operated at 13.56 MHz in He with different admixture concentrations of N 2 and at several driving voltage amplitudes. We find the spatio-temporal dynamics of the light emission of the plasma at various wavelengths to be markedly different. This is understood by revealing the population dynamics of the upper levels of selected emission lines/bands based on comparisons between experimental and simulation results. The populations of these excited states are sensitive to different parts of the electron energy distribution function and to contributions from other excited states. Mode transitions of the electron power absorption dynamics from the Ω- to the Penning-mode are found to be induced by changing the N 2 admixture concentration and the driving voltage amplitude. Our numerical simulations reveal details of this mode transition and provide novel insights into the operation details of the Penning-mode. The characteristic excitation/emission maximum at the time of maximum sheath voltage at each electrode is found to be based on two mechanisms: (i) a direct channel, i.e. excitation/emission caused by electrons generated by Penning ionization inside the sheaths and (ii) an indirect channel, i.e. secondary electrons emitted from the electrode due to the impact of positive ions generated by Penning ionization at the electrodes. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aaf35d
  • 2018 • 147 Fully automated primary particle size analysis of agglomerates on transmission electron microscopy images via artificial neural networks
    Frei, M. and Kruis, F.E.
    Powder Technology 332 120-130 (2018)
    There is a high demand for fully automated methods for the analysis of primary particle size distributions of agglomerates on transmission electron microscopy images. Therefore, a novel method, based on the utilization of artificial neural networks, was proposed, implemented and validated. The training of the artificial neural networks requires large quantities (up to several hundreds of thousands) of transmission electron microscopy images of agglomerates consisting of primary particles with known sizes. Since the manual evaluation of such large amounts of transmission electron microscopy images is not feasible, a synthesis of lifelike transmission electron microscopy images as training data was implemented. The proposed method can compete with state-of-the-art automated imaging particle size methods like the Hough transformation, ultimate erosion and watershed transformation and is in some cases even able to outperform these methods. It is however still outperformed by the manual analysis. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2018.03.032
  • 2018 • 146 In-situ SEM observation of phase transformation and twinning mechanisms in an interstitial high-entropy alloy
    Wang, M. and Li, Z. and Raabe, D.
    Acta Materialia 147 236-246 (2018)
    The recently developed interstitial high-entropy alloys (iHEAs) exhibit an enhanced combination of strength and ductility. These properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic (FCC) γ matrix into the hexagonal close-packed (HCP) ε phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was produced and investigated via in-situ and interrupted in-situ tensile testing in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP ε phase and incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP ε formation. With further straining HCP ε nuclei grow into the adjacent deformed FCC γ matrix. γ regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC γ grains with a size of ∼10 μm can be activated at room temperature at a stress below ∼736 MPa. With increasing deformation, new twin lamellae continuously nucleate. The twin lamellae grow in preferred directions driven by the motion of the mobile partial dislocations. Owing to the individual grain size dependence of the activation of the dislocation-mediated plasticity, of the athermal phase transformation and of mechanical twinning at the different deformation stages, desired strain hardening profiles can be tuned and adjusted over the entire deformation regime by adequate microstructure design, providing excellent combinations of strength and ductility. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.01.036
  • 2018 • 145 Insight into induced charges at metal surfaces and biointerfaces using a polarizable Lennard-Jones potential
    Geada, I.L. and Ramezani-Dakhel, H. and Jamil, T. and Sulpizi, M. and Heinz, H.
    Nature Communications 9 (2018)
    Metallic nanostructures have become popular for applications in therapeutics, catalysts, imaging, and gene delivery. Molecular dynamics simulations are gaining influence to predict nanostructure assembly and performance; however, instantaneous polarization effects due to induced charges in the free electron gas are not routinely included. Here we present a simple, compatible, and accurate polarizable potential for gold that consists of a Lennard-Jones potential and a harmonically coupled core-shell charge pair for every metal atom. The model reproduces the classical image potential of adsorbed ions as well as surface, bulk, and aqueous interfacial properties in excellent agreement with experiment. Induced charges affect the adsorption of ions onto gold surfaces in the gas phase at a strength similar to chemical bonds while ions and charged peptides in solution are influenced at a strength similar to intermolecular bonds. The proposed model can be applied to complex gold interfaces, electrode processes, and extended to other metals. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03137-8
  • 2018 • 144 Kelvin probe force microscopy studies of the charge effects upon adsorption of carbon nanotubes and C60 fullerenes on hydrogen-terminated diamond
    Kölsch, S. and Fritz, F. and Fenner, M.A. and Kurch, S. and Wöhrl, N. and Mayne, A.J. and Dujardin, G. and Meyer, C.
    Journal of Applied Physics 123 (2018)
    Hydrogen-terminated diamond is known for its unusually high surface conductivity that is ascribed to its negative electron affinity. In the presence of acceptor molecules, electrons are expected to transfer from the surface to the acceptor, resulting in p-type surface conductivity. Here, we present Kelvin probe force microscopy (KPFM) measurements on carbon nanotubes and C60 adsorbed onto a hydrogen-terminated diamond(001) surface. A clear reduction in the Kelvin signal is observed at the position of the carbon nanotubes and C60 molecules as compared with the bare, air-exposed surface. This result can be explained by the high positive electron affinity of carbon nanotubes and C60, resulting in electron transfer from the surface to the adsorbates. When an oxygen-terminated diamond(001) is used instead, no reduction in the Kelvin signal is obtained. While the presence of a charged adsorbate or a difference in work function could induce a change in the KPFM signal, a charge transfer effect of the hydrogen-terminated diamond surface, by the adsorption of the carbon nanotubes and the C60 fullerenes, is consistent with previous theoretical studies. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5019486
  • 2018 • 143 Lateral heterostructures of hexagonal boron nitride and graphene: BCN alloy formation and microstructuring mechanism
    Petrović, M. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    Applied Surface Science 455 1086-1094 (2018)
    Integration of individual two-dimensional materials into heterostructures is a crucial step which enables development of new and technologically interesting functional systems of reduced dimensionality. Here, well-defined lateral heterostructures of hexagonal boron nitride and graphene are synthesized on Ir(1 1 1) by performing sequential chemical vapor deposition from borazine and ethylene in ultra-high vacuum. Low-energy electron microscopy (LEEM) and selected-area electron diffraction (μ-LEED) show that the heterostructures do not consist only of hexagonal boron nitride (an insulator) and graphene (a conductor), but that also a 2D alloy made up of B, C, and N atoms (a semiconductor) is formed. Composition and spatial extension of the alloy can be tuned by controlling the parameters of the synthesis. A new method for in situ fabrication of micro and nanostructures based on decomposition of hexagonal boron nitride is experimentally demonstrated and modeled analytically, which establishes a new route for production of BCN and graphene elements of various shapes. In this way, atomically-thin conducting and semiconducting components can be fabricated, serving as a basis for manufacturing more complex devices. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.06.057
  • 2018 • 142 Observation of the generation of multiple electron beams during a single sheath expansion phase in capacitive RF plasmas
    Berger, B. and You, K. and Lee, H.-C. and Mussenbrock, T. and Awakowicz, P. and Schulze, J.
    Plasma Sources Science and Technology 27 (2018)
    The fundamental investigation of different electron heating modes is important in order to fully understand the generation of plasmas, as well as to optimize their technological applications. In this study, a capacitively coupled radio-frequency discharge is operated at its limit of comparably low plasma density. Phase resolved optical emission spectroscopy provides insights into the electron dynamics on a nanosecond time scale under these conditions. At low applied voltage amplitudes, it is observed that more than one electron beam is generated within a single phase of sheath expansion at a given electrode. When the voltage amplitude is increased these beams merge in time to a single electron beam. This effect has been predicted by particle in cell/Monte-Carlo collision simulations before and contradicts existing models that assume the generation of a single beam per sheath expansion phase by stochastic heating (Wilczek et al 2015 Plasma Sources Sci. Technol. 24 024002; Wilczek et al 2016 Phys. Plasmas 23 063514). In this study, results from a systematic experimental study of the effect are presented, which support the theoretically predicted phenomenon. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aaefc7
  • 2018 • 141 On the growth mechanisms of polar (100) surfaces of ceria on copper (100)
    Hackl, J. and Duchoň, T. and Gottlob, D.M. and Cramm, S. and Veltruská, K. and Matolín, V. and Nemšák, S. and Schneider, C.M.
    Surface Science 671 1-5 (2018)
    We present a study of temperature dependent growth of nano-sized ceria islands on a Cu (100) substrate. Low-energy electron microscopy, micro-electron diffraction, X-ray absorption spectroscopy, and photoemission electron microscopy are used to determine the morphology, shape, chemical state, and crystal structure of the grown islands. Utilizing real-time observation capabilities, we reveal a three-way interaction between the ceria, substrate, and local oxygen chemical potential. The interaction manifests in the reorientation of terrace boundaries on the Cu (100) substrate, characteristic of the transition between oxidized and metallic surface. The reorientation is initiated at nucleation sites of ceria islands, whose growth direction is influenced by the proximity of the terrace boundaries. The grown ceria islands were identified as fully stoichiometric CeO2 (100) surfaces with a (2 × 2) reconstruction. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2018.01.008
  • 2018 • 140 Optical properties of In2O3 from experiment and first-principles theory: Influence of lattice screening
    Schleife, A. and Neumann, M.D. and Esser, N. and Galazka, Z. and Gottwald, A. and Nixdorf, J. and Goldhahn, R. and Feneberg, M.
    New Journal of Physics 20 (2018)
    The framework of many-body perturbation theory led to deep insight into electronic structure and optical properties of diverse systems and, in particular, many semiconductors. It relies on an accurate approximation of the screened Coulomb electron-electron interaction W, that in current implementations is usually achieved by describing electronic interband transitions. However, our results for several oxide semiconductors indicate that for polar materials it is necessary to also account for lattice contributions to dielectric screening. To clarify this question in this work, we combine highly accurate experimentation and cutting-edge theoretical spectroscopy to elucidate the interplay of quasiparticle and excitonic effects for cubic bixbyite In2O3 across an unprecedentedly large photon energy range. We then show that the agreement between experiment and theory is excellent and, thus, validate that the physics of quasiparticle and excitonic effects is described accurately by these first-principles techniques, except for the immediate vicinity of the absorption onset. Finally, our combination of experimental and computational data clearly establishes the need for including a lattice contribution to dielectric screening in the screened electron-electron interaction, in order to improve the description of excitonic effects near the absorption edge. © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aabeb0
  • 2018 • 139 Optically excited structural transition in atomic wires on surfaces at the quantum limit: A femtosecond ultrafast surface electron diffraction study
    Horn-von Hoegen, M
    Proceedings of SPIE - The International Society for Optical Engineering 10673 (2018)
    Ultrafast electron diffraction is employed for study of structural dynamics at surfaces in the time domain. Experiments are performed in a pump probe setup with fs-laser excitation and subsequent probing through diffraction of a fs electron pulse at a temporal resolution of 350 fs. The system of interest is one atomic layer of indium atoms on a Si(111) surface: Through self-assembly In atomic wires and form which exhibits a Peierls-like, insulator to metal phase transition which can be driven non-thermally through a femtosecond-laser pulse. Through the transient intensity of the diffraction spots we observe the lifting of the Peierls transition and melting of a charge density wave in only 700 fs, heating of the surface in 6 ps, and formation of a metastable and supercooled phase which exists for nanoseconds. © 2018 SPIE.
    view abstractdoi: 10.1117/12.2312239
  • 2018 • 138 Surface structure modification of single crystal graphite after slow, highly charged ion irradiation
    Alzaher, I. and Akcöltekin, S. and Ban-d'Etat, B. and Manil, B. and Dey, K.R. and Been, T. and Boduch, P. and Rothard, H. and Schleberger, M. and Lebius, H.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 420 23-26 (2018)
    Single crystal graphite was irradiated by slow, highly charged ions. The modification of the surface structure was studied by means of Low-Energy Electron Diffraction. The observed damage cross section increases with the potential energy, i.e. the charge state of the incident ion, at a constant kinetic energy. The potential energy is more efficient for the damage production than the kinetic energy by more than a factor of twenty. Comparison with earlier results hints to a strong link between early electron creation and later target atom rearrangement. With increasing ion fluence, the initially large-scale single crystal is first transformed into μm-sized crystals, before complete amorphisation takes place. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.nimb.2018.01.024
  • 2017 • 137 A linear triple quantum dot system in isolated configuration
    Flentje, H. and Bertrand, B. and Mortemousque, P.-A. and Thiney, V. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Applied Physics Letters 110 (2017)
    The scaling up of electron spin qubit based nanocircuits has remained challenging up till date and involves the development of efficient charge control strategies. Here, we report on the experimental realization of a linear triple quantum dot in a regime isolated from the reservoir. We show how this regime can be reached with a fixed number of electrons. Charge stability diagrams of the one, two, and three electron configurations where only electron exchange between the dots is allowed are observed. They are modeled with the established theory based on a capacitive model of the dot systems. The advantages of the isolated regime with respect to experimental realizations of quantum simulators and qubits are discussed. We envision that the results presented here will make more manipulation schemes for existing qubit implementations possible and will ultimately allow to increase the number of tunnel coupled quantum dots which can be simultaneously controlled. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4984745
  • 2017 • 136 Dynamic Evolution from Negative to Positive Photocharging in Colloidal CdS Quantum Dots
    Feng, D. and Yakovlev, D.R. and Pavlov, V.V. and Rodina, A.V. and Shornikova, E.V. and Mund, J. and Bayer, M.
    Nano Letters 17 2844-2851 (2017)
    The optical properties of colloidal semiconductor nanocrystals are largely influenced by the trapping of charge carriers on the nanocrystal surface. Different concentrations of electron and hole traps and different rates of their capture to the traps provide dynamical charging of otherwise neutral nanocrystals. We study the photocharging formation and evolution dynamics in CdS colloidal quantum dots with native oleic acid surface ligands. A time-resolved technique with three laser pulses (pump, orientation, and probe) is developed to monitor the photocharging dynamics with picosecond resolution on wide time scales ranging from picoseconds to milliseconds. The detection is based on measuring the coherent spin dynamics of electrons, allowing us to distinguish the type of carrier in the QD core (electron or hole). We find that although initially negative photocharging happens because of fast hole trapping, it eventually evolves to positive photocharging due to electron trapping and hole detrapping. The positive photocharging lasts up to hundreds of microseconds at room temperature. These findings give insight into the photocharging process and provide valuable information for understanding the mechanisms responsible for the emission blinking in colloidal nanostructures. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b05305
  • 2017 • 135 Electron transport in stepped Bi2Se3 thin films
    Bauer, S. and Bobisch, C.A.
    Journal of Physics Condensed Matter 29 (2017)
    We analyse the electron transport in a 16 quintuple layer thick stepped Bi2Se3 film grown on Si(1 1 1) by means of scanning tunnelling potentiometry (STP) and multi-point probe measurements. Scanning tunnelling microscopy images reveal that the local structure of the Bi2Se3 film is dominated by terrace steps and domain boundaries. From a microscopic study on the nm scale by STP, we find a mostly linear gradient of the voltage on the Bi2Se3 terraces which is interrupted by voltage drops at the position of the domain boundaries. The voltage drops indicate that the domain boundaries are scatterers for the electron transport. Macroscopic resistance measurements (2PP and in-line 4PP measurement) on the μm scale support the microscopic results. An additional rotational square 4PP measurement shows an electrical anisotropy of the sheet conductance parallel and perpendicular to the Bi2Se3 steps of about 10%. This is a result of the anisotropic step distribution at the stepped Bi2Se3 surface while domain boundaries are distributed isotropically. The determined value of the conductivity of the Bi2Se3 steps of about 1000 S cm-1 verifies the value of an earlier STP study. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa7a3c
  • 2017 • 134 Electronic structure of metastable bcc Cu-Cr alloy thin films: Comparison of electron energy-loss spectroscopy and first-principles calculations
    Liebscher, C.H. and Freysoldt, C. and Dennenwaldt, T. and Harzer, T.P. and Dehm, G.
    Ultramicroscopy 178 96-104 (2017)
    Metastable Cu-Cr alloy thin films with nominal thickness of 300nm and composition of Cu67Cr33 (at%) are obtained by co-evaporation using molecular beam epitaxy. The microstructure, chemical phase separation and electronic structure are investigated by transmission electron microscopy (TEM). The thin film adopts the body-centered cubic crystal structure and consists of columnar grains with ~50nm diameter. Aberration-corrected scanning TEM in combination with energy dispersive X-ray spectroscopy confirms compositional fluctuations within the grains. Cu- and Cr-rich domains with composition of Cu85Cr15 (at%) and Cu42Cr58 (at%) and domain size of 1-5nm are observed. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for (110)-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by electron energy loss spectroscopy (EELS) and is contrasted to an fcc-Cu reference sample. The experimental EEL spectra are compared to spectra computed by density functional theory. The main differences between bcc-and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L3-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10at% Cr confirm the experimental observations. The calculated spectrum for a perfect Cu50Cr50 (at%) random structure shows a shift in the van Hove singularity towards higher energy by developing a Cu-Cr d-band that lies between the delocalized d-bands of Cu and Cr. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2016.07.011
  • 2017 • 133 High-resolution two-dimensional optical spectroscopy of electron spins
    Salewski, M. and Poltavtsev, S.V. and Yugova, I.A. and Karczewski, G. and Wiater, M. and Wojtowicz, T. and Yakovlev, D.R. and Akimov, I.A. and Meier, T. and Bayer, M.
    Physical Review X 7 (2017)
    Multidimensional coherent optical spectroscopy is one of the most powerful tools for investigating complex quantum mechanical systems. While it was conceived decades ago in magnetic resonance spectroscopy using microwaves and radio waves, it has recently been extended into the visible and UV spectral range. However, resolving MHz energy splittings with ultrashort laser pulses still remains a challenge. Here, we analyze two-dimensional Fourier spectra for resonant optical excitation of resident electrons to localized trions or donor-bound excitons in semiconductor nanostructures subject to a transverse magnetic field. Particular attention is devoted to Raman coherence spectra, which allow one to accurately evaluate tiny splittings of the electron ground state and to determine the relaxation times in the electron spin ensemble. A stimulated steplike Raman process induced by a sequence of two laser pulses creates a coherent superposition of the ground-state doublet which can be retrieved only optically because of selective excitation of the same subensemble with a third pulse. This provides the unique opportunity to distinguish between different complexes that are closely spaced in energy in an ensemble. The related experimental demonstration is based on photon-echo measurements in an n-type CdTe=ðCd; MgÞTe quantum-well structure detected by a heterodyne technique. The difference in the sub-μeV range between the Zeeman splittings of donor-bound electrons and electrons localized at potential fluctuations can be resolved even though the homogeneous linewidth of the optical transitions is larger by 2 orders of magnitude.
    view abstractdoi: 10.1103/PhysRevX.7.031030
  • 2017 • 132 Identification and separation of rectifier mechanisms in Si/SiGe ballistic cross junctions
    Von Pock, J.F. and Salloch, D. and Wieser, U. and Hackbarth, T. and Kunze, U.
    Journal of Applied Physics 121 (2017)
    Depending on the detailed geometry, gate voltage, and circuitry, nanoscale Si/SiGe cross junctions at low temperatures exhibit full-wave rectification arising from different mechanisms like change in the number of current-carrying modes, stationary ballistic charging of a current-free voltage lead, and hot-electron thermopower. We study the rectifier structures on high-mobility Si/SiGe heterostructures consisting of a straight voltage stem and oblique current-injecting leads. Local gate electrodes are used to control the electron density in the voltage or current channel. Compared to three-terminal Y-branch junctions, the four-terminal cross junction eliminates the mode effect. A gradual increase of output voltage as gate-voltage is reduced until threshold voltage is identified as contribution of hot-electron thermopower. Heating the initially cold reservoir from a second orthogonal cross junction eliminates the electron temperature gradient and suppresses the thermopower. Even if the operation as six-terminal device re-induces a mode-controlled contribution, we demonstrate that it is negligible. As expected, the ballistic signal can be reliably separated from other mechanisms by measurements under positive gate voltage. The ballistic voltage can be described by a parabolic function of the injected current and is proportional to the cosine of the injection angle. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4973279
  • 2017 • 131 Imaging the Nonlinear Plasmoemission Dynamics of Electrons from Strong Plasmonic Fields
    Podbiel, D. and Kahl, P. and Makris, A. and Frank, B. and Sindermann, S. and Davis, T.J. and Giessen, H. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    Nano Letters 17 6569-6574 (2017)
    We use subcycle time-resolved photoemission microscopy to unambiguously distinguish optically triggered electron emission (photoemission) from effects caused purely by the plasmonic field (termed "plasmoemission"). We find from time-resolved imaging that nonlinear plasmoemission is dominated by the transverse plasmon field component by utilizing a transient standing wave from two counter-propagating plasmon pulses of opposite transverse spin. From plasmonic foci on flat metal surfaces, we observe highly nonlinear plasmoemission up to the fifth power of intensity and quantized energy transfer, which reflects the quantum-mechanical nature of surface plasmons. Our work constitutes the basis for novel plasmonic devices such as nanometer-confined ultrafast electron sources as well as applications in time-resolved electron microscopy. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b02235
  • 2017 • 130 Interfacial Electrochemistry in Liquids Probed with Photoemission Electron Microscopy
    Nemšák, S. and Strelcov, E. and Duchoň, T. and Guo, H. and Hackl, J. and Yulaev, A. and Vlassiouk, I. and Mueller, D.N. and Schneider, C.M. and Kolmakov, A.
