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 • 254 Parametrization protocol and refinement strategies for accurate and transferable analytic bond-order potentials: Application to Re
    Subramanyam, Aparna P. A. and Jenke, Jan and Ladines, Alvin N. and Drautz, Ralf and Hammerschmidt, Thomas
    Physical Review Materials 8 (2024)
    Interatomic potentials provide a means to simulate extended length and time scales that are outside the reach of ab initio calculations. The development of an interatomic potential for a particular material requires the optimization of the parameters of the functional form of the potential. We present a parametrization protocol for analytic bond-order potentials (BOPs) that provides a physically transparent and computationally efficient description of the interatomic interaction. The parametrization protocol of the BOP follows the derivation of the BOP along the coarse-graining of the electronic structure from density-functional theory (DFT) to the tight-binding (TB) bond model to analytic BOPs. In particular, it starts from TB parameters that are obtained by downfolding DFT eigenstates of two-atomic molecules to an sd-valent minimal basis. This sd-valent Hamiltonian is combined with a pairwise repulsion to obtain an initial binding energy relation. The s electrons are then removed from the Hamiltonian and instead represented by an isotropic embedding term. In the final step, the parameters of the remaining d-d interaction, the pair repulsion, and the embedding term are optimized simultaneously. We demonstrate that the application of this parametrization protocol leads to a basic BOP for Re with good transferability. We discuss different strategies to refine the basic BOP towards global transferability or towards local accuracy. We demonstrate that homogeneous samplings of the structural phase space in a map of local atomic environments can be used to systematically increase the global transferability. We also demonstrate the influence of training data weighting on local accuracy refinements with a Pareto-front analysis, and we suggest further requirements to select a final BOP. The local accuracy and global transferability of the final BOP is also shown and compared to DFT. © 2024 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.8.013803
  • 2023 • 253 Atomic cluster expansion for Pt–Rh catalysts: From ab initio to the simulation of nanoclusters in few steps
    Liang, Yanyan and Mrovec, Matous and Lysogorskiy, Yury and Vega-Paredes, Miquel and Scheu, Christina and Drautz, Ralf
    Journal of Materials Research 38 5125 – 5135 (2023)
    Abstract: Insight into structural and thermodynamic properties of nanoparticles is crucial for designing optimal catalysts with enhanced activity and stability. In this work, we present a semi-automated workflow for parameterizing the atomic cluster expansion (ACE) from ab initio data. The main steps of the workflow are the generation of training data from accurate electronic structure calculations, an efficient fitting procedure supported by active learning and uncertainty indication, and a thorough validation. We apply the workflow to the simulation of binary Pt–Rh nanoparticles that are important for catalytic applications. We demonstrate that the Pt–Rh ACE is able to reproduce accurately a broad range of fundamental properties of the elemental metals as well as their compounds while retaining an outstanding computational efficiency. This enables a direct comparison of atomistic simulations to high-resolution experiments. Graphical abstract: [Figure not available: see fulltext.]. © 2023, The Author(s).
    view abstractdoi: 10.1557/s43578-023-01123-5
  • 2023 • 252 How to train a neural network potential
    Tokita, Alea Miako and Behler, Jörg
    Journal of Chemical Physics 159 (2023)
    The introduction of modern Machine Learning Potentials (MLPs) has led to a paradigm change in the development of potential energy surfaces for atomistic simulations. By providing efficient access to energies and forces, they allow us to perform large-scale simulations of extended systems, which are not directly accessible by demanding first-principles methods. In these simulations, MLPs can reach the accuracy of electronic structure calculations, provided that they have been properly trained and validated using a suitable set of reference data. Due to their highly flexible functional form, the construction of MLPs has to be done with great care. In this Tutorial, we describe the necessary key steps for training reliable MLPs, from data generation via training to final validation. The procedure, which is illustrated for the example of a high-dimensional neural network potential, is general and applicable to many types of MLPs. © 2023 Author(s).
    view abstractdoi: 10.1063/5.0160326
  • 2023 • 251 Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion
    Liao, Cheng-Jhe and Tseng, Yu-Ting and Cheng, Yu-An and Dayao, Loise Ann and Iffland-Mühlhaus, Linda and Gee, Leland B. and Ribson, Ryan D. and Chan, Ting-Shan and Apfel, Ulf-Peter and Lu, Tsai-Te
    Journal of the American Chemical Society 145 20389 – 20402 (2023)
    Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of “3 + O2- ⇋ 3-red + O2(g)”, which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2). © 2023 American Chemical Society.
    view abstractdoi: 10.1021/jacs.3c05577
  • 2023 • 250 Machine learning guided high-throughput search of non-oxide garnets
    Schmidt, Jonathan and Wang, Hai-Chen and Schmidt, Georg and Marques, Miguel A. L.
    npj Computational Materials 9 (2023)
    Garnets have found important applications in modern technologies including magnetorestriction, spintronics, lithium batteries, etc. The overwhelming majority of experimentally known garnets are oxides, while explorations (experimental or theoretical) for the rest of the chemical space have been limited in scope. A key issue is that the garnet structure has a large primitive unit cell, requiring a substantial amount of computational resources. To perform a comprehensive search of the complete chemical space for new garnets, we combine recent progress in graph neural networks with high-throughput calculations. We apply the machine learning model to identify the potentially (meta-)stable garnet systems before performing systematic density-functional calculations to validate the predictions. We discover more than 600 ternary garnets with distances to the convex hull below 100 meV ⋅ atom−1. This includes sulfide, nitride, and halide garnets. We analyze their electronic structure and discuss the connection between the value of the electronic band gap and charge balance. © 2023, The Author(s).
    view abstractdoi: 10.1038/s41524-023-01009-4
  • 2023 • 249 Machine learning transferable atomic forces for large systems from underconverged molecular fragments
    Herbold, M. and Behler, J.
    Physical Chemistry Chemical Physics 25 12979-12989 (2023)
    Machine learning potentials (MLP) enable atomistic simulations with first-principles accuracy at a small fraction of the costs of electronic structure calculations. Most modern MLPs rely on constructing the potential energy, or a major part of it, as a sum of atomic energies, which are given as a function of the local chemical environments up to a cutoff radius. Since analytic forces are readily available, nowadays it is common practice to make use of both, reference energies and forces, for training these MLPs. This can be computationally demanding since often large systems are required to obtain structurally converged reference forces experienced by atoms in realistic condensed phase environments. In this work we show how density-functional theory calculations of molecular fragments, which are too small to provide such structurally converged forces, can be used to learn forces exhibiting excellent transferability to extended systems. The general procedure and the accuracy of the method are illustrated for metal-organic frameworks using second-generation high-dimensional neural network potentials. © 2023 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d2cp05976b
  • 2023 • 248 Magnetic bond-order potential for iron-cobalt alloys
    Egorov, Aleksei and Subramanyam, Aparna P. A. and Yuan, Ziyi and Drautz, Ralf and Hammerschmidt, Thomas
    Physical Review Materials 7 (2023)
    For large-scale atomistic simulations of magnetic materials, the interplay of atomic and magnetic degrees of freedom needs to be described with high computational efficiency. Here we present an analytic bond-order potential (BOP) for iron-cobalt, an interatomic potential based on a coarse-grained description of the electronic structure. We fitted BOP parameters to magnetic and non-magnetic density functional theory (DFT) calculations of Fe, Co, and Fe-Co bulk phases. Our BOP captures the electronic structure of magnetic and nonmagnetic Fe-Co phases. It provides accurate predictions of structural stability, elastic constants, phonons, point and planar defects, and structural transformations. It also reproduces the DFT-predicted sequence of stable ordered phases peculiar to Fe-Co and the stabilization of B2 against disordered phases by magnetism. Our Fe-Co BOP is suitable for atomistic simulations with thousands and millions of atoms. © 2023 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.7.044403
  • 2023 • 247 Rationalizing the formation of porosity in mechanochemically-synthesized polymers
    Krusenbaum, Annika and Hinojosa, Steffi Krause and Fabig, Sven and Becker, Valentin and Grätz, Sven and Borchardt, Lars
    Physical Chemistry Chemical Physics 25 16781 – 16789 (2023)
    In this study, we present a matrix of 144 mechanochemically-synthesized polymers. All polymers were constructed by the solvent-free Friedel-Crafts polymerization approach, employing 16 aryl-containing monomers and 9 halide-containing linkers, which were processed in a high-speed ball mill. This Polymer Matrix was utilized to investigate the origin of porosity in Friedel-Crafts polymerizations in detail. By examining the physical state, molecular size, geometry, flexibility, and electronic structure of the utilized monomers and linkers, we identified the most important factors influencing the formation of porous polymers. We analyzed the significance of these factors for both monomers and linkers based on the yield and specific surface area of the generated polymers. Our in-depth evaluation serves as a benchmark study for future targeted design of porous polymers by the facile and sustainable concept of mechanochemistry. © 2023 The Royal Society of Chemistry
    view abstractdoi: 10.1039/d3cp02128a
  • 2023 • 246 Unusual crossover from Bardeen-Cooper-Schrieffer to Bose-Einstein-condensate superconductivity in iron chalcogenides
    Mizukami, Yuta and Haze, Masahiro and Tanaka, Ohei and Matsuura, Kohei and Sano, Daiki and Böker, Jakob and Eremin, Ilya and Kasahara, Shigeru and Matsuda, Yuji and Shibauchi, Takasada
    Communications Physics 6 (2023)
    The BCS-BEC (Bardeen-Cooper-Schrieffer–Bose-Einstein-condensate) crossover from strongly overlapping Cooper pairs to non-overlapping composite bosons in the strong coupling limit has been a long-standing issue of interacting many-body fermion systems. Recently, FeSe semimetal with hole and electron bands emerged as a high-transition-temperature (high-T c) superconductor located in the BCS-BEC crossover regime, owing to its very small Fermi energies. In FeSe, however, an ordinary BCS-like heat-capacity jump is observed at T c, posing a fundamental question on the characteristics of the BCS-BEC crossover. Here we report on high-resolution heat capacity, magnetic torque, and scanning tunneling spectroscopy measurements in FeSe1−xS x. Upon entering the tetragonal phase at x > 0.17, where nematic order is suppressed, T c discontinuously decreases. In this phase, highly non-mean-field behaviours consistent with BEC-like pairing are found in the thermodynamic quantities with giant superconducting fluctuations extending far above T c, implying the change of pairing nature. Moreover, the pseudogap formation, which is expected in BCS-BEC crossover of single-band superconductors, is not observed in the tunneling spectra. These results illuminate highly unusual features of the superconducting states in the crossover regime with multiband electronic structure and competing electronic instabilities. © 2023, The Author(s).
    view abstractdoi: 10.1038/s42005-023-01289-8
  • 2022 • 245 A Hessian-based assessment of atomic forces for training machine learning interatomic potentials
    Herbold, M. and Behler, J.
    Journal of Chemical Physics 156 (2022)
    In recent years, many types of machine learning potentials (MLPs) have been introduced, which are able to represent high-dimensional potential-energy surfaces (PESs) with close to first-principles accuracy. Most current MLPs rely on atomic energy contributions given as a function of the local chemical environments. Frequently, in addition to total energies, atomic forces are also used to construct the potentials, as they provide detailed local information about the PES. Since many systems are too large for electronic structure calculations, obtaining reliable reference forces from smaller subsystems, such as molecular fragments or clusters, can substantially simplify the construction of the training sets. Here, we propose a method to determine structurally converged molecular fragments, providing reliable atomic forces based on an analysis of the Hessian. The method, which serves as a locality test and allows us to estimate the importance of long-range interactions, is illustrated for a series of molecular model systems and the metal-organic framework MOF-5 as an example for a complex organic-inorganic hybrid material. © 2022 Author(s).
    view abstractdoi: 10.1063/5.0082952
  • 2022 • 244 Coupled Cluster Molecular Dynamics of Condensed Phase Systems Enabled by Machine Learning Potentials: Liquid Water Benchmark
    Daru, J. and Forbert, H. and Behler, J. and Marx, D.