    Journal of the American Chemical Society 139 18138-18141 (2017)
    Studies of the electrified solid-liquid interfaces are crucial for understanding biological and electrochemical systems. Until recently, use of photoemission electron microscopy (PEEM) for such purposes has been hampered by incompatibility of the liquid samples with ultrahigh vacuum environment of the electron optics and detector. Here we demonstrate that the use of ultrathin electron transparent graphene membranes, which can sustain large pressure differentials and act as a working electrode, makes it possible to probe electrochemical reactions in operando in liquid environments with PEEM. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b07365
  • 2017 • 129 Interrogation of a PS1-Based Photocathode by Means of Scanning Photoelectrochemical Microscopy
    Zhao, F. and Plumeré, N. and Nowaczyk, M.M. and Ruff, A. and Schuhmann, W. and Conzuelo, F.
    Small 13 (2017)
    In the development of photosystem-based energy conversion devices, the in-depth understanding of electron transfer processes involved in photocurrent generation and possible charge recombination is essential as a basis for the development of photo-bioelectrochemical architectures with increased efficiency. The evaluation of a bio-photocathode based on photosystem 1 (PS1) integrated within a redox hydrogel by means of scanning photoelectrochemical microscopy (SPECM) is reported. The redox polymer acts as a conducting matrix for the transfer of electrons from the electrode surface to the photo-oxidized P700 centers within PS1, while methyl viologen is used as charge carrier for the collection of electrons at the reduced FB site of PS1. The analysis of the modified surfaces by SPECM enables the evaluation of electron-transfer processes by simultaneously monitoring photocurrent generation at the bio-photoelectrode and the associated generation of reduced charge carriers. The possibility to visualize charge recombination processes is illustrated by using two different electrode materials, namely Au and p-doped Si, exhibiting substantially different electron transfer kinetics for the reoxidation of the methyl viologen radical cation used as freely diffusing charge carrier. In the case of p-doped Si, a slower recombination kinetics allows visualization of methyl viologen radical cation concentration profiles from SPECM approach curves. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201604093
  • 2017 • 128 In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy
    Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 695 1583-1590 (2017)
    The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.10.302
  • 2017 • 127 Microanalysis of single-layer hexagonal boron nitride islands on Ir(111)
    Petrović, M. and Hagemann, U. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    Applied Surface Science 420 504-510 (2017)
    Large hexagonal boron nitride (hBN) single-layer islands of high crystalline quality were grown on Ir(111) via chemical vapor deposition (CVD) and have been studied with low-energy electron microscopy (LEEM). Two types of hBN islands have been observed that structurally differ in their shape and orientation with respect to iridium, where the former greatly depends on the iridium step morphology. Photoemission electron microscopy (PEEM) and IV-LEEM spectroscopy revealed that the two island types also exhibit different work functions and bindings to iridium, which provides an explanation for differences in their shape and growth modes. In addition, various temperatures were used for the CVD synthesis of hBN, and it was found that at temperatures higher than ≈950 °C boron atoms, originating either from decomposed borazine molecules or disintegrated hBN islands, can form additional compact reconstructed regions. The presented results are important for advancement in synthesis of high-quality hBN and other boron-based layered materials, and could therefore expedite their technological implementation. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.05.155
  • 2017 • 126 Nanostructure of and structural defects in a Mo2BC hard coating investigated by transmission electron microscopy and atom probe tomography
    Gleich, S. and Fager, H. and Bolvardi, H. and Achenbach, J.-O. and Soler, R. and Pradeep, K.G. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Journal of Applied Physics 122 (2017)
    In this work, the nanostructure of a Mo2BC hard coating was determined by several transmission electron microscopy methods and correlated with the mechanical properties. The coating was deposited on a Si (100) wafer by bipolar pulsed direct current magnetron sputtering from a Mo2BC compound target in Ar at a substrate temperature of 630 °C. Transmission electron microscopy investigations revealed structural features at various length scales: bundles (30 nm to networks of several micrometers) consisting of columnar grains (∼10 nm in diameter), grain boundary regions with a less ordered atomic arrangement, and defects including disordered clusters (∼1.5 nm in diameter) as well as stacking faults within the grains. The most prominent defect with a volume fraction of ∼0.5% is the disordered clusters, which were investigated in detail by electron energy loss spectroscopy and atom probe tomography. The results provide conclusive evidence that Ar is incorporated into the Mo2BC film as disordered Ar-rich Mo-B-C clusters of approximately 1.5 nm in diameter. Hardness values of 28 ± 1 GPa were obtained by nanoindentation tests. The Young's modulus of the Mo2BC coating exhibits a value of 462 ± 9 GPa, which is consistent with ab initio calculations for crystalline and defect free Mo2BC and measurements of combinatorically deposited Mo2BC thin films at a substrate temperature of 900 °C. We conclude that a reduction of the substrate temperature of 270 °C has no significant influence on hardness and Young's modulus of the Mo2BC hard coating, even if its nanostructure exhibits defects. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4999304
  • 2017 • 125 Observations of Surface Mode Influence on Plasma Uniformity in PIC/MCC Simulations of Large Capacitive Discharges
    Eremin, D. and Brinkmann, R.P. and Mussenbrock, T.
    Plasma Processes and Polymers 14 (2017)
    Capacitively coupled plasmas with large electrodes, driven at high frequencies, exhibit new physics compared to small scale CCP devices or at low frequencies. This is due to excitation of two types of surface modes which arise as a result of interaction between the bulk plasma and the plasma sheaths separating the plasma from electrodes. Based on the physical effects that these modes cause, they are labeled as “self-bias” (SB) and “plasma-series resonance” (PSR) modes. Results of electrostatic 2d3v PIC/MCC simulations for a model geometry are used to selectively study the SB modes and demonstrate that they lead to non-uniformities of the plasma density profile owing to the influence of the SB modes on the heating of high- and low-energy electrons. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ppap.201600164
  • 2017 • 124 Probing the electron density in HiPIMS plasmas by target inserts
    Hecimovic, A. and Held, J. and Schulz-Von Der Gathen, V. and Breilmann, W. and Maszl, C. and Von Keudell, A.
    Journal of Physics D: Applied Physics 50 (2017)
    High power impulse magnetron sputtering (HiPIMS) is a versatile technology to deposit thin films with superior properties. During HiPIMS, the power is applied in short pulses of the order of 100 μs at power densities of kW to a magnetron target creating a torus shaped dynamic high density plasma. This plasma torus is not homogeneous, but individual ionization zones become visible, which rotate along the torus with velocities of 10 km . Up to now, however, any direct measurement of the electron density inside these rotating ionization zones is missing. Here, we probe the electron density by measuring the target current locally by using small inserts embedded in an aluminium target facing the plasma torus. By applying simple sheath theory, a plasma density of the order of at the sheath edge can be inferred. The plasma density increases with increasing target current. In addition, the dynamics of the local target current variation is consistent with the dynamics of the traveling ionization zone causing a modulation of the local current density by 25%. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa9914
  • 2017 • 123 Proton-Coupled Reduction of the Catalytic [4Fe-4S] Cluster in [FeFe]-Hydrogenases
    Senger, M. and Laun, K. and Wittkamp, F. and Duan, J. and Haumann, M. and Happe, T. and Winkler, M. and Apfel, U.-P. and Stripp, S.T.
    Angewandte Chemie - International Edition 56 16503-16506 (2017)
    In nature, [FeFe]-hydrogenases catalyze the uptake and release of molecular hydrogen (H2) at a unique iron-sulfur cofactor. The absence of an electrochemical overpotential in the H2 release reaction makes [FeFe]-hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one-electron reduction of [FeFe]-hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton-coupled electron transport during the formation of the reduced state Hred′. Charge compensation stabilizes the excess electron at the [4Fe-4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred′ in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201709910
  • 2017 • 122 Room-temperature electron spin dynamics of Ce3+ ions in a YAG crystal
    Liang, P. and Hu, R.R. and Chen, C. and Belykh, V.V. and Jia, T.Q. and Sun, Z.R. and Feng, D.H. and Yakovlev, D.R. and Bayer, M.
    Applied Physics Letters 110 (2017)
    Circularly polarized optical excitation generates electron spin polarization in the lowest 5d state of rare-earth Ce3+ ions in a YAG crystal. The 5d electron spin dynamics is investigated in transverse and longitudinal magnetic fields by time-resolved pump-probe Faraday rotation. Long lived electron spin coherence with a dephasing time of 2.5 ns is found at room temperature. In a transverse magnetic field of 1 T, the electron spin coherence shows a distinct beating-like amplitude modulation due to several slightly different Larmor frequencies corresponding to different electron g factors of magnetically inequivalent positions of the Ce3+ ions in the crystal lattice. Hyperfine coupling between the 5d electron of Ce3+ ions and environmental nuclear spins dominates the spin relaxation, which can be efficiently suppressed by a longitudinal magnetic field as small as 10 mT. The dependence of electron spin relaxation on both the transverse and longitudinal magnetic fields agrees well with the one predicted theoretically for the hyperfine coupling mechanism. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4984232
  • 2017 • 121 The effect of realistic heavy particle induced secondary electron emission coefficients on the electron power absorption dynamics in single- and dual-frequency capacitively coupled plasmas
    Daksha, M. and Derzsi, A. and Wilczek, S. and Trieschmann, J. and Mussenbrock, T. and Awakowicz, P. and Donkó, Z. and Schulze, J.
    Plasma Sources Science and Technology 26 (2017)
    doi: 10.1088/1361-6595/aa7c88
  • 2017 • 120 Ultrashort high-brightness pulses from storage rings
    Khan, S.
    Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 865 95-98 (2017)
    The brightness of short-wavelength radiation from accelerator-based sources can be increased by coherent emission in which the radiation intensity scales with the number of contributing electrons squared. This requires a microbunched longitudinal electron distribution, which is the case in free-electron lasers. The brightness of light sources based on electron storage rings was steadily improved, but could profit further from coherent emission. The modulation of the electron energy by a continuous-wave laser field may provide steady-state microbunching in the infrared regime. For shorter wavelengths, the energy modulation can be converted into a temporary density modulation by a dispersive chicane. One particular goal is coherent emission from a very short slice within an electron bunch in order to produce ultrashort radiation pulses with high brightness. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.nima.2016.07.048
  • 2017 • 119 Vibrational Density Matrix Renormalization Group
    Baiardi, A. and Stein, C.J. and Barone, V. and Reiher, M.
    Journal of Chemical Theory and Computation 13 3764-3777 (2017)
    Variational approaches for the calculation of vibrational wave functions and energies are a natural route to obtain highly accurate results with controllable errors. Here, we demonstrate how the density matrix renormalization group (DMRG) can be exploited to optimize vibrational wave functions (vDMRG) expressed as matrix product states. We study the convergence of these calculations with respect to the size of the local basis of each mode, the number of renormalized block states, and the number of DMRG sweeps required. We demonstrate the high accuracy achieved by vDMRG for small molecules that were intensively studied in the literature. We then proceed to show that the complete fingerprint region of the sarcosyn-glycin dipeptide can be calculated with vDMRG. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jctc.7b00329
  • 2016 • 118 A combined STM and SPA-LEED study of the "explosive" nucleation and collective diffusion in Pb/Si(111)
    Hattab, H. and Hupalo, M. and Hershberger, M.T. and Horn-von Hoegen, M. and Tringides, M.C.
    Surface Science 646 50-55 (2016)
    A novel type of very fast nucleation was recently found in Pb/Si(111) with 4- to 7-layer high islands becoming crystalline in an "explosive" way, when the Pb deposited amount in the wetting layer is compressed to θc ~ 1.22 ML, well above the metallic Pb(111) density. This "explosive" nucleation is very different from classical nucleation when island growth is more gradual and islands grow in size by single adatom aggregation [8]. In order to identify the key parameters that control the nucleation we used scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). It was found that the number and duration of steps in iterative deposition used to approach θc and the flux rate have dramatic effects on the crystallization process. Larger depositions over shorter times induce greater spatial coverage fluctuations, so local areas can reach the critical coverage θc easier. This can trigger the collective motion of the wetting layer from far away to build the Pb islands "explosively". The SPA-LEED experiments show that even low flux experiments in iterative deposition experiments can trigger transfer of material to the superstable 7-layer islands, as seen from the stronger satellite rings close to the (00) spot. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2015.08.017
  • 2016 • 117 A new setup for the investigation of swift heavy ion induced particle emission and surface modifications
    Meinerzhagen, F. and Breuer, L. and Bukowska, H. and Bender, M. and Severin, D. and Herder, M. and Lebius, H. and Schleberger, M. and Wucher, A.
    Review of Scientific Instruments 87 (2016)
    The irradiation with fast ions with kinetic energies of >10 MeV leads to the deposition of a high amount of energy along their trajectory (up to several ten keV/nm). The energy is mainly transferred to the electronic subsystem and induces different secondary processes of excitations, which result in significant material modifications. A new setup to study these ion induced effects on surfaces will be described in this paper. The setup combines a variable irradiation chamber with different techniques of surface characterizations like scanning probe microscopy, time-of-flight secondary ion, and neutral mass spectrometry, as well as low energy electron diffraction under ultra high vacuum conditions, and is mounted at a beamline of the universal linear accelerator (UNILAC) of the GSI facility in Darmstadt, Germany. Here, samples can be irradiated with high-energy ions with a total kinetic energy up to several GeVs under different angles of incidence. Our setup enables the preparation and in situ analysis of different types of sample systems ranging from metals to insulators. Time-of-flight secondary ion mass spectrometry enables us to study the chemical composition of the surface, while scanning probe microscopy allows a detailed view into the local electrical and morphological conditions of the sample surface down to atomic scales. With the new setup, particle emission during irradiation as well as persistent modifications of the surface after irradiation can thus be studied. We present first data obtained with the new setup, including a novel measuring protocol for time-of-flight mass spectrometry with the GSI UNILAC accelerator. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4939899
  • 2016 • 116 Access to long-term optical memories using photon echoes retrieved from electron spins in semiconductor quantum wells
    Poltavtsev, S.V. and Langer, L. and Yugova, I.A. and Salewski, M. and Kapitonov, Y.V. and Yakovlev, D.R. and Karczewski, G. and Wojtowicz, T. and Akimov, I.A. and Bayer, M.
    Proceedings of SPIE - The International Society for Optical Engineering 9931 (2016)
    We use spontaneous (two-pulse) and stimulated (three-pulse) photon echoes for studying the coherent evolution of optically excited ensemble of trions which are localized in semiconductor CdTe/CdMgTe quantum well. Application of transverse magnetic field leads to the Larmor precession of the resident electron spins, which shuffles optically induced polarization between optically accessible and inaccessible states. This results in several spectacular phenomena. First, magnetic field induces oscillations of spontaneous photon echo amplitude. Second, in three-pulse excitation scheme, the photon echo decay is extended by several orders of magnitude. In this study, short-lived optical excitation which is created by the first pulse is coherently transferred into a long-lived electron spin state using the second optical pulse. This coherent spin state of electron ensemble persists much longer than any optical excitation in the system, preserving information on initial optical field, which can be retrieved as a photon echo by means of third optical pulse. © 2016 SPIE.
    view abstractdoi: 10.1117/12.2237360
  • 2016 • 115 Beam-induced atomic migration at Ag-containing nanofacets at an asymmetric Cu grain boundary
    Peter, N.J. and Liebscher, C.H. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Research 32 968-982 (2016)
    Besides the high spatial resolution achieved in aberration-corrected scanning transmission microscopy, beam-induced dynamic effects have to be considered for quantitative chemical characterization on the level of single atomic columns. The present study investigates the influence of imaging conditions in an aberration-corrected scanning transmission electron microscope on the beam-induced atomic migration at a complex Ag-segregated, nanofaceted Cu grain boundary. Three distinct imaging conditions including static single image and serial image acquisition have been utilized. Chemical information on the Ag column occupation of single atomic columns at the grain boundary was extracted by the evolution of peak intensity ratios and compared to idealized scanning transmission electron microscopy image simulations. The atomic column occupation is underestimated when using conventional single frame acquisition due to an averaging of Ag atomic migration events during acquisition. Possible migration paths for the beam-induced atomic motion at a complex Cu grain boundary are presented. Copyright © Materials Research Society 2016
    view abstractdoi: 10.1557/jmr.2016.398
  • 2016 • 114 Coherent electron-spin-resonance manipulation of three individual spins in a triple quantum dot
    Noiri, A. and Yoneda, J. and Nakajima, T. and Otsuka, T. and Delbecq, M.R. and Takeda, K. and Amaha, S. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Applied Physics Letters 108 (2016)
    Quantum dot arrays provide a promising platform for quantum information processing. For universal quantum simulation and computation, one central issue is to demonstrate the exhaustive controllability of quantum states. Here, we report the addressable manipulation of three single electron spins in a triple quantum dot using a technique combining electron-spin-resonance and a micro-magnet. The micro-magnet makes the local Zeeman field difference between neighboring spins much larger than the nuclear field fluctuation, which ensures the addressable driving of electron-spin-resonance by shifting the resonance condition for each spin. We observe distinct coherent Rabi oscillations for three spins in a semiconductor triple quantum dot with up to 25 MHz spin rotation frequencies. This individual manipulation over three spins enables us to arbitrarily change the magnetic spin quantum number of the three spin system, and thus to operate a triple-dot device as a three-qubit system in combination with the existing technique of exchange operations among three spins. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4945592
  • 2016 • 113 Doping dependence and electron-boson coupling in the ultrafast relaxation of hot electron populations in Ba(Fe1-xCox)2As2
    Avigo, I. and Thirupathaiah, S. and Ligges, M. and Wolf, T. and Fink, J. and Bovensiepen, U.
    New Journal of Physics 18 (2016)
    Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in Ba(Fe1-xCox)2As2 in a range of 0 ≤ x < 0.15 at temperatures slightly above the Nel temperature. By analyzing the time-dependent photoemission intensity of the pump laser excited population as a function of energy, we found that the relaxation times at 0 < E - EF < 0.2 eV are doping dependent and about 100 fs shorter at optimal doping than for overdoped and parent compounds. Analysis of the relaxation rates also reveals the presence of a pump fluence dependent step in the relaxation time at E - EF = 200 meV whichwe explain by coupling of the excited electronic system to a boson of this energy.Wecompare our results with static ARPES and transport measurements and find disagreement and agreement concerning the doping dependence, respectively. We discuss the effect of the electron-boson coupling on the energy dependent relaxation and assign the origin of the boson to a magnetic excitation.
    view abstractdoi: 10.1088/1367-2630/18/9/093028
  • 2016 • 112 Electron heating in technological plasmas
    Schulze, J. and Mussenbrock, T.
    Plasma Sources Science and Technology 25 (2016)
    The Special Issue of Plasma Sources Science and Technology is devoted to merging the state of knowledge and gather new ideas from theory and experiment. It aims to bridge the gap between fundamental research and application. The old, yet unresolved, issue of collisionless electron heating in capacitive RF low pressure plasmas is discussed computationally by Lafleur and Chabert. They find that the level of true collisionless/stochastic electron heating is small under typical operating conditions. Franek and colleagues focus on diagnostics aspects by applying Optical Emission Spectroscopy (OES) to correlate metastable densities, reduced electric fields, and EEDFs. Schweigert and colleagues analyze plasma breakdown in high voltage open discharges. Gosh and colleagues and Siddiqui and colleagues discuss electron heating in helicon plasma sources with and without microwave assistance. Campanell and colleagues introduce a fundamentally new effect of the self-amplification of electrons emitted from surfaces in plasmas with E?B fields, while Tian and Kushner discuss the control and relevance of VUV photon fluxes in ICPs.
    view abstractdoi: 10.1088/0963-0252/25/2/020401
  • 2016 • 111 Heat flow, transport and fluctuations in etched semiconductor quantum wire structures
    Riha, C. and Chiatti, O. and Buchholz, S.S. and Reuter, D. and Wieck, A.D. and Fischer, S.F.