    Physical Review Letters 129 (2022)
    Coupled cluster theory is a general and systematic electronic structure method, but in particular the highly accurate "gold standard"coupled cluster singles, doubles and perturbative triples, CCSD(T), can only be applied to small systems. To overcome this limitation, we introduce a framework to transfer CCSD(T) accuracy of finite molecular clusters to extended condensed phase systems using a high-dimensional neural network potential. This approach, which is automated, allows one to perform high-quality coupled cluster molecular dynamics, CCMD, as we demonstrate for liquid water including nuclear quantum effects. The machine learning strategy is very efficient, generic, can be systematically improved, and is applicable to a variety of complex systems. © 2022 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.129.226001
  • 2022 • 243 Coupling of electronic and structural degrees of freedom in vanadate superlattices
    Radhakrishnan, P. and Geisler, B. and Fürsich, K. and Putzky, D. and Wang, Y. and Christiani, G. and Logvenov, G. and Wochner, P. and Van Aken, P.A. and Pentcheva, R. and Benckiser, E.
    Physical Review B 105 (2022)
    Heterostructuring provides different ways to manipulate the orbital degrees of freedom and to tailor orbital occupations in transition-metal oxides. However, the reliable prediction of these modifications remains a challenge. Here we present a detailed investigation of the relationship between the crystal and electronic structure in YVO3-LaAlO3 superlattices by combining ab initio theory, scanning transmission electron microscopy, and x-ray diffraction. Density functional theory simulations including an on-site Coulomb repulsion term accurately predict the crystal structure and, in conjunction with x-ray diffraction, provide an explanation for the lifting of degeneracy of the vanadium dxz and dyz orbitals that was recently observed in this system. In addition, we unravel the combined effects of electronic confinement and octahedral connectivity by disentangling their impact from that of epitaxial strain. Our results demonstrate that the specific orientation of the substrate and the thickness of the YVO3 slabs in the multilayer can be utilized to reliably engineer orbital polarization. © 2022 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.105.165117
  • 2022 • 242 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 • 241 High-dimensional neural network potentials for accurate vibrational frequencies: the formic acid dimer benchmark
    Shanavas Rasheeda, D. and Martín Santa Daría, A. and Schröder, B. and Mátyus, E. and Behler, J.
    Physical Chemistry Chemical Physics 24 29381-29392 (2022)
    In recent years, machine learning potentials (MLP) for atomistic simulations have attracted a lot of attention in chemistry and materials science. Many new approaches have been developed with the primary aim to transfer the accuracy of electronic structure calculations to large condensed systems containing thousands of atoms. In spite of these advances, the reliability of modern MLPs in reproducing the subtle details of the multi-dimensional potential-energy surface is still difficult to assess for such systems. On the other hand, moderately sized systems enabling the application of tools for thorough and systematic quality-control are nowadays rarely investigated. In this work we use benchmark-quality harmonic and anharmonic vibrational frequencies as a sensitive probe for the validation of high-dimensional neural network potentials. For the case of the formic acid dimer, a frequently studied model system for which stringent spectroscopic data became recently available, we show that high-quality frequencies can be obtained from state-of-the-art calculations in excellent agreement with coupled cluster theory and experimental data. © 2022 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d2cp03893e
  • 2022 • 240 Lower degree of dissociation of pyruvic acid at water surfaces than in bulk
    Lesnicki, D. and Wank, V. and Cyran, J.D. and Backus, E.H.G. and Sulpizi, M.
    Physical Chemistry Chemical Physics 24 13510-13513 (2022)
    Understanding the acid/base behavior of environmentally relevant organic acids is of key relevance for accurate climate modelling. Here we investigate the effect of pH on the (de)protonation state of pyruvic acid at the air-water interface and in bulk by using the analytical techniques surface-specific vibrational sum frequency generation and attenuated total reflection spectroscopy. To provide a molecular interpretation of the observed behavior, simulations are carried out using a free energy perturbation approach in combination with electronic structure-based molecular dynamics. In both the experimental and theoretical results we observe that the protonated form of pyruvic acid is preferred at the air-water interface. The increased proton affinity is the result of the specific microsolvation at the interface. © 2022 The Royal Society of Chemistry
    view abstractdoi: 10.1039/d2cp01293f
  • 2022 • 239 Method to Determine the Bifunctional Index for the Oxygen Electrocatalysis from Theory
    Razzaq, S. and Exner, K.S.
    ChemElectroChem 9 (2022)
    Metal-air batteries are encountered as a promising solution for energy storage due to their high energy density, cost effectiveness, and environmental benefits. Yet, the application of metal-air batteries in practice is still not mature, which is also related to the bifunctional oxygen electrocatalysis at the cathode, comprising the oxygen reduction (ORR) and oxygen evolution (OER) reactions during discharge and charge of the battery, respectively. Experimentally, the performance of electrocatalysts in the OER and ORR is described by bifunctional index (BI), but, so far, there is no direct approach to capture the BI on the atomic scale. Herein, we present a method to ascertain the BI from ab initio theory, thereby combining a data-driven methodology with thermodynamic considerations and microkinetic modeling as a function of the applied overpotential. Our approach allows deriving the BI from simple adsorption free energies, which are easily accessible to electronic structure theory in the density functional theory (DFT) approximation. We outline how our methodology may steer the design of efficient bifunctional catalysts on the atomic scale. © 2022 The Authors. ChemElectroChem published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/celc.202101603
  • 2022 • 238 Modulating the frontier orbitals of L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl, Br) and their facile oxidation to radical cations
    Sharma, M.K. and Chabbra, S. and Wölper, C. and Weinert, H.M. and Reijerse, E.J. and Schnegg, A. and Schulz, S.