    Physica Status Solidi (A) Applications and Materials Science 213 571-581 (2016)
    Low-dimensional transport in semiconductor meso- and nanostructures is a topical field of fundamental research with potential applications in future quantum devices. However, thermal non-equilibrium may destroy phase-coherence and remains to be explored experimentally. Here, we present effects of thermal non-equilibrium in various implementations of low-dimensional (non-interacting) electron systems, fabricated by etching AlGaAs/GaAs heterostructures. These include narrow quasi-two-dimensional (2D) channels, quasi-one-dimensional (1D) waveguide networks, quantum rings (QRs), and single 1D constrictions, such as quantum point contacts (QPCs). Thermal non-equilibrium is realized by current heating. The charge carrier temperature is determined by noise thermometry. The electrical conductance and the voltage-noise are measured with respect to bath temperatures, heating currents, thermal gradients, and electric fields. We determine and discuss heat transport processes, electron-energy loss rates, and electron-phonon interaction, and our results are consistent with the Wiedemann-Franz relation. Additionally, we show how non-thermal current fluctuations can be used to identify electric conductance anomalies due to charge states. This article presents effects of thermal non-equilibrium in various implementations of low-dimensional electron systems, fabricated by etching AlGaAs/GaAs heterostructures. Measurements of the thermal noise allow one to access electron-energy relaxation mechanisms, to investigate nanostructures as thermal contacts and to identify the paths of heat flow in branched 1D waveguide networks. Additionally, non-thermal current fluctuations can be used to identify electric conductance anomalies due to charge states. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532551
  • 2016 • 110 Heat-Induced Phase Transformation of Three-Dimensional Nb3O7(OH) Superstructures: Effect of Atmosphere and Electron Beam
    Betzler, S.B. and Harzer, T. and Ciston, J. and Dahmen, U. and Dehm, G. and Scheu, C.
    Crystal Growth and Design 16 4309-4317 (2016)
    Nanostructured niobium oxides and hydroxides are potential candidates for photochemical applications due to their excellent optical and electronic properties. In the present work the thermal stability of Nb3O7(OH) superstructures prepared by a simple hydrothermal approach is investigated at the atomic scale. Transmission electron microscopy and electron energy-loss spectroscopy provide insights into the phase transformation occurring at elevated temperatures and probe the effect of the atmospheric conditions. In the presence of oxygen, H2O is released from the crystal at temperatures above 500 °C, and the crystallographic structure changes to H-Nb2O5. In addition to the high thermal stability of Nb3O7(OH), the morphology was found to be stable, and first changes in the form of a merging of nanowires are not observed until 850 °C. Under reducing conditions in a transmission electron microscope and during electron beam bombardment, an oxygen-deficient phase is formed at temperatures above 750 °C. This transformation starts with the formation of defects in the crystal lattice at 450 °C and goes along with the formation of pores in the nanowires which accommodate the volume differences of the two crystal phases. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.6b00386
  • 2016 • 109 High resolution electron energy loss spectroscopy of spin waves in ultra-thin cobalt films
    Michel, E. and Ibach, H. and Schneider, C.M.
    Surface and Interface Analysis 48 1104-1107 (2016)
    Electron energy loss spectroscopy has successfully established itself as the experimental method to study exchange-dominated, high-momentum spin waves in ultra-thin films. Because of insufficient energy resolution, previous studies were limited to spin waves in the high energy range and to wave vectors larger than about q|| = 0.4 Å−1. In this regime, spin waves are strongly damped by decay into Stoner excitations. Furthermore, the spin wave signal of a multilayer film consists of several overlapping modes. After implementation of several technical modifications, our electron spectrometer now enables the study of spin waves with an energy resolution down to 2 meV, and thereby the discrimination of several spin wave modes in ultra-thin films as well as the study of the intrinsic width of modes down to 1 meV. Examples are presented for fcc and hcp cobalt films and discussed in terms of current theoretical models. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.6127
  • 2016 • 108 Impact of CoFe buffer layers on the structural and electronic properties of the Co2MnSi/MgO interface
    Fetzer, R. and Liu, H.-X. and Stadtmüller, B. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M.
    49 (2016)
    The latest improvement of MgO-based magnetic tunnel junctions has been achieved by the combination of CoFe buffer layers and potentially half-metallic ultrathin Co2MnSi electrodes. By this, tunnel magnetoresistance ratios of almost 2000% could be obtained. However, a complete understanding of the underlying processes leading to this enhancement is not yet given. We present a comprehensive study regarding the structural and electronic spin properties of the CoFe(30 nm)-buffered Co2MnSi(3 nm)/MgO(2 nm) buried interface identical to the one formed in actual devices. Low energy electron diffraction experiments show that the ultrathin Co2MnSi layer adopts the lattice constant of the underlying CoFe buffer layer, leading to improved structural conditions at the interface to MgO. In contrast, the Co2MnSi/MgO interface spin polarization at the Fermi level is not affected by the magnetic CoFe buffer layer, as found by interface-sensitive spin-resolved extremely low energy photoemission spectroscopy. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/19/195002
  • 2016 • 107 Importance of inclusion of the effect of s electrons into bond-order potentials for transition bcc metals with d-band mediated bonding
    Lin, Y.-S. and Mrovec, M. and Vitek, V.
    Modelling and Simulation in Materials Science and Engineering 24 (2016)
    In bond-order potentials (BOPs) for transition metals only the bonding mediated by the d electrons is included explicitly and the covalent part of the cohesive energy is evaluated using Slater-Koster dd bond integrals. However, the effect of s electrons with orbitals centered on atoms neighboring the corresponding dd bond is not necessarily negligible. As shown in Nguyen-Manh et al (2000 Phys. Rev. Lett. 85 4136) this can be taken into account via screening of the dd bond integrals. In a recent paper (Lin et al 2014 Model. Simul. Mater. Sci. Eng. 22 034002) the dd bond integrals were determined using a projection scheme utilizing atomic orbitals that give the best representation of the electronic wave functions in the calculations based on the density functional theory (DFT) (Madsen et al 2011 Phys. Rev. B 83 4119) and it was inferred that in this case the effect of s electrons was already included. In this paper we analyze this hypothesis by comparing studies employing BOPs with both unscreened and screened dd bond integrals. In all cases results are compared with calculations based on DFT and/or experiments. Studies of structures alternate to the bcc lattice, transformation paths that connect the bcc structure with fcc, simple cubic (sc), body centered tetragonal (bct) and hcp structures via continuously distorted configurations and calculations of γ-surfaces were all found to be insensitive to the screening of bond integrals. On the other hand, when the bond integrals are screened, formation energies of vacancies are improved and calculated phonon dispersion spectra reproduce the experimentally observed ones much better. Most importantly, dislocation core structure and dislocation glide are significantly different without and with screening of dd bond integrals. The latter lead to a much better agreement with available experiments. These findings suggest that the effect of s electrons on dd bonds, emulated by the screening of corresponding bond integrals, is the least significant when the lattice is distorted away from the ideal bcc structure homogeneously even if such distortion is large. On the other hand, when the distortion is local and inhomogeneous the impact of screening of the dd bond integrals is significant. In the studies presented in this paper such local inhomogeneities occur when phonons propagate through the lattice, at point defects and in the cores of dislocations. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/24/8/085001
  • 2016 • 106 Modelling of electron beam induced nanowire attraction
    Bitzer, L.A. and Speich, C. and Schäfer, D. and Erni, D. and Prost, W. and Tegude, F.J. and Benson, N. and Schmechel, R.
    Journal of Applied Physics 119 (2016)
    Scanning electron microscope (SEM) induced nanowire (NW) attraction or bundling is a well known effect, which is mainly ascribed to structural or material dependent properties. However, there have also been recent reports of electron beam induced nanowire bending by SEM imaging, which is not fully explained by the current models, especially when considering the electro-dynamic interaction between NWs. In this article, we contribute to the understanding of this phenomenon, by introducing an electro-dynamic model based on capacitor and Lorentz force interaction, where the active NW bending is stimulated by an electromagnetic force between individual wires. The model includes geometrical, electrical, and mechanical NW parameters, as well as the influence of the electron beam source parameters and is validated using in-situ observations of electron beam induced GaAs nanowire (NW) bending by SEM imaging. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4945674
  • 2016 • 105 On the Crystallography of Silver Nanoparticles with Different Shapes
    Helmlinger, J. and Prymak, O. and Loza, K. and Gocyla, M. and Heggen, M. and Epple, M.
    Crystal Growth and Design 16 3677-3687 (2016)
    The crystallographic properties of silver nanoparticles with different morphologies (two different kinds of spheres, cubes, platelets, and rods) were derived from X-ray powder diffraction and electron microscopy. The size of the metallic particle core was determined by scanning electron microscopy, and the colloidal stability and the hydrodynamic particle diameter were analyzed by dynamic light scattering. The preferred crystallographic orientation (texture) as obtained by X-ray powder diffraction, including pole figure analysis, and high resolution transmission electron microscopy showed the crystallographic nature of the spheres (almost no texture), the cubes (terminated by {100} faces), the platelets (terminated by {111} faces), and the rods (grown from pentagonal twins along [110] and terminated by {100} faces). The crystallite size was determined by Rietveld refinement of X-ray powder diffraction data and agreed well with the transmission electron microscopic data. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.6b00178
  • 2016 • 104 On the mechanism of {332} twinning in metastable β titanium alloys
    Lai, M.J. and Tasan, C.C. and Raabe, D.
    Acta Materialia 111 173-186 (2016)
    {332} twinning, an unusual twinning mode in other body-centered cubic (bcc) metals and alloys, has been demonstrated to be a fundamental deformation mode in bcc metastable β titanium alloys. Recent studies suggest that this twinning mode plays an important role in enhancing the work hardening and thus improving the mechanical properties. Here, we studied the mechanism of this twinning mode in a metastable β Ti-36Nb-2Ta-3Zr (wt.%) alloy. Tensile tests were performed to induce the formation of {332} twins. By using electron backscatter diffraction, transmission electron microscopy and in situ scanning electron microscopy, the surface-to-bulk microstructures and the initiation and propagation of {332} twins were investigated. In addition to the previously reported high densities of straight dislocations within the twin, we have observed that an α″ martensite band is present near the surface adjacent to the twin. During annealing at 900°C, the α″ martensite band transforms into the adjacent twin rather than into the matrix, indicating that {332} twin nucleates within α″ martensite. Further evidence for this is the constitution of the twin in the initial stage of its formation, where the first portion formed consists of α″ martensite. During propagation, the twins propagating to the opposite directions can merge together when their lateral boundaries impinge on each other. Based on the experimental observations, an α″-assisted twinning mechanism is proposed and the origin of the dislocations within {332} twin is discussed accordingly. © 2016 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.03.040
  • 2016 • 103 Optical Blocking of Electron Tunneling into a Single Self-Assembled Quantum Dot
    Kurzmann, A. and Merkel, B. and Labud, P.A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M. P.
    Physical Review Letters 117 (2016)
    Time-resolved resonance fluorescence (RF) is used to analyze electron tunneling between a single self-assembled quantum dot (QD) and an electron reservoir. In equilibrium, the RF intensity reflects the average electron occupation of the QD and exhibits a gate voltage dependence that is given by the Fermi distribution in the reservoir. In the time-resolved signal, however, we find that the relaxation rate for electron tunneling is, surprisingly, independent of the occupation in the charge reservoir - in contrast to results from all-electrical transport measurements. Using a master equation approach, which includes both the electron tunneling and the optical excitation or recombination, we are able to explain the experimental data by optical blocking, which also reduces the electron tunneling rate when the QD is occupied by an exciton. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.117.017401
  • 2016 • 102 Orientation of FePt nanoparticles on top of a-SiO2/Si(001), MgO(001) and sapphire(0001): Effect of thermal treatments and influence of substrate and particle size
    Schilling, M. and Ziemann, P. and Zhang, Z. and Biskupek, J. and Kaiser, U. and Wiedwald, U.
    Beilstein Journal of Nanotechnology 7 591-604 (2016)
    Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: "small" NPs with diameters in the range of 2-3 nm and "large" ones in the range of 5-8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600-650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min. © 2016 Schilling et al.
    view abstractdoi: 10.3762/bjnano.7.52
  • 2016 • 101 Preparation and structure of ultra-thin GaN (0001) layers on In0.11Ga0.89N-single quantum wells
    Alamé, S. and Quezada, A.N. and Skuridina, D. and Reich, C. and Henning, D. and Frentrup, M. and Wernicke, T. and Koslow, I. and Kneissl, M. and Esser, N. and Vogt, P.
    Materials Science in Semiconductor Processing 55 7-11 (2016)
    Multiple surface reconstructions have been observed on ultra-thin GaN (0001) layers of 1-10nm thickness, covering a 3nm thick In0.11Ga0.89N single quantum well in a GaN matrix. Low energy electron diffraction patterns show (2×2) and (√3×√3)-R30° symmetries for samples annealed in nitrogen plasma, and (2×2), (3×3), (4×4), and (6×6) symmetries for samples overgrown with an additional monolayer-thin GaN film by molecular beam epitaxy under Ga-rich growth conditions. Photoelectron spectroscopy shows that the InGaN quantum wells and capping layers are stable for growth temperatures up to 760°C, and do not show formation of indium or gallium droplets on the surface. The photoluminescence emission from the buried InGaN SQWs remains unchanged by the preparation process, demonstrating that the SQWs do not undergo any significant modification. © 2016.
    view abstractdoi: 10.1016/j.mssp.2016.02.013
  • 2016 • 100 Revising secondary electron yields of ion-sputtered metal oxides
    Corbella, C. and Marcak, A. and de los Arcos, T. and von Keudell, A.
    Journal of Physics D-applied Physics 49 16LT01 (2016)
    The emission of secondary electrons (SE) during sputtering of Al and Ti foils by argon ions in an oxygen background has been measured in a particle beam reactor equipped with a SE-collector. This experiment mimics the process of reactive magnetron sputtering. Quantified beams of argon ions with energies between 500 eV and 2000 eV were employed, while simultaneously molecular oxygen fluxes impinged on the surface and caused oxidation. The measured secondary electron emission coefficients (gamma) ranged from approximately 0.1 (for clean aluminium and titanium) to 1.2 and 0.6 (in the case of aluminium oxide and titanium oxide, respectively). The increase of gamma is compared to SE measurements based on the modelling of magnetron plasmas. Moreover, the energy distributions of the emitted SE have been measured by varying the retarding potential of the SE-collector, which allows the monitoring of the oxidation state from the position of the Auger peaks. The origin of the observed SE yields based on the emission of low-and high-energy electrons generated on the oxide surface is discussed.
    view abstractdoi: 10.1088/0022-3727/49/16/16LT01
  • 2016 • 99 Thermal shift of the resonance between an electron gas and quantum dots: What is the origin?
    Brinks, F. and Wieck, A.D. and Ludwig, Ar.
    New Journal of Physics 18 (2016)
    The operation of quantum dots (QDs) at highest possible temperatures is desirable for many applications. Capacitance-voltage spectroscopy (C(V)-spectroscopy) measurements are an established instrument to analyse the electronic structure and energy levels of self-assembled QDs. We perform C(V) in the dark and C(V) under the influence of non-resonant illumination, probing exciton states up to X4+ on InAs QDs embedded in a GaAs matrix for temperatures ranging from 2.5 to 120 K. While a small shift in the charging spectra resonance is observed for the two spin degenerate electron s-state charging voltages with increasing temperature, a huge shift is visible for the electron-hole excitonic states resonance voltages. The s2-peak moves to slightly higher, the s1-peak to slightly lower charging voltages. In contrast, the excitonic states are surprisingly charged at much lower voltages upon increasing temperature. We derive a rate-model allowing to attribute and value different contributions to these shifts. Resonant tunnelling, state degeneracy and hole generation rate in combination with the Fermi distribution function turn out to be of great importance for the observed effects. The differences in the shifting behaviour is connected to different equilibria schemes for the peaks - s-peaks arise when tunnelling-in- and out-rates become equal, while excitonic peaks occur, when electron tunnelling-in- and hole-generation rates are balanced. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aa4f63
  • 2016 • 98 Thickness dependent electron-lattice equilibration in thin Bi films studied by time-resolved MeV electron diffraction
    Sokolowski-Tinten, K. and Li, R.K. and Reid, A.H. and Weathersby, S.P. and Quirin, F. and Chase, T. and Coffee, R. and Corbett, J. and Fry, A. and Hartmann, N. and Hast, C. and Hettel, R. and Horn-von Hoegen, M. and Janoschka, D. ...
    Optics InfoBase Conference Papers Part F20-UP 2016 (2016)
    Using time-resolved electron diffraction the electron-lattice equilibration in laserexcited thin Bi-films has been investigated. Our data reveal a pronounced thickness-dependence which is attributed to cross-interfacial coupling of hot electrons in the Bi-film to substrate phonons.
    view abstractdoi: 10.1364/UP.2016.UTh4A.49
  • 2015 • 97 Ab initio study of compositional trends in solid solution strengthening in metals with low Peierls stresses
    Ma, D. and Friák, M. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 98 367-376 (2015)
    Abstract We identify and analyze general trends governing solid solution strengthening in binary alloys containing solutes across the Periodic table using quantum-mechanical calculations. Here we present calculations for the model system of Al binary solid solutions. The identified trends originate from an approximately parabolic dependence of two strengthening parameters to quantitatively predict the solid solution strengthening effect, i.e. the volume and slip misfit parameters. The volume misfit parameter shows a minimum (concave-up behavior) as a function of the solute element group number in the periodic table, whereas the slip misfit parameter shows a maximum (concave-down behavior). By analyzing reported data, a similar trend is also found in Ni and Mg (basal slip) binary systems. Hence, these two strengthening parameters are strongly anti-correlated, which can be understood in terms of the Fermi level shift in the framework of free electron model. The chemical trends identified in this study enable a rapid and efficient identification of the solutes that provide optimum solid-solution strengthening. The approach described here may thus serve as basis for ab initio guided metallurgical materials design. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.054
  • 2015 • 96 Adsorption phenomena of cubane-type tetranuclear Ni(II) complexes with neutral, thioether-functionalized ligands on Au(111)
    Heß, V. and Matthes, F. and Bürgler, D.E. and Monakhov, K.Y. and Besson, C. and Kögerler, P. and Ghisolfi, A. and Braunstein, P. and Schneider, C.M.
    Surface Science 641 210-215 (2015)
    Abstract The controlled and intact deposition of molecules with specific properties onto surfaces is an emergent field impacting a wide range of applications including catalysis, molecular electronics, and quantum information processing. One strategy is to introduce grafting groups functionalized to anchor to a specific surface. While thiols and disulfides have proven to be quite effective in combination with gold surfaces, other S-containing groups have received much less attention. Here, we investigate the surface anchoring and organizing capabilities of novel charge-neutral heterocyclic thioether groups as ligands of polynuclear nickel(II) complexes. We report on the deposition of a cubane-type {Ni<inf>4</inf>} (= [Ni(μ<inf>3</inf>-Cl)Cl(HL·S)]<inf>4</inf>) single-molecule magnet from dichloromethane solution on a Au(111) surface, investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction, both immediately after deposition and after subsequent post-annealing. The results provide strong evidence for partial decomposition of the coordination complex upon deposition on the Au(111) surface that, however, leaves the magnetic {Ni<inf>4</inf>Cl<inf>4n</inf>} (n = 1 or 2) core intact. Only post-annealing above 480 K induces further decomposition and fragmentation of the {Ni<inf>4</inf>Cl<inf>4n</inf>} core. The detailed insight into the chemisorption-induced decomposition pathway not only provides guidelines for the deposition of thioether-functionalized Ni(II) complexes on metallic surfaces but also reveals opportunities to use multidentate organic ligands decorated with thioether groups as transporters for highly unstable inorganic structures onto conducting surfaces, where they are stabilized retaining appealing electronic and magnetic properties. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2015.06.026
  • 2015 • 95 Analytic model of the energy distribution function for highly energetic electrons in magnetron plasmas
    Gallian, S. and Trieschmann, J. and Mussenbrock, T. and Brinkmann, R.P. and Hitchon, W.N.G.