    Chemical Science (2022)
    Modulating the electronic structures of main group element compounds is crucial to control their chemical reactivity. Herein we report on the synthesis, frontier orbital modulation, and one-electron oxidation of two L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl 2a, Br 2b; L = HC[C(Me)N(Ar)]2, Ar = 2,6-i-Pr2C6H3). Photolysis of L(Cl)GaPCO 1 gave [L(Cl)GaP]22a, which reacted with Me3SiBr with halide exchange to [L(Br)GaP]22b. Reactions with MeNHC (MeNHC = 1,3,4,5-tetramethylimidazol-2-ylidene) gave the corresponding carbene-coordinated complexes L(X)GaPP(MeNHC)Ga(X)L (X = Cl 3a, Br 3b). DFT calculations revealed that the carbene coordination modulates the frontier orbitals (i.e. HOMO/LUMO) of diphosphenes 2a and 2b, thereby affecting the reactivity of 3a and 3b. In marked contrast to diphosphenes 2a and 2b, the cyclic voltammograms (CVs) of the carbene-coordinated complexes each show one reversible redox event at E1/2 = −0.65 V (3a) and −0.36 V (3b), indicating their one-electron oxidation to the corresponding radical cations as was confirmed by reactions of 3a and 3b with the [FeCp2][B(C6F5)4], yielding the radical cations [L(X)GaPP(MeNHC)Ga(X)L]B(C6F5)4 (X = Cl 4a, Br 4b). The unpaired spin in 4a (79%) and 4b (80%) is mainly located at the carbene-uncoordinated phosphorus atoms as was revealed by DFT calculations and furthermore experimentally proven in reactions with nBu3SnH, yielding the diphosphane cations [L(X)GaPHP(MeNHC)Ga(X)L]B(C6F5)4 (X = Cl 5a, Br 5b). Compounds 2-5 were fully characterized by NMR and IR spectroscopy as well as by single crystal X-ray diffraction (sc-XRD), and compounds 4a and 4b were further studied by EPR spectroscopy, while their bonding nature was investigated by DFT calculations. © 2022 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d2sc04207j
  • 2022 • 237 Multilayer atomic cluster expansion for semilocal interactions
    Bochkarev, Anton and Lysogorskiy, Yury and Ortner, Christoph and Csányi, Gábor and Drautz, Ralf
    Physical Review Research 4 (2022)
    Traditionally, interatomic potentials assume local bond formation supplemented by long-range electrostatic interactions when necessary. This ignores intermediate-range multiatom interactions that arise from the relaxation of the electronic structure. Here, we present the multilayer atomic cluster expansion (ml-ACE) that includes collective, semi-local multiatom interactions naturally within its remit. We demonstrate that ml-ACE significantly improves fit accuracy and efficiency compared to a local expansion on selected examples and provide physical intuition to understand this improvement. © 2022 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.
    view abstractdoi: 10.1103/PhysRevResearch.4.L042019
  • 2022 • 236 Synthesis and redox activity of carbene-coordinated group 13 metal radicals
    Li, B. and Geoghegan, B.L. and Weinert, H.M. and Wölper, C. and Cutsail, G.E. and Schulz, S.
    Chemical Communications 58 4372-4375 (2022)
    Carbenes are known to stabilize main group element compounds with unusual electronic properties. Herein, we report the synthesis of carbene-stabilized group 13 metal radicals (cAAC)MX2(IPr) (M = Al, X = Br 3; M = Ga, X = Cl 4) and the corresponding cations [(cAAC)MX2(IPr)][B(C6F5)4] (M = Al, X = Br 5; M = Ga, X = Cl 6), which were characterized spectroscopically and by sc-XRD. Quantum chemical calculation gave insights into their electronic structures. © 2022 The Royal Society of Chemistry
    view abstractdoi: 10.1039/d2cc00216g
  • 2022 • 235 Uniaxially Aligned 1D Sandwich-Molecular Wires: Electronic Structure and Magnetism
    Kraus, S. and Herman, A. and Huttmann, F. and Bianchi, M. and Stan, R.-M. and Holt, A.J. and Tsukamoto, S. and Rothenbach, N. and Ollefs, K. and Dreiser, J. and Bischof, K. and Wende, H. and Hofmann, P. and Atodiresei, N. and Michely, T.
    Journal of Physical Chemistry C 126 3140-3150 (2022)
    Sandwich-molecular wires consisting of europium and cyclooctatetraene (Cot) were grown in situ on the moiré of graphene with Ir(110). The moiré templates a uniaxial alignment of monolayer EuCot nanowire carpets and multilayer films with the EuCot wire axis along the [001] direction of the Ir substrate. Using angle-resolved photoemission spectroscopy, we investigate the band structure of the wire carpet films. While π-derived bands were not observed experimentally, we find a flat band 1.85 eV below the Fermi energy. Using density-functional theory and X-ray photoelectron spectroscopy and replacing europium through barium in the sandwich-molecular wires, it is concluded that the flat band is derived from Eu 4f states weakly mixed with Eu 5d states and slightly overlapping with Cot π states. X-ray magnetic circular dichroism is employed to characterize the magnetic properties of the EuCot wire carpet films at low temperatures. Clear evidence for an easy-axis magnetization along the wires is found. © 2022 American Chemical Society
    view abstractdoi: 10.1021/acs.jpcc.1c10625
  • 2022 • 234 X-Band Parallel-Mode and Multifrequency Electron Paramagnetic Resonance Spectroscopy of S = 1/2 Bismuth Centers
    Haak, J. and Krüger, J. and Abrosimov, N.V. and Helling, C. and Schulz, S. and Cutsail Iii, G.E.