    Journal of Applied Physics 117 (2015)
    This paper analyzes a situation which is common for magnetized technical plasmas such as dc magnetron discharges and high power impulse magnetron sputtering (HiPIMS) systems, where secondary electrons enter the plasma after being accelerated in the cathode fall and encounter a nearly uniform bulk. An analytic calculation of the distribution function of hot electrons is presented; these are described as an initially monoenergetic beam that slows down by Coulomb collisions with a Maxwellian distribution of bulk (cold) electrons, and by inelastic collisions with neutrals. Although this analytical solution is based on a steady-state assumption, a comparison of the characteristic time-scales suggests that it may be applicable to a variety of practical time-dependent discharges, and it may be used to introduce kinetic effects into models based on the hypothesis of Maxwellian electrons. The results are verified for parameters appropriate to HiPIMS discharges, by means of time-dependent and fully kinetic numerical calculations. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4905943
  • 2015 • 94 Charging effect reduction in electron beam lithography and observation of single nanopillars on highly insulating substrates
    Tirpanci, Ş. and Bürgler, D.E. and Schneider, C.M. and Rameev, B. and Aktaş, B.
    Microelectronic Engineering 140 33-37 (2015)
    Electron beam writing and imaging of nanoscale structures on highly insulating substrates severely suffer from charging effects, which cause reduction in pattern resolution, positioning precision, and imaging quality. Conductive layers deposited above or below the resist layer can effectively reduce charge accumulation, but often give rise to contamination impairing the physical and chemical properties of functional nanostructures. Here we deal with top and bottom contacted, sub-micron-sized nanopillars made from multilayer stacks comprising ferromagnetic and non-magnetic materials for the study of current-induced magnetization dynamics. We show how the charging effects in a previously established fabrication process for single-crystalline nanopillars by H. Dassow et al. (2006) [1] can be significantly reduced by using the bottom electrode layer as charge dissipater and only isolating and disconnecting the bottom electrodes from ground after the fabrication of the delicate nanopillar structure by electron beam lithography. The modified process is successfully applied to Co<inf>2</inf>MnSi/Ag/Co<inf>2</inf>MnSi(001) multilayer stacks grown on highly insulating MgO substrates. Ellipsoidal nanopillars with a cross-section of 75 × 120 nm2 reveal 2% giant magnetoresistance and angular dependent magnetization behavior due to the magnetic anisotropy of the elliptical nanomagnets. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.mee.2015.05.007
  • 2015 • 93 Combining structural and chemical information at the nanometer scale by correlative transmission electron microscopy and atom probe tomography
    Herbig, M. and Choi, P. and Raabe, D.
    Ultramicroscopy 153 32-39 (2015)
    In many cases, the three-dimensional reconstructions from atom probe tomography (APT) are not sufficiently accurate to resolve crystallographic features such as lattice planes, shear bands, stacking faults, dislocations or grain boundaries. Hence, correlative crystallographic characterization is required in addition to APT at the exact same location of the specimen. Also, for the site-specific preparation of APT tips containing regions of interest (e.g. grain boundaries) correlative electron microscopy is often inevitable. Here we present a versatile experimental setup that enables performing correlative focused ion beam milling, transmission electron microscopy (TEM), and APT under optimized characterization conditions. The setup was designed for high throughput, robustness and practicability. We demonstrate that atom probe tips can be characterized by TEM in the same way as a standard TEM sample. In particular, the use of scanning nanobeam diffraction provides valuable complementary crystallographic information when being performed on atom probe tips. This technique enables the measurement of orientation and phase maps as known from electron backscattering diffraction with a spatial resolution down to one nanometer. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.02.003
  • 2015 • 92 Controlling the polarity of metalorganic vapor phase epitaxy-grown GaP on Si(111) for subsequent III-V nanowire growth
    Paszuk, A. and Brückner, S. and Steidl, M. and Zhao, W. and Dobrich, A. and Supplie, O. and Kleinschmidt, P. and Prost, W. and Hannappel, T.
    Applied Physics Letters 106 (2015)
    Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111). © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4922275
  • 2015 • 91 Cyclic bending experiments on free-standing Cu micron lines observed by electron backscatter diffraction
    Wimmer, A. and Heinz, W. and Detzel, T. and Robl, W. and Nellessen, M. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 83 460-469 (2015)
    Polycrystalline Cu samples 20 × 20 μm2 in size were cyclically bent inside a scanning electron microscope until fracture occurred. The microstructural changes were investigated by secondary electron imaging and electron backscatter diffraction. The in situ experiments revealed that, for the coarse-grained samples, it is not the external stress that dominates the cyclic deformation, but the local internal strength. This is in strong contrast to macroscopic bending samples, where deformation always happens near the fixed end of the bending beam and decreases constantly with increasing distance from the fixation. For micron-sized polycrystalline samples, the grain dimensions, dislocation density evolution and grain orientation (Taylor factor) can define the location of failure if the grain size and sample diameter become similar in size. A comparison with cyclic in situ tension-tension experiments (ratio of minimum stress to maximum stress R ≈ 0) reveals that cyclic bending experiments (R ≈ -1) undergo bulk-like fatigue deformation with extrusions/intrusions, in contrast to the experiments with R ≈ 0. Both the cyclic tension-tension and bending experiments can be described by a Basquin equation, although different mechanisms lead to failure of the samples. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.10.012
  • 2015 • 90 Effects of strain amplitude, cycle number and orientation on low cycle fatigue microstructures in austenitic stainless steel studied by electron channelling contrast imaging
    Nellessen, J. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 87 86-99 (2015)
    Substructure analysis on cyclically deformed metals is typically performed by time-consuming transmission electron microscopy probing, thus limiting such studies often to a single parameter. Here, we present a novel approach which consists in combining electron backscatter diffraction (EBSD), digital image correlation and electron channelling contrast imaging (ECCI), enabling us to systematically probe a large matrix of different parameters with the aim of correlating and comparing their interdependence. The main focus here is to identify the influence of cycle number, initial grain orientation and local strain amplitude on the evolving dislocation patterns. Therefore, experiments up to 100 cycles were performed on a polycrystalline austenitic stainless steel with local strain amplitudes between 0.35% and 0.95%. EBSD and ECCI maps reveal the individual influence of each parameter while the others remained constant. We find that the dislocation structures strongly depend on grain orientation. Dislocation structures in grains with double-slip (〈1 1 2〉 // LD, 〈1 2 2〉 // LD and 〈0 1 2〉 // LD) and multiple-slip (〈1 1 1〉 // LD, M 〈0 1 1〉 // LD and 〈0 0 1〉 // LD) orientations with respect to the loading direction (LD) are characterized under the variation of strain amplitude and cycle number. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.024
  • 2015 • 89 Electron collection in host-guest nanostructured hematite photoanodes for water splitting: The influence of scaffold doping density
    Kondofersky, I. and Dunn, H.K. and Müller, A. and Mandlmeier, B. and Feckl, J.M. and Fattakhova-Rohlfing, D. and Scheu, C. and Peter, L.M. and Bein, T.
    ACS Applied Materials and Interfaces 7 4623-4630 (2015)
    Nanostructuring has proven to be a successful strategy in overcoming the trade-off between light absorption and hole transport to the solid/electrolyte interface in hematite photoanodes for water splitting. The suggestion that poor electron (majority carrier) collection hinders the performance of nanostructured hematite electrodes has led to the emergence of host-guest architectures in which the absorber layer is deposited onto a transparent high-surface-area electron collector. To date, however, state of the art nanostructured hematite electrodes still outperform their host-guest counterparts, and a quantitative evaluation of the benefits of the host-guest architecture is still lacking. In this paper, we examine the impact of host-guest architectures by comparing nanostructured tin-doped hematite electrodes with hematite nanoparticle layers coated onto two types of conducting macroporous SnO2 scaffolds. Analysis of the external quantum efficiency spectra for substrate (SI) and electrolyte side (EI) illumination reveals that the electron diffusion length in the host-guest electrodes based on an undoped SnO2 scaffold is increased substantially relative to the nanostructured hematite electrode without a supporting scaffold. Nevertheless, electron collection is still incomplete for EI illumination. By contrast, an electron collection efficiency of 100% is achieved by fabricating the scaffold using antimony-doped SnO2, showing that the scaffold conductivity is crucial for the device performance. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/am5078667
  • 2015 • 88 Electron lifetimes in a 2D electron-gas with rashba SO-coupling: Screening properties
    Vollmar, S. and Ruffing, A. and Jakobs, S. and Baral, A. and Kaltenborn, S. and Cinchetti, M. and Aeschlimann, M. and Mathias, S. and Schneider, H.C.
    159 175-178 (2015)
    We calculate lifetimes due to electron-electron scattering in a 2D Rashba band structure and study the influence of the substrate screening. A comparison with measurements on the quantum-well system Bi/Cu(111) is presented. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-07743-7_56
  • 2015 • 87 Electron-phonon interaction and thermal boundary resistance at the crystal-amorphous interface of the phase change compound GeTe
    Campi, D. and Donadio, D. and Sosso, G.C. and Behler, J. and Bernasconi, M.
    Journal of Applied Physics 117 (2015)
    Phonon dispersion relations and electron-phonon coupling of hole-doped trigonal GeTe have been computed by density functional perturbation theory. This compound is a prototypical phase change material of interest for applications in phase change non-volatile memories. The calculations allowed us to estimate the electron-phonon contribution to the thermal boundary resistance at the interface between the crystalline and amorphous phases present in the device. The lattice contribution to the thermal boundary resistance has been computed by non-equilibrium molecular dynamics simulations with an interatomic potential based on a neural network scheme. We find that the electron-phonon term contributes to the thermal boundary resistance to an extent which is strongly dependent on the concentration and mobility of the holes. Further, for measured values of the holes concentration and electrical conductivity, the electron-phonon term is larger than the contribution from the lattice. It is also shown that the presence of Ge vacancies, responsible for the p-type degenerate character of the semiconductor, strongly affects the lattice thermal conductivity of the crystal. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4904910
  • 2015 • 86 Error analysis of the crystal orientations and disorientations obtained by the classical electron backscatter diffraction technique
    Ram, F. and Zaefferer, S. and Jäpel, T. and Raabe, D.
    Journal of Applied Crystallography 48 797-813 (2015)
    The fidelity - that is, the error, precision and accuracy - of the crystallographic orientations and disorientations obtained by the classical two-dimensional Hough-transform-based analysis of electron backscatter diffraction patterns (EBSPs) is studied. Using EBSPs simulated based on the dynamical electron diffraction theory, the fidelity analysis that has been previously performed using the patterns simulated based on the theory of kinematic electron diffraction is improved. Using the same patterns, the efficacy of a Fisher-distribution-based analytical accuracy measure for orientation and disorientation is verified.
    view abstractdoi: 10.1107/S1600576715005762
  • 2015 • 85 Experimental investigations of electron heating dynamics and ion energy distributions in capacitive discharges driven by customized voltage waveforms
    Berger, B. and Brandt, S. and Franek, J. and Schüngel, E. and Koepke, M. and Mussenbrock, T. and Schulze, J.
    Journal of Applied Physics 118 (2015)
    Capacitively coupled radio frequency plasmas driven by customized voltage waveforms provide enhanced opportunities to control process-relevant energy distributions of different particle species. Here, we present an experimental investigation of the spatio-temporal electron heating dynamics probed by Phase-Resolved Optical Emission Spectroscopy (PROES) in an argon discharge driven by up to three consecutive harmonics of 13.56 MHz with individually adjustable harmonics' amplitudes and phases. PROES and voltage measurements are performed at fixed total voltage amplitudes as a function of the number of driving harmonics, their relative phases, and pressure to study the effects of changing the applied voltage waveform on the heating dynamics in collisionless and collisional regimes. Additionally, the ion energy distribution function (IEDF) is measured at low pressure. In this collisionless regime, the discharge is operated in the α-mode. The velocity of energetic electron beams generated by the expanding sheaths is found to be affected by the number of driving harmonics and their relative phases. This is understood based on the sheath dynamics obtained from a model that determines sheath voltage waveforms. The formation of the measured IEDFs is understood and found to be directly affected by the observed changes in the electron heating dynamics. It is demonstrated that the mean ion energy can be controlled by adjusting the harmonics' phases. In the collisional regime at higher pressures changing the number of harmonics and their phases at fixed voltage is found to induce heating mode transitions from the α- to the γ-mode. Finally, a method to use PROES as a non-invasive diagnostic to monitor and detect changes of the ion flux to the electrodes is developed. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4937403
  • 2015 • 84 Formation and Movement of Cationic Defects during Forming and Resistive Switching in SrTiO3 Thin Film Devices
    Lenser, C. and Koehl, A. and Slipukhina, I. and Du, H. and Patt, M. and Feyer, V. and Schneider, C.M. and Lezaic, M. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 6360-6368 (2015)
    The resistance switching phenomenon in many transition metal oxides is described by ion motion leading to the formation of oxygen-deficient, highly electron-doped filaments. In this paper, the interface and subinterface region of electroformed and switched metal-insulator-metal structures fabricated from a thin Fe-doped SrTiO3 (STO) film on n-conducting Nb-doped SrTiO3 crystals are investigated by photoemission electron microscopy, transmission electron microscopy, and hard X-ray photoelectron spectroscopy in order to gain a deeper understanding of cation movement in this specific system. During electroforming, the segregation of Sr to the top interface and the generation of defect-rich cones in the film are observed, apparently growing from the anode toward the cathode during electroforming. An unusual binding energy component of the Sr 3d emission line is observed which can be assigned to Sr Ti-VO∗ defect complexes by performing ab initio calculations. Since this Sr component can be reversibly affected by an external electrical bias, the movement of both oxygen and Sr point defects and the formation of defect complexes Sr Ti-VO∗ during resistive switching are suggested. These findings are discussed with regard to the point defect structure of the film and the local oxidation of the donor-doped substrate. In particular, the apparent dichotomy between the observation of acceptor-type defects and increased electronic conductivity in STO is addressed. A low binding energy component of the Sr 3d photoemission line is observed in Fe-doped SrTiO3 memristive devices and assigned to Sr′Ti-V∗O defect complexes by ab initio calculations. Since this Sr component can be reversibly affected by an electrical bias, the movement of both oxygen and Sr vacancies and the formation of Sr′Ti-V∗O defect complexes during resistive switching are suggested. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201500851
  • 2015 • 83 Grain boundary segregation in multicrystalline silicon: Correlative characterization by EBSD, EBIC, and atom probe tomography
    Stoffers, A. and Cojocaru-Mirédin, O. and Seifert, W. and Zaefferer, S. and Riepe, S. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 1742-1753 (2015)
    This study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron-beam-induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a non-coherent (random) high-angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high-angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface-property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon. Copyright © 2015 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2614
  • 2015 • 82 High-throughput screening of thin-film semiconductor material libraries II: Characterization of Fe-W-O libraries
    Meyer, R. and Sliozberg, K. and Khare, C. and Schuhmann, W. and Ludwig, Al.
    ChemSusChem 8 1279-1285 (2015)
    Metal oxides are promising materials for solar water splitting. To identify suitable materials within the ternary system Fe-W-O, thin-film material libraries with combined thickness and compositional gradients were synthesized by combinatorial reactive magnetron sputtering. These libraries (>1000 different samples) were investigated by means of structural and functional high-throughput characterization techniques to establish correlations between composition, crystallinity, morphology, thickness, and photocurrent density in the compositional range between (Fe<inf>6</inf>W<inf>94</inf>)O<inf>x</inf> and (Fe<inf>61</inf>W<inf>39</inf>)O<inf>x</inf>. In addition to the well-known phase WO<inf>3</inf>, the binary phase W<inf>5</inf>O<inf>14</inf> and the ternary phase Fe<inf>2</inf>O<inf>6</inf>W show enhanced photoelectrochemical activity. The highest photocurrent density of 65 μA cm-2 was achieved for the composition (Fe<inf>15</inf>W<inf>85</inf>)O<inf>x</inf>, which contains the W<inf>5</inf>O<inf>14</inf> phase and has a thickness of 1060 nm. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201402918
  • 2015 • 81 Implications of electron heating and non-uniformities in a VHF-CCP for sterilization of medical instruments
    Stapelmann, K. and Fiebrandt, M. and Styrnoll, T. and Baldus, S. and Bibinov, N. and Awakowicz, P.
    Plasma Sources Science and Technology 24 (2015)
    A capacitively coupled plasma driven at a frequency of 81.36 MHz from the VHF-band is investigated by means of optical emission spectroscopy (OES) and multipole resonance probe (MRP). The discharge is operated with hydrogen, yielding an electropositive discharge, as well as oxygen, yielding an electronegative discharge, and mixtures of both. Pressure is varied from p = 5 Pa to p = 25 Pa. Homogeneity of the discharge is investigated by CCD camera recordings as well as spatially resolved multipole resonance probe measurements. The results indicate the presence of electromagnetic edge effects as well as standing wave effects. Furthermore, a largely homogeneous discharge can be achieved with hydrogen as process gas at a pressure of p = 5-10 Pa. With increasing pressure as well as with increasing oxygen content, the discharge appears less homogeneously. The transition from an electropositive to an electronegative discharge leads to a change in electron heating mechanisms, with pronounced local maxima of electron density at the sheath edges. A comparison of OES and MRP results reveal a significant difference in electron density, which can be explained by a non-Maxwellian distribution function of electrons. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/24/3/034014
  • 2015 • 80 Interplay of Electron and Nuclear Spin Noise in n -Type GaAs
    Berski, F. and Hübner, J. and Oestreich, M. and Ludwig, Ar. and Wieck, A.D. and Glazov, M.
    Physical Review Letters 115 (2015)
    We present spin-noise spectroscopy measurements on an ensemble of donor-bound electrons in ultrapure GaAs:Si covering temporal dynamics over 6 orders of magnitude from milliseconds to nanoseconds. The spin-noise spectra detected at the donor-bound exciton transition show the multifaceted dynamical regime of the ubiquitous mutual electron and nuclear spin interaction typical for III-V-based semiconductor systems. The experiment distinctly reveals the finite Overhauser shift of an electron spin precession at zero external magnetic field and a second contribution around zero frequency stemming from the electron spin components parallel to the nuclear spin fluctuations. Moreover, at very low frequencies, features related with time-dependent nuclear spin fluctuations are clearly resolved making it possible to study the intricate nuclear spin dynamics at zero and low magnetic fields. The findings are in agreement with the developed model of electron and nuclear spin noise. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.176601
  • 2015 • 79 Light Induced H2 Evolution from a Biophotocathode Based on Photosystem 1 - Pt Nanoparticles Complexes Integrated in Solvated Redox Polymers Films
    Zhao, F. and Conzuelo, F. and Hartmann, V. and Li, H. and Nowaczyk, M.M. and Plumeré, N. and Rögner, M. and Schuhmann, W.
    Journal of Physical Chemistry B 119 13726-13731 (2015)
    We report on a biophotocathode based on photosystem 1 (PS1)-Pt nanoparticle complexes integrated in a redox hydrogel for photoelectrocatalytic H2 evolution at low overpotential. A poly(vinyl)imidazole Os(bispyridine)2Cl polymer serves as conducting matrix to shuttle the electrons from the electrode to the PS1-Pt complexes embedded within the hydrogel. Light induced charge separation at the PS1-Pt complexes results in the generation of photocurrents (4.8 ± 0.4 μA cm-2) when the biophotocathodes are exposed to anaerobic buffer solutions. Under these conditions, the protons are the sole possible electron acceptors, suggesting that the photocurrent generation is associated with H2 evolution. Direct evidence for the latter process is provided by monitoring the H2 production with a Pt microelectrode in scanning electrochemical microscopy configuration over the redox hydrogel film containing the PS1-Pt complexes under illumination. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.5b03511
  • 2015 • 78 Mechanism of protection of catalysts supported in redox hydrogel films
    Fourmond, V. and Stapf, S. and Li, H. and Buesen, D. and Birrell, J. and Rüdiger, O. and Lubitz, W. and Schuhmann, W. and Plumeré, N. and Léger, C.
    Journal of the American Chemical Society 137 5494-5505 (2015)
    The use of synthetic inorganic complexes as supported catalysts is a key route in energy production and in industrial synthesis. However, their intrinsic oxygen sensitivity is sometimes an issue. Some of us have recently demonstrated that hydrogenases, the fragile but very efficient biological catalysts of H<inf>2</inf> oxidation, can be protected from O<inf>2</inf> damage upon integration into a film of a specifically designed redox polymer. Catalytic oxidation of H<inf>2</inf> produces electrons which reduce oxygen near the film/solution interface, thus providing a self-activated protection from oxygen [Plumeré et al., Nat Chem. 2014, 6, 822-827]. Here, we rationalize this protection mechanism by examining the time-dependent distribution of species in the hydrogenase/polymer film, using measured or estimated values of all relevant parameters and the numerical and analytical solutions of a realistic reaction-diffusion scheme. Our investigation sets the stage for optimizing the design of hydrogenase-polymer films, and for expanding this strategy to other fragile catalysts. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b01194
  • 2015 • 77 Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory
    Weathersby, S.P. and Brown, G. and Centurion, M. and Chase, T.F. and Coffee, R. and Corbett, J. and Eichner, J.P. and Frisch, J.C. and Fry, A.R. and Gühr, M. and Hartmann, N. and Hast, C. and Hettel, R. and Jobe, R.K. and Jongewa...