    Inorganic Chemistry 61 11173-11181 (2022)
    The recent successes in the isolation and characterization of several bismuth radicals inspire the development of new spectroscopic approaches for the in-depth analysis of their electronic structure. Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for the characterization of main group radicals. However, the large electron-nuclear hyperfine interactions of Bi (209Bi, I = 9/2) have presented difficult challenges to fully interpret the spectral properties for some of these radicals. Parallel-mode EPR (B1B0) is almost exclusively employed for the study of S > 1/2 systems but becomes feasible for S = 1/2 systems with large hyperfine couplings, offering a distinct EPR spectroscopic approach. Herein, we demonstrate the application of conventional X-band parallel-mode EPR for S = 1/2, I = 9/2 spin systems: Bi-doped crystalline silicon (Si:Bi) and the molecular Bi radicals [L(X)Ga]2Bi >(X = Cl or I) and [L(Cl)GaBi(MecAAC)]+(L = HC[MeCN(2,6-iPr2C6H3)]2). In combination with multifrequency perpendicular-mode EPR (X-, Q-, and W-band frequencies), we were able to fully refine both the anisotropic g-and A-Tensors of these molecular radicals. The parallel-mode EPR experiments demonstrated and discussed here have the potential to enable the characterization of other S = 1/2 systems with large hyperfine couplings, which is often challenging by conventional perpendicular-mode EPR techniques. Considerations pertaining to the choice of microwave frequency are discussed for relevant spin-systems. © 2022 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/acs.inorgchem.2c01141
  • 2021 • 233 A bin and hash method for analyzing reference data and descriptors in machine learning potentials
    Paleico, M.L. and Behler, J.
    Machine Learning: Science and Technology 2 (2021)
    In recent years the development of machine learning potentials (MLPs) has become a very active field of research. Numerous approaches have been proposed, which allow one to perform extended simulations of large systems at a small fraction of the computational costs of electronic structure calculations. The key to the success of modern MLPs is the close-To first principles quality description of the atomic interactions. This accuracy is reached by using very flexible functional forms in combination with high-level reference data from electronic structure calculations. These data sets can include up to hundreds of thousands of structures covering millions of atomic environments to ensure that all relevant features of the potential energy surface are well represented. The handling of such large data sets is nowadays becoming one of the main challenges in the construction of MLPs. In this paper we present a method, the bin-And-hash (BAH) algorithm, to overcome this problem by enabling the efficient identification and comparison of large numbers of multidimensional vectors. Such vectors emerge in multiple contexts in the construction of MLPs. Examples are the comparison of local atomic environments to identify and avoid unnecessary redundant information in the reference data sets that is costly in terms of both the electronic structure calculations as well as the training process, the assessment of the quality of the descriptors used as structural fingerprints in many types of MLPs, and the detection of possibly unreliable data points. The BAH algorithm is illustrated for the example of high-dimensional neural network potentials using atom-centered symmetry functions for the geometrical description of the atomic environments, but the method is general and can be combined with any current type of MLP. © 2021 Machine Learning: Science and Technology. All rights reserved.
    view abstractdoi: 10.1088/2632-2153/abe663
  • 2021 • 232 A Career in Catalysis: Robert Schlögl
    Bao, X. and Behrens, M. and Ertl, G. and Fu, Q. and Knop-Gericke, A. and Lunkenbein, T. and Muhler, M. and Schmidt, C.M. and Trunschke, A.
    ACS Catalysis 11 6243-6260 (2021)
    "Why?"is the question that initiates science. "Why?"is also the answer that maintains science. This interrogative adverb fuels the scientific career of Robert Schlögl. Robert is a dedicated solid-state chemist who has found his specialization in untangling the working principles of heterogeneous catalysts under realistic conditions. As such he combines the full complexity of real catalysts with tailor-made operando experiments to overcome pressure, material, and complexity gaps. His ability to quickly abstract the meaning of spectroscopic and microscopic data, his talent to ask the right question paired with curiosity, diligence, and creativity have made him a world-leading expert in heterogeneous catalysis and energy science. His scientific passion is focused on untangling chemical dynamics as well as working principles and understanding the important interplay of geometric and electronic structures in functional materials. Thereby his research interests involve ammonia and methanol synthesis, carbon materials in catalysis, hydrogenation, and dehydrogenation, selective oxidation, and the development of operando setups for microscopy and spectroscopy. He also has a strong commitment to society in scientifically accelerating the energy transition ("Energiewende") in Europe, where he focuses on CO2 utilization and hydrogen as an energy carrier. This is manifested in three recent large Germany-wide projects: Carbon2Chem, CatLab, and TransHyDe. ©
    view abstractdoi: 10.1021/acscatal.1c01165
  • 2021 • 231 Ab initio based models for temperature-dependent magnetochemical interplay in bcc Fe-Mn alloys
    Schneider, A. and Fu, C.-C. and Waseda, O. and Barreteau, C. and Hickel, T.
    Physical Review B 103 (2021)
    Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.024421
  • 2021 • 230 Ab initio study of the structural response to magnetic disorder and van der Waals interactions in FeSe
    Lochner, F. and Eremin, I.M. and Hickel, T. and Neugebauer, J.
    Physical Review B 103 (2021)
    The electronic structure in unconventional superconductors holds a key to understanding the momentum-dependent pairing interactions and the resulting superconducting gap function. In superconducting Fe-based chalcogenides, there have been controversial results regarding the importance of the kz dependence of the electronic dispersion, the gap structure, and the pairing mechanisms. Here, we use density functional theory to investigate the underlying structural properties in combination with a sophisticated real-space treatment of magnetic disorder for the prototype system FeSe. Our calculations demonstrate that interlayer and intralayer interactions need to be considered and that charge-driven van der Waals interactions between Se atoms instead of magnetic coupling effects drive the interlayer binding. The methodological advances and physical insights are important for upcoming investigations of the three-dimensional effects, including nontrivial topology, of FeSe1-xTex and FeSe1-xSx systems. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.054506
  • 2021 • 229 An automatized workflow from molecular dynamic simulation to quantum chemical methods to identify elementary reactions and compute reaction constants
    Schmitz, G. and Yönder, Ö. and Schnieder, B. and Schmid, R. and Hättig, C.
    Journal of Computational Chemistry 42 2264-2282 (2021)
    We present an automatized workflow which, starting from molecular dynamics simulations, identifies reaction events, filters them, and prepares them for accurate quantum chemical calculations using, for example, Density Functional Theory (DFT) or Coupled Cluster methods. The capabilities of the automatized workflow are demonstrated by the example of simulations for the combustion of some polycyclic aromatic hydrocarbons (PAHs). It is shown how key elementary reaction candidates are filtered out of a much larger set of redundant reactions and refined further. The molecular species in question are optimized using DFT and reaction energies, barrier heights, and reaction rates are calculated. The setup is general enough to include at this stage configurational sampling, which can be exploited in the future. Using the introduced machinery, we investigate how the observed reaction types depend on the gas atmosphere used in the molecular dynamics simulation. For the re-optimization on the DFT level, we show how the additional information needed to switch from reactive force-field to electronic structure calculations can be filled in and study how well ReaxFF and DFT agree with each other and shine light on the perspective of using more accurate semi-empirical methods in the MD simulation. © 2021 The Authors. Journal of Computational Chemistry published by Wiley Periodicals LLC.
    view abstractdoi: 10.1002/jcc.26757
  • 2021 • 228 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 • 227 Controlling Oxygen Reduction Selectivity through Steric Effects: Electrocatalytic Two-Electron and Four-Electron Oxygen Reduction with Cobalt Porphyrin Atropisomers
    Lv, B. and Li, X. and Guo, K. and Ma, J. and Wang, Y. and Lei, H. and Wang, F. and Jin, X. and Zhang, Q. and Zhang, W. and Long, R. and Xiong, Y. and Apfel, U.-P. and Cao, R.
    Angewandte Chemie - International Edition 60 12742-12746 (2021)
    Achieving a selective 2 e− or 4 e− oxygen reduction reaction (ORR) is critical but challenging. Herein, we report controlling ORR selectivity of Co porphyrins by tuning only steric effects. We designed Co porphyrin 1 with meso-phenyls each bearing a bulky ortho-amido group. Due to the resulted steric hinderance, 1 has four atropisomers with similar electronic structures but dissimilar steric effects. Isomers αβαβ and αααα catalyze ORR with n=2.10 and 3.75 (n is the electron number transferred per O2), respectively, but ααββ and αααβ show poor selectivity with n=2.89–3.10. Isomer αβαβ catalyzes 2 e− ORR by preventing a bimolecular O2 activation path, while αααα improves 4 e− ORR selectivity by improving O2 binding at its pocket, a feature confirmed by spectroscopy methods, including O K-edge near-edge X-ray absorption fine structure. This work represents an unparalleled example to improve 2 e− and 4 e− ORR by tuning only steric effects without changing molecular and electronic structures. © 2021 Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202102523
  • 2021 • 226 Determination of spin-wave stiffness in the Fe-Si system using first-principles calculations
    Rinaldi, M. and Mrovec, M. and Fähnle, M. and Drautz, R.
    Physical Review B 104 (2021)
    The behavior of magnetic materials can be simulated at the macroscale using the micromagnetic model whose key parameters, such as exchange stiffness constants and magnetic anisotropies, can be derived from first-principles electronic structure calculations. In this work we employed the Korringa-Kohn-Rostoker (KKR) Green's function method with the coherent potential approximation (CPA) to investigate the dependence of the spin-wave stiffness on the Si concentration for the three magnetic phases of FeSi, namely A2, B2, and D03. Based on the structural, magnetic, and electronic structure analysis using the KKR-CPA methodology, the changes in the spin-wave stiffness caused by the addition of Si are primarily governed by the variations in the electronic structure. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.104.064413
  • 2021 • 225 Evidence for a large Rashba splitting in PtPb4 from angle-resolved photoemission spectroscopy
    Lee, K. and Mou, D. and Jo, N.H. and Wu, Y. and Schrunk, B. and Wilde, J.M. and Kreyssig, A. and Estry, A. and Bud'Ko, S.L. and Nguyen, M.C. and Wang, L.-L. and Wang, C.-Z. and Ho, K.-M. and Canfield, P.C. and Kaminski, A.
    Physical Review B 103 (2021)
    We studied the electronic structure of PtPb4 using laser angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. This material is closely related to PtSn4, which exhibits exotic topological properties such as Dirac node arcs. The Fermi surface (FS) of PtPb4 consists of two electron pockets at the center of the Brillouin zone (BZ) and several hole pockets around the zone boundaries. Our ARPES data reveal significant Rashba splitting at the Γ point, in agreement with DFT calculations. The presence of Rashba splitting may render this material of potential interest for spintronic applications. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.085125
  • 2021 • 224 Ferrous to Ferric Transition in Fe-Phthalocyanine Driven by NO2 Exposure
    Cojocariu, I. and Carlotto, S. and Sturmeit, H.M. and Zamborlini, G. and Cinchetti, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Jugovac, M. and Puschnig, P. and Piamonteze, C. and Casarin, M. and Feyer, V. and Schneider, C.M.