    Review of Scientific Instruments 86 (2015)
    Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4926994
  • 2015 • 76 Mode-selected heat flow through a one-dimensional waveguide network
    Riha, C. and Miechowski, P. and Buchholz, S.S. and Chiatti, O. and Wieck, A.D. and Reuter, D. and Fischer, S.F.
    Applied Physics Letters 106 (2015)
    Cross-correlated measurements of thermal noise are performed to determine the electron temperature in nanopatterned channels of a GaAs/AlGaAs heterostructure at 4.2K. Two-dimensional (2D) electron reservoirs are connected via an extended one-dimensional (1D) electron waveguide network. Hot electrons are produced using a current Ih in a source 2D reservoir, are transmitted through the ballistic 1D waveguide, and relax in a drain 2D reservoir. We find that the electron temperature increase, ΔTe, in the drain is proportional to the square of the heating current Ih, as expected from Joule's law. No temperature increase is observed in the drain when the 1D waveguide does not transmit electrons. Therefore, we conclude that electron-phonon interaction is negligible for heat transport between 2D reservoirs at temperatures below 4.2K. Furthermore, mode control of the 1D electron waveguide by application of a top-gate voltage reveals that ΔTe is not proportional to the number of populated subbands N, as previously observed in single 1D conductors. This can be explained with the splitting of the heat flow in the 1D waveguide network. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4908052
  • 2015 • 75 Model-independent measurement of the charge density distribution along an Fe atom probe needle using off-axis electron holography without mean inner potential effects
    Migunov, V. and London, A. and Farle, M. and Dunin-Borkowski, R.E.
    Journal of Applied Physics 117 134301 (2015)
    The one-dimensional charge density distribution along an electrically biased Fe atom probe needle is measured using a model-independent approach based on off-axis electron holography in the transmission electron microscope. Both the mean inner potential and the magnetic contribution to the phase shift are subtracted by taking differences between electron-optical phase images recorded with different voltages applied to the needle. The measured one-dimensional charge density distribution along the needle is compared with a similar result obtained using model-based fitting of the phase shift surrounding the needle. On the assumption of cylindrical symmetry, it is then used to infer the three-dimensional electric field and electrostatic potential around the needle with ∼10 nm spatial resolution, without needing to consider either the influence of the perturbed reference wave or the extension of the projected potential outside the field of view of the electron hologram. The present study illustrates how a model-independent approach can be used to measure local variations in charge density in a material using electron holography in the presence of additional contributions to the phase, such as those arising from changes in mean inner potential and specimen thickness. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4916609
  • 2015 • 74 Nonlocal behavior of the excitation rate in highly collisional RF discharges
    Eremin, D. and Hemke, T. and Mussenbrock, T.
    Plasma Sources Science and Technology 24 (2015)
    The present work focuses on the fundamental aspects of atmospheric pressure plasma electropositive discharges operated in the ohmically heated Ω mode, the electron heating and the excitation (ionization) rate. We find that the two do not necessarily have similar profiles and can show peaks at different locations, the ionization rate being much more sensitive to the electric field compared to the sensitivity to the electric field of the electron heating. This suggests an explanation for the discrepancies between the profiles of the power absorbed by electrons and the excitation patterns previously reported in the literature and observed in the present study. The excitation rate profile can then be explained by analyzing overlapping of the electron heating and the electric field profiles. Surprisingly, it has been discovered that the excitation dynamics exhibits nonlocal behavior having maxima spatially separated from the maxima of the electric field and the electron heating rate, a new effect in discharges operated in the Ω mode. The strong electric field in such discharges leads to large displacements of the electron component. This can produce significant charge separation close to the sheath or even in the bulk plasma because electrons are not able to follow the electric field adiabatically and maintain quasineutrality owing to the high collisionality. In particular, this leads to a significant distortion of the sheath structure and increase in the electric field there. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/24/4/044004
  • 2015 • 73 Note: Ion-induced secondary electron emission from oxidized metal surfaces measured in a particle beam reactor
    Marcak, A. and Corbella, C. and de los Arcos, T. and von Keudell, A.
    Review of Scientific Instruments 86 106102 (2015)
    The secondary electron emission of metals induced by slow ions is characterized in a beam chamber by means of two coaxial semi-cylindrical electrodes with different apertures. The voltages of the outer electrode (screening), inner electrode (collector), and sample holder (target) were set independently in order to measure the effective yield of potential and kinetic electron emissions during ion bombardment. Aluminum samples were exposed to quantified beams of argon ions up to 2000 eV and to oxygen atoms and molecules in order to mimic the plasma-surface interactions on metallic targets during reactive sputtering. The variation of electron emission yield was correlated to the ion energy and to the oxidation state of Al surfaces. This system provides reliable measurements of the electron yields in real time and is of great utility to explore the fundamental surface processes during target poisoning occurring in reactive magnetron sputtering applications. (C) 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4932309
  • 2015 • 72 Open volume defects and magnetic phase transition in Fe60Al40 transition metal aluminide
    Liedke, M.O. and Anwand, W. and Bali, R. and Cornelius, S. and Butterling, M. and Trinh, T.T. and Wagner, A. and Salamon, S. and Walecki, D. and Smekhova, A. and Wende, H. and Potzger, K.
    Journal of Applied Physics 117 (2015)
    Magnetic phase transition in the Fe<inf>60</inf>Al<inf>40</inf> transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000 °C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and Mössbauer spectroscopy. The positron annihilation spectroscopy has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate the importance of the open volume defects onto the magnetic phase transition. No evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400-600 °C) has been found, whereas higher temperatures showed an increase in the vacancy concentration. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4919014
  • 2015 • 71 Quantum Manipulation of Two-Electron Spin States in Isolated Double Quantum Dots
    Bertrand, B. and Flentje, H. and Takada, S. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Physical Review Letters 115 (2015)
    We studied experimentally the dynamics of the exchange interaction between two antiparallel electron spins in an isolated double quantum dot where coupling to the electron reservoirs can be ignored. We demonstrate that the level of control of such a double dot is higher than in conventional double dots. In particular, it allows us to couple coherently two electron spins in an efficient manner following a scheme initially proposed by Loss and DiVincenzo [Phys. Rev. A 57, 120 (1998)]. The present study demonstrates that isolated quantum dots are a possible route to increase the number of coherently coupled quantum dots. © 2015 American Physical Society. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.096801
  • 2015 • 70 Relaxation dynamics of majority and minority electrons after ultrashort laser excitation
    Mueller, B.Y. and Cinchetti, M. and Aeschlimann, M. and Schneider, H.C. and Rethfeld, B.
    159 116-119 (2015)
    We analyze the ultrafast demagnetization after ultrashort laser excitation by a kinetic model based on Elliott-Yafet scattering processes. By applying complete Boltzmann scattering integrals we trace the majority and minority electrons as well as phonons in the sample. Additionally, we allow for dynamical changes in the exchange splitting between majority and minority electrons. We find that our model has the potential to describe the magnetization dynamics and provides insights in the relaxation dynamics of the non-equilibrium electron system. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-07743-7_37
  • 2015 • 69 The effect of charged quantum dots on the mobility of a two-dimensional electron gas: How important is the Coulomb scattering?
    Kurzmann, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Journal of Applied Physics 117 (2015)
    We have investigated the influence of a layer of charged self-assembled quantum dots (QDs) on the mobility of a nearby two-dimensional electron gas (2DEG). Time-resolved transconductance spectroscopy was used to separate the two contributions of the change in mobility, which are: (i) The electrons in the QDs act as Coulomb scatterers for the electrons in the 2DEG. (ii) The screening ability and, hence, the mobility of the 2DEG decreases when the charge carrier density is reduced by the charged QDs, i.e., the mobility itself depends on the charge carrier concentration. Surprisingly, we find a negligible influence of the Coulomb scattering on the mobility for a 2DEG, separated by a 30nm tunneling barrier to the layer of QDs. This means that the mobility change is completely caused by depletion, i.e., reduction of the charge carrier density in the 2DEG, which indirectly influences the mobility. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4907217
  • 2015 • 68 The effect of the driving frequency on the confinement of beam electrons and plasma density in low-pressure capacitive discharges
    Wilczek, S. and Trieschmann, J. and Schulze, J. and Schuengel, E. and Brinkmann, R.P. and Derzsi, A. and Korolov, I. and Donkó, Z. and Mussenbrock, T.
    Plasma Sources Science and Technology 24 (2015)
    The effect of changing the driving frequency on the plasma density and the electron dynamics in a capacitive radio-frequency argon plasma operated at low pressures of a few Pa is investigated by particle-in-cell/Monte-Carlo collision simulations and analytical modeling. In contrast to previous assumptions, the plasma density does not follow a quadratic dependence on the driving frequency in this non-local collisionless regime. Instead, a step-like increase at a distinct driving frequency is observed. Based on an analytical power balance model, in combination with a detailed analysis of the electron kinetics, the density jump is found to be caused by an electron heating mode transition from the classical -mode into a low-density resonant heating mode characterized by the generation of two energetic electron beams at each electrode per sheath expansion phase. These electron beams propagate through the bulk without collisions and interact with the opposing sheath. In the low-density mode, the second beam is found to hit the opposing sheath during its collapse. Consequently, a large number of energetic electrons is lost at the electrodes resulting in a poor confinement of beam electrons in contrast to the classical -mode observed at higher driving frequencies. Based on the analytical model this modulated confinement quality and the related modulation of the energy lost per electron lost at the electrodes is demonstrated to cause the step-like change of the plasma density. The effects of a variation of the electrode gap, the neutral gas pressure, the electron sticking and secondary electron emission coefficients of the electrodes on this step-like increase of the plasma density are analyzed based on the simulation results. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/24/2/024002
  • 2015 • 67 Thickness-dependent electron-lattice equilibration in laser-excited thin bismuth films
    Sokolowski-Tinten, K. and Li, R.K. and Reid, A.H. and Weathersby, S.P. and Quirin, F. and Chase, T. and Coffee, R. and Corbett, J. and Fry, A. and Hartmann, N. and Hast, C. and Hettel, R. and Horn-von Hoegen, M. and Janoschka, D. ...
    New Journal of Physics 17 (2015)
    Electron-phonon coupling processes determine electronic transport properties of materials and are responsible for the transfer of electronic excess energy to the lattice. With decreasing device dimensions an understanding of these processes in nanoscale materials is becoming increasingly important. Here we use time-resolved electron diffraction to directly study energy relaxation in thin bismuth films after optical excitation. Precise measurements of the transient Debye-Waller-effect for various film thicknesses and over an extended range of excitation fluences allow to separate different contributions to the incoherent lattice response. While phonon softening in the electronically excited state is responsible for an immediate increase of the r.m.s. atomic displacement within a few hundred fs, 'ordinary' electron-phonon coupling leads to subsequent heating of the material on a few ps time-scale. The data reveal distinct changes in the energy transfer dynamics which becomes faster for stronger excitation and smaller film thickness, respectively. The latter effect is attributed to a cross-interfacial coupling of excited electrons to phonons in the substrate. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/17/11/113047
  • 2014 • 66 Antiferromagnetism in iron-based superconductors: Magnetic order in the model of delocalized electrons
    Eremin, I.M.
    Physics-Uspekhi 57 807-813 (2014)
    A theoretical analysis of the magnetic ordering mechanisms in parent ferropnictides (FPs) was investigated. The study takes into account that a ferropnictide remains metallic when it resides a magnetic state, and relies on a model that describes AFM order in terms of the spin density wave (SDW) for itinerant electrons. The reason is that optical conductivity measurements reveal a transfer of spectral weight from the Drude peak to the middle of the infrared peak, in accordance with the itinerant electron model that leads to AFM order. The SDW order parameter also becomes finite when the Fermi surface disappears. This results from the fact that an electron-hole loop formed by α and Β fermions is similar to the particle-particle loop because the α and Βband dispersions differ in sign. The calculated Fermi contours, ARPES spectral intensity, and the band dispersion near the Fermi level are consistent with the experimental data.
    view abstractdoi: 10.3367/UFNe.0184.201408g.0875
  • 2014 • 65 D-lactate-selective amperometric biosensor based on the cell debris of the recombinant yeast Hansenula polymorpha
    Smutok, O.V. and Dmytruk, K.V. and Karkovska, M.I. and Schuhmann, W. and Gonchar, M.V. and Sibirny, A.A.
    Talanta 125 227-232 (2014)
    A d-lactate-selective biosensor has been developed using cellsdebris of recombinant thermotolerant methylotrophic yeast Hansenula polymorpha, overproducing d-lactate: cytochrome c-oxidoreductase (EC, d-lactate dehydrogenase (cytochrome), DlDH). The H. polymorpha DlDH-producer was constructed in two steps. First, the gene CYB2 was deleted on the background of the C-105 (gcr1 catX) strain of H. polymorpha impaired in glucose repression and devoid of catalase activity to avoid specific l-lactate-cytochrome c oxidoreductase activity. Second, the homologous gene DLD1 coding for DlDH was overexpressed under the control of the strong H. polymorpha alcohol oxidase promoter in the frame of a plasmid for multicopy integration in the Δcyb2 strain. The selected recombinant strain possesses 6-fold increased DlDH activity as compared to the initial strain. The cellsdebris was used as a biorecognition element of a biosensor, since DlDH is strongly bound to mitochondrial membranes. The cellsdebris, prepared by mechanic disintegration of recombinant cells, was immobilized on a graphite working electrode in an electrochemically generated layer using an Os-complex modified cathodic electrodeposition polymer. Cytochrome c was used as additional native electron mediator to improve electron transfer from reduced DlDH to the working electrode. The constructed d-lactate-selective biosensors are characterized by a high sensitivity (46.3-61.6 A M-1 m-2), high selectivity and sufficient storage stability. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.talanta.2014.02.041
  • 2014 • 64 Engineered electron-transfer chain in photosystem 1 based photocathodes outperforms electron-transfer rates in natural photosynthesis
    Kothe, T. and Pöller, S. and Zhao, F. and Fortgang, P. and Rögner, M. and Schuhmann, W. and Plumeré, N.
    Chemistry (Weinheim an der Bergstrasse, Germany) 20 11029-11034 (2014)
    Photosystem 1 (PS1) triggers the most energetic light-induced charge-separation step in nature and the in vivo electron-transfer rates approach 50 e(-)  s(-1)  PS1(-1). Photoelectrochemical devices based on this building block have to date underperformed with respect to their semiconductor counterparts or to natural photosynthesis in terms of electron-transfer rates. We present a rational design of a redox hydrogel film to contact PS1 to an electrode for photocurrent generation. We exploit the pH-dependent properties of a poly(vinyl)imidazole Os(bispyridine)2Cl polymer to tune the redox hydrogel film for maximum electron-transfer rates under optimal conditions for PS1 activity. The PS1-containing redox hydrogel film displays electron-transfer rates of up to 335±14 e(-)  s(-1)  PS1(-1), which considerably exceeds the rates observed in natural photosynthesis or in other semiartificial systems. Under O2 supersaturation, photocurrents of 322±19 μA cm(-2) were achieved. The photocurrents are only limited by mass transport of the terminal electron acceptor (O2). This implies that even higher electron-transfer rates may be achieved with PS1-based systems in general. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201402585
  • 2014 • 63 Epitaxial Cu(001) films grown on a Cr/Ag/Fe/GaAs(001) buffer system
    Gottlob, D.M. and Jansen, T. and Hoppe, M. and Bürgler, D.E. and Schneider, C.M.
    Thin Solid Films 562 250-253 (2014)
    We present a procedure to prepare single-crystalline, high-purity Cu(001) films (templates) suitable as substrates for subsequent epitaxial thin-film growth. The template films were grown in a dedicated molecular-beam epitaxy system on a Cr/Ag/Fe/GaAs(001) buffer layer system. Low-energy electron diffraction and X-ray diffraction were applied to determine the surface orientation and the epitaxial relationship between all layers of the stack. Post-annealing at moderate temperatures enhances the quality of the film as shown by low-energy electron diffraction and atomic force microscopy. X-ray photoemission and Auger electron spectroscopy confirm that no atoms of the buffer layers diffuse into the Cu film during the initial preparation and the post-annealing treatment. The completed Cu(001) template system can be exposed to air and afterwards refurbished by Ar+-ion bombardment and annealing, enabling the transfer between vacuum systems. The procedure provides suitable conductive thin film templates for studies of epitaxial thin films, e.g. on the magnetic and magnetotransport properties of Co and Ni based films and multilayers. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.04.078
  • 2014 • 62 In-situ high-resolution low energy electron diffraction study of strain relaxation in heteroepitaxy of Bi(111) on Si(001): Interplay of strain state, misfit dislocation array and lattice parameter
    Hattab, H. and Jnawali, G. and Horn-von Hoegen, M.
    Thin Solid Films 159-163 (2014)
    The relief of lattice mismatch-induced strain in Bi(111) on Si(001) heteroepitaxial system was investigated in real time as the Bi film relaxes, by means of high resolution low-energy electron diffraction (LEED). The inherent lattice mismatch of 2.5% at room temperature is accommodated through the formation of an ordered misfit dislocation array confined to the interface. The strain fields of the dislocations cause a periodic height undulation of the surface in the sub-Ångström regime, which is observed through spot splitting in LEED. From a simultaneous measurement of the position of the first-order LEED spots, which corresponds to the lattice parameter of the film, and of the separation of satellite spots, which corresponds to the ordering of the dislocation array, the evolution of the strain state during annealing of a 6 nm Bi film was determined. The strain is solely relaxed by full edge-type dislocations arranged in the ordered array at the interface. From the remaining strain of ε = 0.6% the critical thickness for generation of misfit dislocations under equilibrium conditions can be derived. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2014.08.013
  • 2014 • 61 The adsorption behavior of octafluoropropane at the water/gas interface
    Giebel, F. and Paulus, M. and Nase, J. and Bieder, S. and Kiesel, I. and Tolan, M.
    Journal of Applied Physics 116 (2014)
    We studied the adsorption behavior of the gas octafluoropropane at the water/gas interface as a function of different pressures. In a custom-made measurement cell, the gas pressure was varied in a range between 1 bar and close to the condensation pressure of octafluoropropane. The electron density profiles of the adsorption layers show that the layer thickness increases with pressure. The evolution of the layer electron density indicates that the bulk electron density is reached if a layer consisting of more than one monolayer of octafluoropropane is adsorbed on the water surface. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4902961
  • 2013 • 60 Al-induced faceting of Si(113)
    Klein, C. and Heidmann, I. and Nabbefeld, T. and Speckmann, M. and Schmidt, T. and Meyer zu Heringdorf, F.-J. and Falta, J. and Horn-von Hoegen, M.
    Surface Science 618 109-114 (2013)
    Adsorption of Al on a Si(113) substrate at elevated temperatures causes a faceting transition of the initially flat surface. The (113) surface decomposes into a quasi-periodic sequence of Al terminated (115)- and (112)-facets. The resulting surface morphology is characterized in-situ by reciprocal space maps obtained with in-situ spot profile analyzing low-energy electron diffraction and ex-situ atomic force microscopy. The periodicity length of the faceted surface increases with adsorption temperature from 7 nm at 650 C to 80 nm at 800 C. The stability of the Al terminated Si(112) surface is the driving force for the faceting transition. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.08.007
  • 2013 • 59 Application of SECM in tracing of hydrogen peroxide at multicomponent non-noble electrocatalyst films for the oxygen reduction reaction
    Dobrzeniecka, A. and Zeradjanin, A. and Masa, J. and Puschhof, A. and Stroka, J. and Kulesza, P.J. and Schuhmann, W.
    Catalysis Today 202 55-62 (2013)
    The redox competition mode of scanning electrochemical microscopy (RC-SECM) was used to study the electrocatalytic activity of three different non-noble metal O2 reduction catalysts at a pH value of 7.4, namely; multi-walled carbon nanotubes (MWCNTs), cobalt protoporphyrin (CoP) and a composite of MWCNTs/CoP. The collection efficiency of a scanning electrochemical microscopy (SECM) tip for the H2O2 generated by the reduction of O2 at the catalyst layer was almost 100%. Consequently, SECM experiments in a combined redox competition and generator/collector mode could be applied for the determination of the number of electrons exchanged during O2 reduction, leading to improved understanding of the intrinsic features of catalyst activity. This approach avoids the typical limitations encountered with rotating ring disk electrode (RRDE) voltammetry, notably, the variation of the quantity of H2O2 in the proximity of the electrode with the speed of electrode rotation or the chemical decomposition of reaction intermediates on the Pt ring, which often introduce inconsistencies and errors in the measured values of the number of exchanged electrons. It is commonly assumed that the O2 reduction reaction on most non-noble metal catalysts proceeds via formation of H2O 2 as an intermediate. The follow-up reaction of H2O 2, typically chemical decomposition or electrochemical reduction, influences the overall number of electrons exchanged during O2 reduction. In this study, we have confirmed by comparing the rate of electrochemical reduction of H2O2 using rotating disk electrode (RDE) measurements with its rate of chemical decomposition studied using a positioned SECM tip, that for the MWCNTs/CoP catalyst, chemical decomposition is predominantly determining the overall number of exchanged electrons per O2 molecule. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2012.03.060
  • 2013 • 58 Direct electron transfer of Trametes hirsuta laccase adsorbed at unmodified nanoporous gold electrodes
    Salaj-Kosla, U. and Pöller, S. and Schuhmann, W. and Shleev, S. and Magner, E.
    Bioelectrochemistry 91 15-20 (2013)
    The enzyme Trametes hirsuta laccase undergoes direct electron transfer at unmodified nanoporous gold electrodes, displaying a current density of 28μA/cm2. The response indicates that ThLc was immobilised at the surface of the nanopores in a manner which promoted direct electron transfer, in contrast to the absence of a response at unmodified polycrystalline gold electrodes. The bioelectrocatalytic activity of ThLc modified nanoporous gold electrodes was strongly dependent on the presence of halide ions. Fluoride completely inhibited the enzymatic response, whereas in the presence of 150mM Cl-, the current was reduced to 50% of the response in the absence of Cl-. The current increased by 40% when the temperature was increased from 20°C to 37°C. The response is limited by enzymatic and/or enzyme electrode kinetics and is 30% of that observed for ThLc co-immobilised with an osmium redox polymer. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.bioelechem.2012.11.001
  • 2013 • 57 Influence of diffusion on space-charge-limited current measurements in organic semiconductors
    Kirchartz, T.
    Beilstein Journal of Nanotechnology 4 180-188 (2013)
    Numerical simulations of current-voltage curves in electron-only devices are used to discuss the influence of charged defects on the information derived from fitting space-charge-limited current models to the data. Charged, acceptor-like defects lead to barriers impeding the flow of electrons in electron-only devices and therefore lead to a reduced current that is similar to the situation where the device has a built-in voltage. This reduced current will lead to an underestimation of the mobilities and an overestimation of characteristic tail slopes if analytical equations are used to analyze the data. Correcting for the barrier created by the charged defects can, however, be a successful way to still be able to obtain reasonably accurate mobility values. © 2013 Kirchartz; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.4.18
  • 2013 • 56 Long electron spin coherence in ion-implanted GaN: The role of localization
    Bu, J.H. and Rudolph, J. and Shvarkov, S. and Hardtdegen, H. and Wieck, A.D. and Hägele, D.
    Applied Physics Letters 102 (2013)
    The impact of Ga and Au ion implantation on the electron spin dynamics in bulk wurtzite GaN is studied by time-resolved Kerr-rotation spectroscopy. The spin relaxation time increases strongly by up to a factor of 20 for increasing implantation doses. This drastic increase is caused by a transition from delocalized to localized electrons. We find a characteristic change in the magnetic field dependence of spin relaxation that can be used as a sensitive probe for the degree of localization. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4804558
  • 2013 • 55 Magneto-optical studies of Gd-implanted GaN: No spin alignment of conduction band electrons
    Buß, J.H. and Rudolph, J. and Shvarkov, S. and Semond, F. and Reuter, D. and Wieck, A.D. and Hägele, D.
    Applied Physics Letters 103 (2013)
    Gd-implanted wurtzite GaN as a candidate for a ferromagnetic dilute magnetic semiconductor is investigated by time-resolved magneto-optical spectroscopy. We observe a strong increase of the electron spin lifetimes for increasing Gd doses, while the electron spin Larmor precession frequency is independent of the Gd concentration. These findings are well explained by carrier localization at defects and a negligible interaction with Gd ions. The data show that Gd-implanted GaN cannot be used for an electron spin aligner. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4819767
  • 2013 • 54 Process diagnostics and monitoring using the multipole resonance probe in an inhomogeneous plasma for ion-assisted deposition of optical coatings
    Styrnoll, T. and Harhausen, J. and Lapke, M. and Storch, R. and Brinkmann, R.P. and Foest, R. and Ohl, A. and Awakowicz, P.
    Plasma Sources Science and Technology 22 (2013)
    The application of a multipole resonance probe (MRP) for diagnostic and monitoring purposes in a plasma ion-assisted deposition (PIAD) process is reported. Recently, the MRP was proposed as an economical and industry compatible plasma diagnostic device (Lapke et al 2011 Plasma Sources Sci. Technol. 20 042001). The major advantages of the MRP are its robustness against dielectric coating and its high sensitivity to measure the electron density. The PIAD process investigated is driven by the advanced plasma source (APS), which generates an ion beam in the deposition chamber for the production of high performance optical coatings. With a background neutral pressure of p 0 ~ 20 mPa the plasma expands from the source region into the recipient, leading to an inhomogeneous spatial distribution. Electron density and electron temperature vary over the distance from substrate (ne ~ 109 cm-3 and Te,eff ~ 2 eV) to the APS (ne >~ 1012 cm-3 and Te,eff ~ 20 eV) (Harhausen et al 2012 Plasma Sources Sci. Technol. 21 035012). This huge variation of the plasma parameters represents a big challenge for plasma diagnostics to operate precisely for all plasma conditions. The results obtained by the MRP are compared to those from a Langmuir probe chosen as reference diagnostics. It is demonstrated that the MRP is suited for the characterization of the PIAD plasma as well as for electron density monitoring. The latter aspect offers the possibility to develop new control schemes for complex industrial plasma environments. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/22/4/045008
  • 2013 • 53 To tilt or not to tilt: Correction of the distortion caused by inclined sample surfaces in low-energy electron diffraction
    Sojka, F. and Meissner, M. and Zwick, C. and Forker, R. and Vyshnepolsky, M. and Klein, C. and Horn-von Hoegen, M. and Fritz, T.
    Ultramicroscopy 133 35-40 (2013)
    Low-energy electron diffraction (LEED) is a widely employed technique for the structural characterization of crystalline surfaces and epitaxial adsorbates. For technical reasons the accessible reciprocal space is limited at a given primary electron energy E. This limitation may be overcome by sweeping E to observe higher diffraction orders decisively enhancing the quantitative examination. Yet, in many cases, such as molecular films with rather large unit cells, the adsorbate reflexes become less pronounced at energies high enough to observe substrate reflexes. One possibility to overcome this problem is an intentional inclination of the sample surface during the measurement at the expense of the quantitative interpretability of then severely distorted diffraction patterns. Here, we introduce a correction method for the axially symmetric distortion in LEED images of tilted samples. We provide experimental confirmation for micro-channel plate LEED and spot-profile analysis LEED instruments using the (7×7) reconstructed surface of a Si(111) single crystal as a reference sample. Finally, we demonstrate that the correction of this distortion considerably improves the quantitative analysis of diffraction patterns of adsorbates since substrate and adsorbate reflexes can be evaluated simultaneously. As an illustrative example we have chosen an epitaxial monolayer of 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111) that is known to form a commensurate superstructure. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2013.04.005
  • 2012 • 52 A few-electron quadruple quantum dot in a closed loop
    Thalineau, R. and Hermelin, S. and Wieck, A.D. and Bäuerle, C. and Saminadayar, L. and Meunier, T.
    Applied Physics Letters 101 (2012)
    We report the realization of a quadruple quantum dot device in a square-like configuration where a single electron can be transferred on a closed path free of other electrons. By studying the stability diagrams of this system, we demonstrate that we are able to reach the few-electron regime and to control the electronic population of each quantum dot with gate voltages. This allows us to control the transfer of a single electron on a closed path inside the quadruple dot system. This work opens the route towards electron spin manipulation using spin-orbit interaction by moving an electron on complex paths free of electrons. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4749811
  • 2012 • 51 DFT calculations suggest a new type of self-protection and self-inhibition mechanism in the mammalian heme enzyme myeloperoxidase: Nucleophilic addition of a functional water rather than one-electron reduction
    Sicking, W. and Somnitz, H. and Schmuck, C.
    Chemistry - A European Journal 18 10937-10948 (2012)
    The mammalian heme enzyme myeloperoxidase (MPO) catalyzes the reaction of Cl- to the antimicrobial-effective molecule HOCl. During the catalytic cycle, a reactive intermediate "Compound I" (Cpd I) is generated. Cpd I has the ability to destroy the enzyme. Indeed, in the absence of any substrate, Cpd I decays with a half-life of 100 ms to an intermediate called Compound II (Cpd II), which is typically the one-electron reduced Cpd I. However, the nature of Cpd II, its spectroscopic properties, and the source of the additional electron are only poorly understood. On the basis of DFT and time-dependent (TD)-DFT quantum chemical calculations at the PBE0/6-31G* level, we propose an extended mechanism involving a new intermediate, which allows MPO to protect itself from self-oxidation or self-destruction during the catalytic cycle. Because of its similarity in electronic structure to Cpd II, we named this intermediate Cpd IIa'. However, the suggested mechanism and our proposed functional structure of Cpd IIa' are based on the hypothesis that the heme is reduced by charge separation caused by reaction with a water molecule, and not, as is normally assumed, by the transfer of an electron. In the course of this investigation, we found a second intermediate, the reduced enzyme, towards which the new mechanism is equally transferable. In analogy to Cpd II′, we named it FeIIa'. The proposed new intermediates Cpd IIa' and FeIIa' allow the experimental findings, which have been well documented in the literature for decades but not so far understood, to be explained for the first time. These encompass a) the spontaneous decay of Cpd I, b) the unusual (chlorin-like) UV/Vis, circular dichroism (CD), and resonance Raman spectra, c) the inability of reduced MPO to bind CO, d) the fact that MPO-Cpd II reduces SCN- but not Cl-, and e) the experimentally observed auto-oxidation/auto-reduction features of the enzyme. Our new mechanism is also transferable to cytochromes, and could well be viable for heme enzymes in general. Heme mechanisms explained: Direct visual comparison with Cpd II demonstrates that Cpd II′ is a one-electron reduced intermediate with respect to the heme system. In both cases an electron is transferred: in Cpd II from an external donor, and in Cpd II′ through charge separation caused by reaction with a water molecule (see figure). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201103477
  • 2012 • 50 Dislocation density measurement by electron channeling contrast imaging in a scanning electron microscope
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 66 343-346 (2012)
    We have measured the average dislocation density by electron channeling contrast imaging (ECCI) in a scanning electron microscope under controlled diffraction conditions in a Fe-3 wt.% Si alloy tensile deformed to a macroscopic stress of 500 MPa. Under optimal diffraction conditions, ECCI provides an average dislocation density close to that obtained by bright-field transmission electron microscopy. This result confirms that ECCI is a powerful technique for determining dislocation densities in deformed bulk metals. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.11.027
  • 2012 • 49 Electron transport in partially filled iron carbon nanotubes
    Migunov, V. and Li, Z.-A. and Spasova, M. and Farle, M.
    Solid State Phenomena 190 498-501 (2012)
    We report electron transport properties of iron filled multiwalled carbon nanotubes (MWCNT) with outer diameters of 30 to 80 nm and lengths of 1 to 10 μm. Our study is combined with a structural investigation of the iron core using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It was found that high current densities of 1.8×107A/cm2 increase the conductivity of the MWCNT by a factor of two at 300 K, while the Fe core disappears probably forming defect states in the carbon shells. The enhanced diffusion of iron is most probably the result of local heating of the iron followed by implantation of iron atoms in the nanotube layers. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 48 Electron-nuclei spin coupling in GaAs-Free versus localized electrons
    Huang, J. and Chen, Y.S. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Bacher, G.
    Applied Physics Letters 100 (2012)
    We report on an experimental evidence of a significantly different dynamic nuclear polarization (DNP) for localized and itinerant electrons in n-GaAs. Optically injected spin-polarized electrons are used to generate dynamic nuclear polarization via electron-nucleus hyperfine interaction. Using time resolved Kerr rotation measurements for probing the transient Overhauser field, the DNP time constants for itinerant and localized electrons are extracted to be 10 min and less than 1 min, respectively. This is attributed to a rapid DNP occurring in the vicinity of the donors followed by a delayed nuclear spin polarization in between the donor sites. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.3699261
  • 2012 • 47 New insights into hard phases of CoCrMo metal-on-metal hip replacements
    Liao, Y. and Pourzal, R. and Stemmer, P. and Wimmer, M.A. and Jacobs, J.J. and Fischer, A. and Marks, L.D.
    Journal of the Mechanical Behavior of Biomedical Materials 12 39-49 (2012)
    The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M23C6-type (M=Cr, Mo, Co) and M6C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ~100nm fine grains. The nanosized grains were identified to be mostly of M23C6 type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M23C6 structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ~15.7GPa, while the M23C6 carbides in the wrought alloy were twice as hard (~30.7GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2°C/s, whereas single phase carbides were formed at a cooling rate of 50°C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2012.03.013
  • 2012 • 46 Surface damage of silicon after swift heavy ion irradiation
    Peters, T. and Alzaher, I. and Ban D'Etat, B. and Cassimi, A. and Monnet, I. and Lebius, H. and Schleberger, M.
    Journal of Physics: Conference Series 388 (2012)
    In order to answer the long-standing question, if silicon surfaces can be damaged by swift heavy ions, a set-up to study ion-irradiation damage of reactive surfaces is presented. This set-up allows for the first time to avoid oxidization of the silicone surface during the experimental study. Scanning tunneling microscopy as well as low-energy electron diffraction was used to study the surfaces before and after irradiation. Silicon surfaces were prepared by flash-heating before irradiation with swift heavy ions (Xenon at 0.9 MeV/u). The targets stayed in ultra-high vacuum during preparation, irradiation and surface imaging. No surface damage was detected, at normal as well as at grazing incidence angle. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/388/13/132035
  • 2012 • 45 The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain
    Chan, W.-L. and Ligges, M. and Zhu, X.-Y.
    Nature Chemistry 4 840-845 (2012)
    One strategy to improve solar-cell efficiency is to generate two excited electrons from just one photon through singlet fission, which is the conversion of a singlet (S 1) into two triplet (T 1) excitons. For efficient singlet fission it is believed that the cumulative energy of the triplet states should be no more than that of S 1. However, molecular analogues that satisfy this energetic requirement do not show appreciable singlet fission, whereas crystalline tetracene displays endothermic singlet fission with near-unity quantum yield. Here we probe singlet fission in tetracene by directly following the intermediate multiexciton (ME) state. The ME state is isoenergetic with 2×T 1, but fission is not activated thermally. Rather, an S 1 ⇔ ME superposition formed through a quantum-coherent process allows access to the higher-energy ME. We attribute entropic gain in crystalline tetracene as the driving force for the subsequent decay of S 1 ⇔ ME into 2×T 1, which leads to a high singlet-fission yield. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nchem.1436
  • 2011 • 44 A molecular dynamics investigation of kinetic electron emission from silver surfaces under varying angle of projectile impact
    Duvenbeck, A. and Hanke, S. and Weidtmann, B. and Wucher, A.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 269 1661-1664 (2011)
    We present a computer simulation study on the influence of the impact angle of the projectile on kinetic electron emission yields for 5-keV Ag → Ag bombardment. By means of a hybrid computer simulation model incorporating (i) the particle dynamics following the primary particle impact, (ii) the kinetically induced electronic substrate excitations via electronic friction and electron promotion and (iii) the transport of excitation energy away from the spot of generation, a full three-dimensional electron temperature profile within the volume affected by the atomic collision cascade is calculated. This profile is evaluated at the very surface of the target and taken as input for a thermionic model ('hot-spot-model') for kinetic electron emission. Averaging the results for different choices of the polar angle of incidence Θ over a large set of impact points, the obtained kinetic electron emission yields can be compared with experimental data and predictions from simple geometrical calculations. The presented simulation results appear to be reasonable in comparison with experimental data as well as with simple geometrical considerations of kinetic electron emission under oblique incidence. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.nimb.2010.11.082
  • 2011 • 43 Characterization of dielectric barrier discharge (DBD) on mouse and histological evaluation of the plasma-treated tissue
    Rajasekaran, P. and Opländer, C. and Hoffmeister, D. and Bibinov, N. and Suschek, C.V. and Wandke, D. and Awakowicz, P.
    Plasma Processes and Polymers 8 246-255 (2011)
    Atmospheric-pressure dielectric barrier discharge (DBD) in air is investigated for medical applications, especially for skin treatment. When the DBD was tested on mouse skin, a homogeneous discharge accompanied by filamentary microdischarges is observed. For characterization of the homogeneous discharge, averaged plasma parameters (namely electron density and electron velocity distribution function) and gas temperature are determined by optical emission spectroscopy, microphotography and numerical simulation. Chemical kinetics in the active plasma volume and in the afterglow is simulated. Fluxes of biologically useful molecules like nitric oxide (NO) and ozone reaching the treated surface and irradiation by UV photons are determined. Skin biopsy results show that DBD treatment causes no inflammation and no changes in the skin-collagen. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppap.201000122
  • 2011 • 42 Deformation mechanisms in micron-sized PST TiAl compression samples: Experiment and model
    Rester, M. and Fischer, F.D. and Kirchlechner, C. and Schmoelzer, T. and Clemens, H. and Dehm, G.
    Acta Materialia 59 3410-3421 (2011)
    Titanium aluminides are the most promising intermetallics for use in aerospace and automotive applications. Consequently, it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is extensively used for such studies are polysynthetically twinned (PST) TiAl crystals, which consist predominantly of parallel γ-TiAl and, fewer, α2-Ti3Al lamellae. In the present study, PST TiAl crystals with a nominal composition of Ti-50 at.% Al were machined by means of the focused ion beam (FIB) technique into miniaturized compression samples with a square cross-section of approximately 9 μm × 9 μm. Compression tests on the miniaturized samples were performed in situ inside a scanning electron microscope using a microindenter equipped with a diamond flat punch. After deformation, thin foils were cut from the micro-compression samples and thinned to electron transparency using a FIB machine in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at strains of 5.4%, while at strains of 8.3% dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the γ-TiAl lamella. A kinematic and thermodynamic model is developed to describe the twin-related length change of the micro-compression sample at small strains as well as the relationship of an increase of twin width with increasing γ-TiAl lamella thickness. The developed twin model predicts a width of the twins in the range of a few nanometers, which is in agreement with experimental findings. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.02.016
  • 2011 • 41 Dislocation and twin substructure evolution during strain hardening of an Fe-22 wt.% Mn-0.6 wt.% C TWIP steel observed by electron channeling contrast imaging
    Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 59 6449-6462 (2011)
    We study the kinetics of the substructure evolution and its correspondence to the strain hardening evolution of an Fe-22 wt.% Mn-0.6 wt.% C TWIP steel during tensile deformation by means of electron channeling contrast imaging (ECCI) combined with electron backscatter diffraction (EBSD). The contribution of twin and dislocation substructures to strain hardening is evaluated in terms of a dislocation mean free path approach involving several microstructure parameters, such as the characteristic average twin spacing and the dislocation substructure size. The analysis reveals that at the early stages of deformation (strain below 0.1 true strain) the dislocation substructure provides a high strain hardening rate with hardening coefficients of about G/40 (G is the shear modulus). At intermediate strains (below 0.3 true strain), the dislocation mean free path refinement due to deformation twinning results in a high strain rate with a hardening coefficient of about G/30. Finally, at high strains (above 0.4 true strain), the limited further refinement of the dislocation and twin substructures reduces the capability for trapping more dislocations inside the microstructure and, hence, the strain hardening decreases. Grains forming dislocation cells develop a self-organized and dynamically refined dislocation cell structure which follows the similitude principle but with a smaller similitude constant than that found in medium to high stacking fault energy alloys. We attribute this difference to the influence of the stacking fault energy on the mechanism of cell formation. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.07.009
  • 2011 • 40 Dislocation storage in single slip-oriented Cu micro-tensile samples: New insights via X-ray microdiffraction
    Kirchlechner, C. and Kiener, D. and Motz, C. and Labat, S. and Vaxelaire, N. and Perroud, O. and Micha, J.-S. and Ulrich, O. and Thomas, O. and Dehm, G. and Keckes, J.