    Chemistry - A European Journal 27 3526-3535 (2021)
    Due to its unique magnetic properties offered by the open-shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d-states of FePc and the sp-band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature. © 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202004932
  • 2021 • 223 Finite-size correction for slab supercell calculations of materials with spontaneous polarization
    Yoo, S.-H. and Todorova, M. and Wickramaratne, D. and Weston, L. and Walle, C.G.V. and Neugebauer, J.
    npj Computational Materials 7 (2021)
    The repeated slab approach has become a de facto standard to accurately describe surface properties of materials by density functional theory calculations with periodic boundary conditions. For materials exhibiting spontaneous polarization, we show that the conventional scheme of passivation with pseudo hydrogen is unable to realize a charge-neutral surface. The presence of a net surface charge induces via Gauss’s law a macroscopic electric field through the slab and results in poor size convergence with respect to the thickness of the slab. We propose a modified passivation method that accounts for the effect of spontaneous polarization, describes the correct bulk limits and boosts convergence with respect to slab thickness. The robustness, reliability, and superior convergence of energetics and electronic structure achieved by the proposed method are demonstrated using the example of polar ZnO surfaces. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41524-021-00529-1
  • 2021 • 222 Four Generations of High-Dimensional Neural Network Potentials
    Behler, J.
    Chemical Reviews 121 10037-10072 (2021)
    Since their introduction about 25 years ago, machine learning (ML) potentials have become an important tool in the field of atomistic simulations. After the initial decade, in which neural networks were successfully used to construct potentials for rather small molecular systems, the development of high-dimensional neural network potentials (HDNNPs) in 2007 opened the way for the application of ML potentials in simulations of large systems containing thousands of atoms. To date, many other types of ML potentials have been proposed continuously increasing the range of problems that can be studied. In this review, the methodology of the family of HDNNPs including new recent developments will be discussed using a classification scheme into four generations of potentials, which is also applicable to many other types of ML potentials. The first generation is formed by early neural network potentials designed for low-dimensional systems. High-dimensional neural network potentials established the second generation and are based on three key steps: First, the expression of the total energy as a sum of environment-dependent atomic energy contributions; second, the description of the atomic environments by atom-centered symmetry functions as descriptors fulfilling the requirements of rotational, translational, and permutation invariance; and third, the iterative construction of the reference electronic structure data sets by active learning. In third-generation HDNNPs, in addition, long-range interactions are included employing environment-dependent partial charges expressed by atomic neural networks. In fourth-generation HDNNPs, which are just emerging, in addition, nonlocal phenomena such as long-range charge transfer can be included. The applicability and remaining limitations of HDNNPs are discussed along with an outlook at possible future developments. © 2021 The Author. Published by American Chemical Society.
    view abstractdoi: 10.1021/acs.chemrev.0c00868
  • 2021 • 221 Free Molecule Studies by Perturbed γ-γ Angular Correlation: A New Path to Accurate Nuclear Quadrupole Moments
    Haas, H. and Röder, J. and Correia, J.G. and Schell, J. and Fenta, A.S. and Vianden, R. and Larsen, E.M.H. and Aggelund, P.A. and Fromsejer, R. and Hemmingsen, L.B.S. and Sauer, S.P.A. and Lupascu, D.C. and Amaral, V.S.
    Physical Review Letters 126 (2021)
    Accurate nuclear quadrupole moment values are essential as benchmarks for nuclear structure models and for the interpretation of experimentally determined nuclear quadrupole interactions in terms of electronic and molecular structure. Here, we present a novel route to such data by combining perturbed γ-γ angular correlation measurements on free small linear molecules, realized for the first time within this work, with state-of-the-art ab initio electronic structure calculations of the electric field gradient at the probe site. This approach, also feasible for a series of other cases, is applied to Hg and Cd halides, resulting in Q(Hg199,5/2-)=+0.674(17) b and Q(Cd111,5/2+)=+0.664(7) b. © 2021 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.126.103001
  • 2021 • 220 Importance of catalyst–photoabsorber interface design configuration on the performance of Mo-doped BiVO4 water splitting photoanodes
    Krysiak, O.A. and Junqueira, J.R.C. and Conzuelo, F. and Bobrowski, T. and Masa, J. and Wysmolek, A. and Schuhmann, W.
    Journal of Solid State Electrochemistry 25 173-185 (2021)
    Photoelectrochemical water splitting is mostly impeded by the slow kinetics of the oxygen evolution reaction. The construction of photoanodes that appreciably enhance the efficiency of this process is of vital technological importance towards solar fuel synthesis. In this work, Mo-modified BiVO4 (Mo:BiVO4), a promising water splitting photoanode, was modified with various oxygen evolution catalysts in two distinct configurations, with the catalysts either deposited on the surface of Mo:BiVO4 or embedded inside a Mo:BiVO4 film. The investigated catalysts included monometallic, bimetallic, and trimetallic oxides with spinel and layered structures, and nickel boride (NixB). In order to follow the influence of the incorporated catalysts and their respective properties, as well as the photoanode architecture on photoelectrochemical water oxidation, the fabricated photoanodes were characterised for their optical, morphological, and structural properties, photoelectrocatalytic activity with respect to evolved oxygen, and recombination rates of the photogenerated charge carriers. The architecture of the catalyst-modified Mo:BiVO4 photoanode was found to play a more decisive role than the nature of the catalyst on the performance of the photoanode in photoelectrocatalytic water oxidation. Differences in the photoelectrocatalytic activity of the various catalyst-modified Mo:BiVO4 photoanodes are attributed to the electronic structure of the materials revealed through differences in the Fermi energy levels. This work thus expands on the current knowledge towards the design of future practical photoanodes for photoelectrocatalytic water oxidation. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10008-020-04636-9
  • 2021 • 219 Nanoscopic Surface Decomposition of Pr0.5Ba0.5CoO3-δPerovskites Turns Performance Descriptors Ambiguous
    Mueller, D.N. and Giesen, M. and Duchoň, T. and Cramm, S. and Gunkel, F. and Jugovac, M. and Zamborlini, G. and Feyer, V. and Schneider, C.M.