    Philosophical Magazine 91 1256-1264 (2011)
    Synchrotron X-ray microdiffraction was used to characterize the deformation structure of single crystalline Cu micro-tensile specimens which were oriented for single slip. The 3-m thick samples were strained in situ in a scanning electron microscope (SEM). Electron microscopy observations revealed glide steps at the surface indicating single slip. While the slip steps at the surface must have formed by the predominant activation of the primary glide system, analysis of Laue peak streaking directions revealed that, even at low strains, dislocations had been activated and stored on an unpredicted slip system. Furthermore, the Laue scans showed that multiple slip takes over at a later state of deformation. © 2011 Taylor & Francis.
    view abstractdoi: 10.1080/14786431003785639
  • 2011 • 39 Driving force of ultrafast magnetization dynamics
    Mueller, B.Y. and Roth, T. and Cinchetti, M. and Aeschlimann, M. and Rethfeld, B.
    13 (2011)
    Irradiating a ferromagnetic material with an ultrashort laser pulse leads to demagnetization on the femtosecond timescale. We implement Elliott-Yafet-type spin-flip scattering, mediated by electron-electron and electron-phonon collisions, in the framework of a spin-resolved Boltzmann equation. Considering three mutually coupled reservoirs, (i) spin-up electrons, (ii) spin-down electrons and (iii) phonons, we trace non-equilibrium electron distributions during and after laser excitation. We identified the driving force for ultrafast magnetization dynamics as the equilibration of temperatures and chemical potentials between electronic subsystems. This principle can be used to easily predict the maximum quenching of magnetization upon ultrashort laser irradiation in any material, as we show for the case of 3d-ferromagnetic nickel. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/13/12/123010
  • 2011 • 38 Effect of fermi surface nesting on resonant spin excitations in Ba 1-xKxFe2As2
    Castellan, J.-P. and Rosenkranz, S. and Goremychkin, E.A. and Chung, D.Y. and Todorov, I.S. and Kanatzidis, M.G. and Eremin, I. and Knolle, J. and Chubukov, A.V. and Maiti, S. and Norman, M.R. and Weber, F. and Claus, H. and Guidi...
    Physical Review Letters 107 (2011)
    We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1-xKxFe2As2 over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s±-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.107.177003
  • 2011 • 37 Electronic excitations induced by hydrogen surface chemical reactions on gold
    Schindler, B. and Diesing, D. and Hasselbrink, E.
    Journal of Chemical Physics 134 (2011)
    Associated with chemical reactions at surfaces energy may be dissipated exciting surface electronic degrees of freedom. These excitations are detected using metal-insulator-metal (MIM) heterostructures (Ta-TaOx-Au) and the reactions of H with and on a Au surface are probed. A current corresponding to 510-5 electrons per adsorbing H atom and a marked isotope effect are observed under steady-state conditions. Analysis of the current trace when the H atom flux is intermitted suggests that predominantly the recombination reaction creates electronic excitations. Biasing the front versus the back electrode of the MIM structure provides insights into the spectrum of electronic excitations. The observed spectra differ for the two isotopes H and D and are asymmetric when comparing negative and positive bias voltages. Modeling indicates that the excited electrons and the concurrently created holes differ in their energy distributions. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3523647
  • 2011 • 36 From attraction to repulsion: Anion-π interactions between bromide and fluorinated phenyl groups
    Giese, M. and Albrecht, M. and Bannwarth, C. and Raabe, G. and Valkonen, A. and Rissanen, K.
    Chemical Communications 47 8542-8544 (2011)
    Anion-π interactions in crystals of fluorobenzyl ammonium salts depend on the degree of fluorination at the aromatics. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c1cc12667a
  • 2011 • 35 Hierarchical microstructure of explosive joints: Example of titanium to steel cladding
    Song, J. and Kostka, A. and Veehmayer, M. and Raabe, D.
    Materials Science and Engineering A 528 2641-2647 (2011)
    The microstructure of explosive cladding joints formed among parallel Ti and steel plates was examined by electron microscopy. The bonding interface and the bulk materials around it form pronounced hierarchical microstructures. This hierarchy is characterized by the following features: at the mesoscopic scale of the hierarchy a wavy course of the interface characterizes the interface zone. This microstructure level is formed by heavy plastic shear waves (wavelength≈0.5mm) which expand within the two metal plates during the explosion parallel to the bonding interface. At the micro-scale range, intermetallic inclusions (size≈100-200μm) are formed just behind the wave crests on the steel side as a result of partial melting. Electron diffraction revealed FeTi and metastable Fe9.64Ti0.36. Most of the observed phases do not appear in the equilibrium Fe-Ti phase diagram. These intermetallic inclusions are often accompanied by micro-cracks of similar dimension. At the smallest hierarchy level we observe a reaction layer of about 100-300nm thickness consisting of nano-sized grains formed along the entire bonding interface. Within that complex hierarchical micro- and nanostructure, the mesoscopic regime, more precisely the type and brittleness of the intermetallic zones, seems to play the dominant role for the mechanical behavior of the entire compound. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.11.092
  • 2011 • 34 Influence of channel width on the performance of an injection-type ballistic rectifier: Carrier injection versus hot-electron thermopower
    Salloch, D. and Wieser, U. and Kunze, U. and Hackbarth, T.
    Microelectronic Engineering 88 2386-2389 (2011)
    Injection-type ballistic rectification is achieved in an asymmetric Si/SiGe cross junction made from narrow channels, where the injectors are inclined with respect to the straight voltage stem. In this geometry an inertial-ballistic signal establishes due to the momentum direction of the injected electrons towards the lower part of the central voltage stem. Additionally, a diffusion hot-electron thermopower signal is superimposed which arises from an electron temperature gradient over a gate-confined region of the stem channel. We investigate the influence of the stem width on the rectifier performance. The inertial-ballistic signal disappears at a stem width larger than the 2D electron mean free path in contrast to the diffusion thermopower, which is independent of the stem width. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.mee.2011.02.079
  • 2011 • 33 Interdigitating biocalcite dendrites form a 3-D jigsaw structure in brachiopod shells
    Goetz, A.J. and Steinmetz, D.R. and Griesshaber, E. and Zaefferer, S. and Raabe, D. and Kelm, K. and Irsen, S. and Sehrbrock, A. and Schmahl, W.W.
    Acta Biomaterialia 7 2237-2243 (2011)
    We report a newly discovered dense microstructure of dendrite-like biocalcite that is formed by marine organisms. High spatial resolution electron backscatter diffraction (EBSD) was carried out under specific analytical conditions (15 and 10 kV) on the primary layer of the modern brachiopod Gryphus vitreus. The primary layer of modern brachiopods, previously termed nanocrystalline, is formed by an array of concave/convex calcite grains with interdigitated recesses and protrusions of abutting crystals without any cavities in or between the dendrites. The interface topology of this structure ranges from a few tens of nanometres to tens of micrometres, giving a nanoscale structure to the material fabric. The dendritic grains show a spread of crystallographic orientation of several degrees and can thus be referred to as mesocrystals. Individual dendritic mesocrystals reach sizes in one dimension larger than 20 μm. The preferred crystallographic orientation is similar in the primary and adjacent fibrous shell layers, even though these two layers show completely different crystal morphologies and grain boundary topologies. This observation indicates that two separate control mechanisms are active when the primary and the fibrous shell layers are formed. We propose a growth model for the interdigitated dendritic calcite grain structure based on a precursor of vesicles filled with amorphous calcium carbonate (ACC). © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2011.01.035
  • 2011 • 32 Long room-temperature electron spin lifetimes in bulk cubic GaN
    Buß, J.H. and Rudolph, J. and Schupp, T. and As, D.J. and Lischka, K. and Hägele, D.
    Proceedings of SPIE - The International Society for Optical Engineering 7937 (2011)
    We report on very long electron spin lifetimes in cubic GaN measured by time-resolved Kerr-rotation-spectroscopy. The spin coherence times with and without external magnetic field exceed 500 ps at room temperature, despite a high n-type doping level of more than 1019 cm-3 in the bulk sample under investigation. Our findings are therefore highly relevant for spin optoelectronics in the blue wavelength regime. The spin lifetimes are found to be almost temperature independent in accord with a prediction for degenerate electron gases of Dyakonov and Perel from 1972. These results are discussed also in comparison to wurtzite GaN, which shows much shorter spin lifetimes and a dependence of spin lifetimes on the spin orientation. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstractdoi: 10.1117/12.873395
  • 2011 • 31 Nanoscale photoelectron ionisation detector based on lanthanum hexaboride
    Zimmer, C.M. and Schubert, J. and Hamann, S. and Kunze, U. and Doll, T.
    Physica Status Solidi (A) Applications and Materials Science 208 1241-1245 (2011)
    A nanoscale ioniser is presented exceeding the limitation of conventional photoionisation detectors. It employs accelerated photoelectrons that allow obtaining molecule specificity by the tuning of ionisation energies. The material lanthanum hexaboride (LaB 6) is used as air stable photo cathode. Thin films of that material deposited by pulsed laser deposition (PLD) show quantum efficiency (QE) in the range of 10 -5 which is comparable to laser photo stimulation results. A careful treatment of the material yields reasonable low work functions even after surface reoxidation which opens up the possibility of using ultraviolet light emitting diodes (UV LEDs) in replacement of discharge lamps. Schematic diagram of a photoelectron ionisation detector (PeID) operating by an electron emitter based on the photoelectric effect of lanthanum hexaboride. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201000966
  • 2011 • 30 Possibilities and limitations of different analytical methods for the size determination of a bimodal dispersion of metallic nanoparticles
    Mahl, D. and Diendorf, J. and Meyer-Zaika, W. and Epple, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 377 386-392 (2011)
    Silver nanoparticles (about 70. nm) and gold nanoparticles (about 15. nm) were prepared and colloidally stabilized with poly(vinylpyrrolidone) (PVP). The pure nanoparticles as well as a 1:1 mixture (w:w) were analysed with a variety of methods which probe the size distribution: Scanning electron microscopy, transmission electron microscopy, dynamic light scattering, analytical disc centrifugation, and Brownian motion analysis (nanoparticle tracking analysis). The differences between the methods are highlighted and their ability to distinguish between silver and gold nanoparticles in the mixture is demonstrated. The size distribution data from the different methods were clearly different, therefore it is recommended to apply more than one method to characterize the nanoparticle dispersion. In particular, the smaller particles were undetectable by dynamic light scattering and nanoparticle tracking analysis in the presence of the large particles. For the 1:1 mixture, only electron microscopy and analytical disc centrifugation were able to give quantitative data on the size distribution. On the other hand, it is not possible to make statements about an agglomeration in dispersion with electron microscopy because an agglomeration may also have occurred during the drying process. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2011.01.031
  • 2011 • 29 Scanning transmission electron microscope observations of defects in as-grown and pre-strained Mo alloy fibers
    Phani, P.S. and Johanns, K.E. and Duscher, G. and Gali, A. and George, E.P. and Pharr, G.M.
    Acta Materialia 59 2172-2179 (2011)
    Compression testing of micro-pillars has recently been of great interest to the small-scale mechanics community. Previous compression tests on single crystal Mo alloy micro-pillars produced by directional solidification of eutectic alloys showed that as-grown pillars yield at strengths close to the theoretical strength while pre-strained pillars yield at considerably lower stresses. In addition, the flow behavior changes from stochastic to deterministic with increasing pre-strain. In order to gain a microstructural insight into this behavior, an aberration corrected scanning transmission electron microscope was used to study the defect structures in as-grown and pre-strained single crystal Mo alloy fibers. The as-grown fibers were found to be defect free over large lengths while the highly pre-strained (16%) fibers had high defect densities that were uniform throughout. Interestingly, the fibers with intermediate pre-strain (4%) exhibited an inhomogeneous defect distribution. The observed defect structures and their distributions are correlated with the previously reported stress-strain behavior. Some of the mechanistic interpretations of Bei et al. are examined in the light of new microstructural observations. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.12.018
  • 2011 • 28 Solvation dynamics of surface-trapped electrons at NH3 and D2O crystallites adsorbed on metals: from femtosecond to minute timescales
    Stähler, J. and Meyer, M. and Bovensiepen, U. and Wolf, M.
    Chemical Science 2 907-916 (2011)
    The creation and stabilization of localized, low-energy electrons is investigated in polar molecular environments. We create such excess electrons in excited states in ice and ammonia crystallites adsorbed on metal surfaces and observe their relaxation in real time using time-resolved photoelectron spectroscopy. The observed dynamics proceed up to minute timescales and are therefore slowed down considerably compared to ultrafast excited state relaxation in front of metal surfaces, which proceeds typically on femto- or picosecond time scales. It is the highly efficient wave function constriction of the electrons from the metal that ultimately enables the investigation of the relaxation dynamics over a large range of timescales (up to 17 orders of magnitude). Therefore, it gives novel insight into the solvated electron ground state formation at interfaces. As these long-lived electrons are observed for both, D2O and NH3 crystallites, they appear to be of general character for polar molecule-metal interfaces. Their time- and temperature-dependent relaxation is analyzed for both, crystalline ice and ammonia, and compared using an empirical model that yields insight into the fundamental solvation processes of the respective solvent. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c0sc00644k
  • 2011 • 27 Space-resolved characterization of high frequency atmospheric-pressure plasma in nitrogen, applying optical emission spectroscopy and numerical simulation
    Rajasekaran, P. and Ruhrmann, C. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 44 (2011)
    Averaged plasma parameters such as electron distribution function and electron density are determined by characterization of high frequency (2.4GHz) nitrogen plasma using both experimental methods, namely optical emission spectroscopy (OES) and microphotography, and numerical simulation. Both direct and step-wise electron-impact excitation of nitrogen emissions are considered. The determination of space-resolved electron distribution function, electron density, rate constant for electron-impact dissociation of nitrogen molecule and the production of nitrogen atoms, applying the same methods, is discussed. Spatial distribution of intensities of neutral nitrogen molecule and nitrogen molecular ion from the microplasma is imaged by a CCD camera. The CCD images are calibrated using the corresponding emissions measured by absolutely calibrated OES, and are then subjected to inverse Abel transformation to determine space-resolved intensities and other parameters. The space-resolved parameters are compared, respectively, with the averaged parameters, and an agreement between them is established. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/48/485205
  • 2011 • 26 The oxidation of tyrosine and tryptophan studied by a molecular dynamics normal hydrogen electrode
    Costanzo, F. and Sulpizi, M. and Valle, R.G.D. and Sprik, M.
    Journal of Chemical Physics 134 (2011)
    The thermochemical constants for the oxidation of tyrosine and tryptophan through proton coupled electron transfer in aqueous solution have been computed applying a recently developed density functional theory (DFT) based molecular dynamics method for reversible elimination of protons and electrons. This method enables us to estimate the solvation free energy of a proton (H+) in a periodic model system from the free energy for the deprotonation of an aqueous hydronium ion (H3O+). Using the computed solvation free energy of H+ as reference, the deprotonation and oxidation free energies of an aqueous species can be converted to pKa and normal hydrogen electrode (NHE) potentials. This conversion requires certain thermochemical corrections which were first presented in a similar study of the oxidation of hydrobenzoquinone [J. Cheng, M. Sulpizi, and M. Sprik, J. Chem. Phys. 131, 154504 (2009)]10.1063/1.3250438. Taking a different view of the thermodynamic status of the hydronium ion, these thermochemical corrections are revised in the present work. The key difference with the previous scheme is that the hydronium is now treated as an intermediate in the transfer of the proton from solution to the gas-phase. The accuracy of the method is assessed by a detailed comparison of the computed pKa, NHE potentials and dehydrogenation free energies to experiment. As a further application of the technique, we have analyzed the role of the solvent in the oxidation of tyrosine by the tryptophan radical. The free energy change computed for this hydrogen atom transfer reaction is very similar to the gas-phase value, in agreement with experiment. The molecular dynamics results however, show that the minimal solvent effect on the reaction free energy is accompanied by a significant reorganization of the solvent. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3597603
  • 2011 • 25 Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots
    Marquardt, B. and Geller, M. and Baxevanis, B. and Pfannkuche, D. and Wieck, A.D. and Reuter, D. and Lorke, A.
    Nature Communications 2 (2011)
    Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms1205
  • 2011 • 24 Ultrafast dynamics at the Na/D2O/Cu(111) interface: Electron solvation in Ice layers and Na+-mediated surface solvation
    Meyer, M. and Bertin, M. and Bovensiepen, U. and Wegkamp, D. and Krenz, M. and Wolf, M.
    Journal of Physical Chemistry C 115 204-209 (2011)
    We have studied the influence of sodium ions bound near the ice/vacuum interface on the electron solvation dynamics in amorphous D2O ice layers by means of femtosecond time-resolved two-photon photoelectron spectroscopy. Adsorption of submonolayer coverages of sodium on top of multilayers of amorphous ice leads to the formation of Na+ ions and to pronounced changes in the observed ultrafast dynamics compared to pure amorphous ice. We identify a Na+-induced species of excess electrons which exhibits a much longer lifetime compared to excess electrons in pure D2O ice and approximate the decay of the Na-induced contribution by two decay times τ2 = 880 fs and τ3 = 9.6 ps. In addition, a faster energetic stabilization of the excited electrons with a rate of σ = 0.73 eV/ps is observed. The population of these electrons depends nonlinearly on the sodium coverage. We attribute the Na-induced contribution to a transient electron/ion/water complex which is located at the ice/vacuum interface. This interpretation is corroborated by coverage-dependent measurements and by overlayer experiments. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp107253g
  • 2011 • 23 X-ray photoelectron spectroscopy on implanted argon as a tool to follow local structural changes in thin films
    Lahrood, A. R. and de los Arcos, T. and Prenzel, M. and von Keudell, A. and Winter, J.
    Thin Solid Films 520 1625--1630 (2011)
    Argon ions were implanted in metallic, semiconducting or insulating substrates, and investigated with X-ray photoelectron spectroscopy. Analysis of the Ar2p core level of argon showed clear differences in binding energy position and width as function of the matrix material, implantation energy, and post-annealing treatment. Although argon is not expected to form chemical bonds with the host matrix, the electronic shells within the gas atom can react to their environment according to different effects. It is shown that the precise determination and correct interpretation of the binding energy levels of the embedded gas atoms provides information about the local environment of the matrix such as amorphization of the crystalline structure, defect healing or gas bubble formation. (C) 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2011.07.040
  • 2011 • 22 XeF2 gas-assisted focused-electron-beaminduced etching of GaAs with 30 nm resolution
    Ganczarczyk, A. and Geller, M. P. and Lorke, A.
    Nanotechnology 22 (2011)
    We demonstrate the gas-assisted focused-electron-beam (FEB)-induced etching of GaAs with a resolution of 30 nm at room temperature. We use a scanning electron microscope (SEM) in a dual beam focused ion beam together with xenon difluoride (XeF2) that can be injected by a needle directly onto the sample surface. We show that the FEB-induced etching with XeF2 as a precursor gas results in isotropic and smooth etching of GaAs, while the etch rate depends strongly on the beam current and the electron energy. The natural oxide of GaAs at the sample surface inhibits the etching process; hence, oxide removal in combination with chemical surface passivation is necessary as a strategy to enable this high-resolution etching alternative for GaAs. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/4/045301
  • 2010 • 21 A 2D electron gas for studies on tunneling dynamics and charge storage in self-assembled quantum dots
    Marquardt, B. and Moujib, H. and Lorke, A. and Reuter, D. and Wieck, A.D. and Geller, M.
    Lecture Notes of the Institute for Computer Sciences, Social-Informatics and Telecommunications Engineering 36 LNICST 180-188 (2010)
    The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using time-resolved conductance measurements of a nearby two-dimensional electron gas (2DEG). The coupling strength (tunneling time) between the QDs and the 2DEG is adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance on the 2DEG, even for very weak coupling, where standard C-V spectroscopy is unsuitable to investigate the electronic structure of these QDs. © Institute for Computer Sciences, Social-Informatics and Telecommunications Engineering 2010.
    view abstractdoi: 10.1007/978-3-642-11731-2_22
  • 2010 • 20 A hemispherical photoelectron spectrometer with 2-dimensional delay-line detector and integrated spin-polarization analysis
    Plucinski, L. and Oelsner, A. and Matthes, F. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 181 215-219 (2010)
    Photoelectron spectrometers usually allow detection of either spin-resolved energy-distribution curves (EDCs) at single emission angle, or 2D angle-vs.-energy images without spin-resolution. We have combined the two detection schemes into one spectrometer system which permits simultaneous detection of a 1D spin-resolved EDC and a 2D angular map. A state-of-the-art hemispherical analyzer is used as an energy filter. Its original scintillator detector has been replaced by a delay-line-detector (DLD), and part of the electron beam is allowed to pass through to reach the spin-polarized low energy electron diffraction (SPLEED) spin-detector mounted subsequently. The electron-optics between DLD and SPLEED contains a 90° deflector to feature simultaneous detection of in-plane and out-of-plane spin components. These electron-optics have been optimized for high transmission to reduce acquisition times in the spin-resolved mode. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elspec.2010.05.005
  • 2010 • 19 A two-dimensional electron gas as a sensitive detector for time-resolved tunneling measurements on self-assembled quantum dots
    Geller, M. and Marquardt, B. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Nanoscale Research Letters 5 829-833 (2010)
    A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients. © 2010 The Author(s).
    view abstractdoi: 10.1007/s11671-010-9569-2
  • 2010 • 18 A two-dimensional electron gas as a sensitive detector to observe the charge carrier dynamics of self-assembled QDs
    Marquardt, B. and Geller, M. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Physica E: Low-Dimensional Systems and Nanostructures 42 2598-2601 (2010)
    The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using a time-resolved conductance measurement of a nearby two-dimensional electron gas (2DEG). The investigated heterostructures consist of a layer of QDs with different coupling strengths to a 2DEG, adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG, even for very weak coupling between the QD layer and the 2D system, where standard capacitance (C)voltage (V) spectroscopy is unsuitable to investigate the electronic structure of these QDs. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.physe.2010.02.010
  • 2010 • 17 A voltage-tunable in-plane diode in a two-dimensional-electron system
    Ganczarczyk, A. and Voßen, S. and Geller, M. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Physica E: Low-Dimensional Systems and Nanostructures 42 1216-1219 (2010)
    We present a voltage-tunable in-plane diode in a nanoscale, two-dimensional electron system, whose functionality is mainly determined by the sample geometry. The diode consists of a narrow semiconductor channel, confined by etched insulating trenches. An applied voltage along the channel modulates the effective width of the conducting channel, depending on the sign of the applied voltage. This behavior results in a diode-like IV -characteristic. The tunability of the device is achieved by two in-plane side gates, which are able to widely tune the I (V) -characteristic of this rectifier. In the normally-off regime, this tunable in-plane diode works as a half-wave rectifier with sharply defined turn-on voltage. The value of the turn-on voltage depends on the side-gate voltage. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.physe.2009.10.018
  • 2010 • 16 Aharonov-Bohm oscillation phase shift in a multi-terminal asymmetric quantum ring
    Buchholz, S.S. and Fischer, S.F. and Kunze, U. and Reuter, D. and Wieck, A.D.
    Physica E: Low-Dimensional Systems and Nanostructures 42 1099-1102 (2010)
    We investigate coherent electron transport in a four-terminal asymmetric waveguide quantum ring with the focus on a controllable electrostatic Aharonov-Bohm (AB) oscillation phase shift. In the AlGaAs/GaAs device, electron wave interference has been detected in all possible four-terminal measurement configuration at a temperature as high as T = 1.5 K. We present a series of AB measurements in small magnetic fields for successive global gate voltages, which strongly suggest an electrostatic AB phase shift. Such a phase shift can be explained by the asymmetry of the ring and indicates that the design and positioning of voltage and current probes to the ring have a significant influence. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.physe.2009.11.123
  • 2010 • 15 Anisotropy of Ag diffusion on vicinal Si surfaces
    Sindermann, S. and Wall, D. and Roos, K.R. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    e-Journal of Surface Science and Nanotechnology 8 372-376 (2010)
    Photoemission electron microscopy (PEEM) is used to study Ag surface diffusion on vicinal Si surfaces. The diffusion field is represented by Iso-Coverage Zones around Ag islands during desorption. By analyzing the shape and radius of the Iso-Coverage Zone we can determine diffusion parameters. For anisotropic diffusion the zone has an elliptical shape and the aspect ratio gives a measure for the anisotropy. Using this technique, we study the degree of anisotropy of Ag diffusion on vicinal Si(001) and Si(111). With increasing miscut angles, starting from Si(001) as well as from Si(111), we find a gradually increasing anisotropy, caused by the higher step density. On higher index surfaces, like Si(119), Si(115) and Si(113), we find isotropic diffusion for surfaces with comparable dimer and (double) step structure as on Si(001)-4°, where diffusion is strongly anisotropic. © 2010 The Surface Science Society of Japan.
    view abstractdoi: 10.1380/ejssnt.2010.372
  • 2010 • 14 Annealing behavior of ferritic-martensitic 9%Cr-ODS-Eurofer steel
    Sandim, H.R.Z. and Renzetti, R.A. and Padilha, A.F. and Raabe, D. and Klimenkov, M. and Lindau, R. and Möslang, A.
    Materials Science and Engineering A 527 3602-3608 (2010)
    Oxide dispersion strengthened ferritic-martensitic steels are potential candidates for applications in future fusion power plants. High creep resistance, good oxidation resistance, reduced neutron activation and microstructural long-term stability at temperatures of about 650-700°C are required in this context. In order to evaluate its thermal stability in the ferritic phase field, samples of the reduced activation ferritic-martensitic 9%Cr-ODS-Eurofer steel were cold rolled to 50% and 80% reductions and further annealed in vacuum from 300 to 800°C for 1h. The characterization in the annealed state was performed by scanning electron microscopy in the backscattered electron mode, high-resolution electron backscatter diffraction and transmission electron microscopy. Results show that the fine dispersion of Y-based particles (about 10nm in size) is effective to prevent recrystallization. The low recrystallized volume fraction (< 0.1) is associated to the nuclei found at prior grain boundaries and around large M23C6 particles. Static recovery was found to be the predominant softening mechanism of this steel in the investigated temperature range. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.02.051
  • 2010 • 13 Dissociation of oxygen on Ag(100) induced by inelastic electron tunneling
    Sprodowski, C. and Mehlhorn, M. and Morgenstern, K.
    Journal of Physics Condensed Matter 22 (2010)
    Scanning tunneling microscopy (STM) is used to study the dissociation of molecular oxygen on Ag(100) induced by inelastic electron tunneling (IET) at 5 K. This dissociation is possible above 3.3 V with a yield of (3.63 ± 0.47) × 10-9 per electron. Dissociation leads to three different types of hot atom motion: lateral motion, a cannon ball mechanism, and abstractive dissociation. Analysis of the I -t characteristics during dissociation suggests that the dissociation is proceeded by an adsorption site change. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/26/264005
  • 2010 • 12 Dissolution kinetics of Si into Ge (111) substrate on the nanoscale
    Balogh, Z. and Erdélyi, Z. and Beke, D.L. and Wiedwald, U. and Pfeiffer, H. and Tschetschetkin, A. and Ziemann, P.
    Thin Solid Films 519 952-955 (2010)
    In this paper we present experiments and simulations on the dissolution of Si into single crystalline Ge(111) substrates. The interface shift during the dissolution was tracked by X-ray Photoelectron Spectroscopy. It was obtained that the interface remained sharp and shifted according to anomalous kinetics similarly to our previous measurement in the Si/amorphous-Ge system. The interface shift, x, can be described by a power function of time x ∞ t kc with a kinetic exponent, kc, of 0.85 ± 0.1, larger than the one measured for the amorphous system (0.7 ± 0.1). Both exponents, however, are different from the kc = 0.5 Fickian (parabolic) value and it is interpreted as a nanoscale diffusional anomaly caused by the strong composition dependence of the diffusion coefficients. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.08.146
  • 2010 • 11 Energy dissipation in insulators induced by swift heavy ions: A parameter study
    Osmani, O. and Lebius, H. and Rethfeld, B. and Schleberger, M.
    Laser and Particle Beams 28 229-234 (2010)
    The irradiation of solids with high energy laser or particle beams has led to a deeper understanding of the relaxation processes inside the target material. However, a lot of open questions remain. In the present paper, we will examine the irradiation of the model system Xe23+ @ 93 MeV → SrTiO3 within the framework of the two-temperature-model and study the electron-phonon-coupling g and the electron diffusivity De as well as the lattice diffusivity Dp. These are crucial parameters for which no experimental data is available. Experimentally, g is very difficult to measure and therefore theoretical predictions are of great importance. With the approach presented here it is possible to determine the coupling-constant by one order of magnitude. © 2010 Cambridge University Press.
    view abstractdoi: 10.1017/S0263034609990632
  • 2010 • 10 Field effects on SnOx and SnO2 nanoparticles synthesized in the gas phase
    Roy Chowdhury, D. and Ivaturi, A. and Nedic, A. and Einar Kruis, F. and Schmechel, R.
    Physica E: Low-Dimensional Systems and Nanostructures 42 2471-2476 (2010)
    The present study reports for the first time the influence of stoichiometry of SnO2 nanoparticles synthesized in the gas phase at atmospheric pressure towards the field effect behaviour. The field effect was measured by using the nanoparticles as active material in a transistor channel. The transistors fabricated from the stoichiometric SnO2 nanoparticles (∼20 nm) obtained by post-deposition low-temperature (300 °C) oxidation of the SnO nanoparticles clearly demonstrate n-type behaviour in contrast to the high electrical conductance exhibited by the non-stoichiometric SnOx nanoparticles obtained by high temperature (650 °C) in-flight oxidation. X-ray Photoelectron Spectroscopy (XPS) studies confirm the stoichiometry of the in-flight as well as the post-oxidized nanoparticles.
    view abstractdoi: 10.1016/j.physe.2010.06.005
  • 2010 • 9 Focused ion beam/scanning electron microscopy tomography and conventional transmission electron microscopy assessment of Ni4Ti3 morphology in compression-aged Ni-rich Ni-Ti single crystals
    Cao, S. and Somsen, C. and Croitoru, M. and Schryvers, D. and Eggeler, G.
    Scripta Materialia 62 399-402 (2010)
    The size, morphology and configuration of Ni4Ti3 precipitates in a single-crystal Ni-Ti alloy have been investigated by two-dimensional transmission electron microscopy-based image analysis and three-dimensional reconstruction from slice-and-view images obtained in a focused ion beam/scanning electron microscopy (FIB/SEM) dual-beam system. Average distances between the precipitates measured along the compression direction correlate well between both techniques, while particle shape and configuration data is best obtained from FIB/SEM. Precipitates form pockets of B2 of 0.54 μm in the compression direction and 1 μm perpendicular to the compression direction. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2009.11.040
  • 2010 • 8 High resolution, low hν photoelectron spectroscopy with the use of a microwave excited rare gas lamp and ionic crystal filters
    Suga, S. and Sekiyama, A. and Funabashi, G. and Yamaguchi, J. and Kimura, M. and Tsujibayashi, M. and Uyama, T. and Sugiyama, H. and Tomida, Y. and Kuwahara, G. and Kitayama, S. and Fukushima, K. and Kimura, K. and Yokoi, T. and M...
    Review of Scientific Instruments 81 (2010)
    The need for not only bulk sensitive but also extremely high resolution photoelectron spectroscopy for studying detailed electronic structures of strongly correlated electron systems is growing rapidly. Moreover, easy access to such a capability in one's own laboratory is desirable. Demonstrated here is the performance of a microwave excited rare gas (Xe, Kr, and Ar) lamp combined with ionic crystal filters (sapphire, CaF2, and LiF), which can supply three strong lines near the photon energy of hnyu hν=8.4, 10.0, and 11.6 eV, with the hν resolution of better than 600 μeV for photoelectron spectroscopy. Its performance is demonstrated on some materials by means of both angle-integrated and angle-resolved measurements. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3488367
  • 2010 • 7 Intramolecular electronic interactions between nonconjugated arene and quinone chromophores
    Jansen, G. and Kahlert, B. and Klärner, F.-G. and Boese, R. and Bläser, D.
    Journal of the American Chemical Society 132 8581-8592 (2010)
    The novel surprisingly colorful dark blue and orange-red molecular clips 1 and 2 containing a central p-benzoquinone spacer-unit and anthracene or napththalene sidewalls were synthesized by DDQ oxidation of the corresponding colorless hydroquinone clips 7 and 8. The colors of the quinone clips result from broad absorption bands in the visible range (1, λ<inf>max</inf> = 537 nm and 2, λ<inf>max</inf> = 423 and λ<inf>shoulder</inf> =515 nm) showing bathochromic shifts of 112 and 90 nm, respectively, compared to the similarly tetraalkyl-substituted duroquinone 31, even though the clips 1 and 2 only contain insulated π systems as chromophores, a central tetraalkyl-substituted p-benzoquinone spacer-unit and two anthracene or two naphthalene sidewalls. To elucidate the electronic properties of these clips, we prepared the compound 3, the anti-configured isomer of clip 2, and the benzene-, naphthalene-, and anthracene-substituted quinones 4, 5, and 6, the so-called "half-clips". The "half-clips" 6 and 5 show a similar color change and the same trend in the UV/vis absorption spectra as the anthracene and naphthalene clip 1 and 2. This finding already rules out that the color of these systems is a result of "through-space" π-π interactions between the aromatic sidewalls in the molecular clips 1 and 2. Quantum chemical ab initio calculations provide good evidence that the bathochromic shift of the absorption band at the longest wavelength observed in the UV/vis spectra of the clip quinones 2, 3, and 1 and the "half- clip" quinones 4, 5, and 6 with an increasing number of rings in the anellated aromatic unit (from benzene to anthracene) is the result of an increasing configuration interaction between a n → π* excitation of the quinoid component and a π → π* excitation with intramolecular charge transfer (CT) character. The initial π orbitals involved here and in higher lying transitions mainly stem from through-space interactions between π orbitals of the aromatic sidewalls and π orbitals of the quinone moiety with varying degree of mixing. The configuration interaction in the excited states can be considered to be a homoconjugation, that is, the relevant charge transfer states are formed across an allegedly insulating aliphatic bridge. The UV/vis spectra of the molecular clips 1-3, the "half-clips" 4-6, and the quinones 32 and 33 simulated by means of quantum chemical ab initio calculations agree well with the experimental spectra. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja910362j
  • 2010 • 6 Long room-temperature electron spin lifetimes in highly doped cubic GaN
    Buß, J.H. and Rudolph, J. and Schupp, T. and As, D.J. and Lischka, K. and Hägele, D.
    Applied Physics Letters 97 (2010)
    We report on very long electron spin relaxation times in highly n -doped bulk zincblende GaN exceeding 500 ps up to room-temperature. Time-resolved Kerr-rotation measurements show an almost temperature independent spin relaxation time between 80 and 295 K confirming an early prediction of Dyakonov and Perel for a degenerate electron gas. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3478838
  • 2010 • 5 Non-thermal atmospheric pressure HF plasma source: Generation of nitric oxide and ozone for bio-medical applications
    Kühn, S. and Bibinov, N. and Gesche, R. and Awakowicz, P.
    Plasma Sources Science and Technology 19 (2010)
    A new miniature high-frequency (HF) plasma source intended for bio-medical applications is studied using nitrogen/oxygen mixture at atmospheric pressure. This plasma source can be used as an element of a plasma source array for applications in dermatology and surgery. Nitric oxide and ozone which are produced in this plasma source are well-known agents for proliferation of the cells, inhalation therapy for newborn infants, disinfection of wounds and blood ozonation. Using optical emission spectroscopy, microphotography and numerical simulation, the gas temperature in the active plasma region and plasma parameters (electron density and electron distribution function) are determined for varied nitrogen/oxygen flows. The influence of the gas flows on the plasma conditions is studied. Ozone and nitric oxide concentrations in the effluent of the plasma source are measured using absorption spectroscopy and electro-chemical NO-detector at variable gas flows. Correlations between plasma parameters and concentrations of the particles in the effluent of the plasma source are discussed. By varying the gas flows, the HF plasma source can be optimized for nitric oxide or ozone production. Maximum concentrations of 2750 ppm and 400 ppm of NO and O3, correspondingly, are generated. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/19/1/015013
  • 2010 • 4 On the characterization of recrystallized fraction using electron backscatter diffraction: A direct comparison to local hardness in an IF steel using nanoindentation
    Dziaszyk, S. and Payton, E.J. and Friedel, F. and Marx, V. and Eggeler, G.
    Materials Science and Engineering A 527 7854-7864 (2010)
    Recrystallized fraction was characterized in a Ti-stabilized interstitial-free (IF) steel by electron backscatter diffraction (EBSD), optical metallography, and hardness-based techniques. EBSD and nanoindentation were performed on overlapping areas to assess the agreement between standard methods of EBSD analysis of deformation microstructures and local hardness. The results of the study indicate that carefully implemented misorientation-based techniques may be used to effectively determine recrystallization fraction, to better agreement with the local recrystallization state than EBSD image quality, optical metallography, or microhardness-based techniques. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.08.063
  • 2010 • 3 Optical probing of spin dynamics of two-dimensional and bulk electrons in a GaAs/AlGaAs heterojunction system
    Rizo, P.J. and Pugzlys, A. and Slachter, A. and Denega, S.Z. and Reuter, D. and Wieck, A.D. and Van Loosdrecht, P.H.M. and Van Der Wal, C.H.
    New Journal of Physics 12 (2010)
    The electron spin dynamics in a GaAs/AlGaAs heterojunction system containing a high-mobility two-dimensional electron gas (2DEG) have been studied in this paper by using pump-probe time-resolved Kerr rotation experiments. Owing to the complex layer structure of this material, the transient Kerr response contains information about electron spins in the 2DEG, an epilayer and the substrate. We analyzed the physics that underlies this Kerr response, and established the conditions under which it is possible to unravel the signatures of the various photo-induced spin populations. This was used to explore how the electron spin dynamics of the various populations depend on the temperature, magnetic field and pump-photon density. The results show that the D'Yakonov-Perel' mechanism for spin dephasing (by spin-orbit fields) plays a prominent role in both the 2DEG and bulk populations over a wide range of temperatures and magnetic fields. Our results are of importance for future studies on the 2DEG in this type of heterojunction system, which offers interesting possibilities for spin manipulation and control of spin relaxation via tunable spin-orbit effects. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/11/113040
  • 2010 • 2 The influence of the relative phase between the driving voltages on electron heating in asymmetric dual frequency capacitive discharges
    Ziegler, D. and Trieschmann, J. and Mussenbrock, T. and Brinkmann, R.P. and Schulze, J. and Czarnetzki, U. and Semmler, E. and Awakowicz, P. and O'Connell, D. and Gans, T.
    Plasma Sources Science and Technology 19 (2010)
    The influence of the relative phase between the driving voltages on electron heating in asymmetric phase-locked dual frequency capacitively coupled radio frequency plasmas operated at 2 and 14 MHz is investigated. The basis of the analysis is a nonlinear global model with the option to implement a relative phase between the two driving voltages. In recent publications it has been reported that nonlinear electron resonance heating can drastically enhance the power dissipation to electrons at moments of sheath collapse due to the self-excitation of nonlinear plasma series resonance (PSR) oscillations of the radio frequency current. This work shows that depending on the relative phase of the driving voltages, the total number and exact moments of sheath collapse can be influenced. In the case of two consecutive sheath collapses a substantial increase in dissipated power compared with the known increase due to a single PSR excitation event per period is observed. Phase resolved optical emission spectroscopy (PROES) provides access to the excitation dynamics in front of the driven electrode. Via PROES the propagation of beam-like energetic electrons immediately after the sheath collapse is observed. In this work we demonstrate that there is a close relation between moments of sheath collapse, and thus excitation of the PSR, and beam-like electron propagation. A comparison of simulation results to experiments in a single and dual frequency discharge shows good agreement. In particular the observed influence of the relative phase on the dynamics of a dual frequency discharge is described by means of the presented model. Additionally, the analysis demonstrates that the observed gain in dissipation is not accompanied by an increase in the electrode's dc-bias voltage which directly addresses the issue of separate control of ion flux and ion energy in dual frequency capacitively coupled radio frequency plasmas. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/19/4/045001
  • 2009 • 1 The electron attachment energy of the aqueous hydroxyl radical predicted from the detachment energy of the aqueous hydroxide anion
    Adriaanse, C. and Sulpizi, M. and VandeVondele, J. and Sprik, M.
    Journal of the American Chemical Society 131 6046-6047 (2009)
    Combining photoemission and electrochemical data from the literature we argue that the difference between the vertical and adiabatic ionization energy of the aqueous hydroxide anion is 2.9 eV. We then use density functional theory based molecular dynamics to show that the solvent response to ionization is nonlinear. Adding this to the experimental data we predict a 4.1 eV difference between the energy for vertical attachment of an electron to the aqueous hydroxyl radical and the corresponding adiabatic electron affinity. This places the state accepting the electron only 2.2 eV below vacuum or 7.7 eV above the edge of the valence band of water. © 2009 American Chemical Society.
    view abstractdoi: 10.1021/ja809155k