    Journal of Physical Chemistry C 125 10043-10050 (2021)
    The surface electronic structure of a material is frequently used to identify simple descriptors for its catalytic efficacy and other properties. To harness the predictive ability of such descriptors, structural and chemical evolutions of the material when exposed to operating conditions, such as oxidizing environments and high temperatures, need to be considered. These evolutions occur at length scales not easily observable, leading to averaging over short-range variations and thus misinterpretation of the property in question. Here, we investigate perovskite Pr0.5Ba0.5CoO3-δ as a prototypical mixed ionic-electronic conductor that exhibits promising catalytic properties toward the oxygen evolution reaction in electrochemical cells, which have been characterized by such descriptors. We employ spatially resolved X-ray absorption spectroscopy and find a Cahn-Hilliard-type decomposition process at sub-micrometer length scales after mere hours at operating or processing conditions. The observation is in contrast to the thermodynamic stability of the Pr0.5Ba0.5CoO3-δ bulk, suggesting the decomposition to be confined to the surface. Our results showcase a considerable lateral inhomogeneity of the surface electronic structure, emphasizing that descriptors derived through spatially averaging techniques have to be heavily scrutinized. ©
    view abstractdoi: 10.1021/acs.jpcc.1c00976
  • 2021 • 218 On the structure-property relationships of (Al, Ga, In)-doped spinel cobalt ferrite compounds: A combined experimental and DFT study
    Naveed-Ul-Haq, M. and Hussain, S. and Webers, S. and Salamon, S. and Ahmad, I. and Bibi, T. and Hameed, A. and Wende, H.
    Physical Chemistry Chemical Physics 23 18112-18124 (2021)
    We report a combined experimental and theoretical study of pure and doped cobalt ferrite where 25% of Fe3+ ions were replaced by Al3+, Ga3+, and In3+ ions, respectively, i.e., CoFe1.5X0.5O4 (X = Al, Ga, and In). The ferrite compositions were successfully synthesized using the solid-state reaction method. The X-ray powder diffraction method established that all ferrite samples had a spinel unit cell structure with the Fd3m (No. 227) space group. The lattice constants of ferrites increased from 8.382 Å (for undoped CoFe2O4) to 8.520 Å (for In-doped cobalt ferrite) in direct relation to the dopant ion size. The magnetic properties were obtained at 4.3 K and 300 K. At 4.3 K, the In-doped CoFe2O4 showed the highest saturation magnetic moment of 4.68 μB f.u.-1, while Al-doped CoFe2O4 showed the smallest value of 2.72 μB f.u.-1. The Fe3+ distribution among the spinel tetrahedral and octahedral sites was determined from the Mössbauer spectra. From ultraviolet-visible diffuse reflectance spectroscopy the direct optical bandgaps were determined, which have values between 1.20 eV and 1.28 eV for these ferrites. The ferrite compositions were also studied theoretically using plane-wave density functional theory using the CASTEP code where it was revealed that arrangements of the non-magnetic cations at the tetrahedral and octahedral sites strongly influence the electronic structure, the bandgap value, and the net magnetic moment per formula unit. Light Al3+ ions at the octahedral site give a low value of the net magnetic moment while the heavier Ga3+ and In3+ ions at the tetrahedral sites of the spinel give an enhanced magnetic moment. The magnetic moment values obtained from theoretical calculations match very well with the experimental values. Moreover, the theoretical calculations reveal that there exists a strong p-d hybridization among the oxygen and magnetic ions, which is affected by the non-magnetic dopant ions. The change in hybridization with the non-magnetic ion doping is responsible for the altered magnetic moments of the doped ferrites. Thus, our study provides a comprehensive investigation covering the synthesis and characterization of ferrites along with a good understanding of the phenomenon of how non-magnetic ion doping into spinel ferrites provides a method to tune the electronic and magnetic properties of the spinel ferrite. This journal is © the Owner Societies.
    view abstractdoi: 10.1039/d1cp02625a
  • 2021 • 217 Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi4Te7 and MnBi6Te10
    Vidal, R.C. and Bentmann, H. and Facio, J.I. and Heider, T. and Kagerer, P. and Fornari, C.I. and Peixoto, T.R.F. and Figgemeier, T. and Jung, S. and Cacho, C. and Büchner, B. and Van Den Brink, J. and Schneider, C.M. and Plucins...
    Physical Review Letters 126 (2021)
    Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi4Te7 and MnBi6Te10, the n=1 and 2 members of a modular (Bi2Te3)n(MnBi2Te4) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi2Te3-terminated surfaces but remains preserved for MnBi2Te4-terminated surfaces. Our results firmly establish the topologically nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.126.176403
  • 2021 • 216 Orbital contributions in the element-resolved valence electronic structure of Bi2Se3
    Kuo, C.-T. and Lin, S.-C. and Rueff, J.-P. and Chen, Z. and Aguilera, I. and Bihlmayer, G. and Plucinski, L. and Graff, I.L. and Conti, G. and Vartanyants, I.A. and Schneider, C.M. and Fadley, C.S.
    Physical Review B 104 (2021)
    In this work, we studied the bulk band structure of a topological insulator (TI) Bi2Se3 and determined the contributions of the Bi and Se orbital states to the valence bands using standing-wave excited hard x-ray photoemission spectroscopy (SW-HAXPES). This SW technique can provide the element-resolved information and extract individual Bi and Se contributions to the Bi2Se3 valence band. Comparisons with density-functional theory calculations (local density approximation and GW) reveal that the Bi 6s, Bi 6p, and Se 4p states are dominant in the Bi2Se3 HAXPES valence band. These findings pave a way for studying the element-resolved band structure and orbital contributions of this class of TIs. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.104.245105
  • 2021 • 215 Orbital engineering in YVO3-LaAl O3 superlattices
    Radhakrishnan, P. and Geisler, B. and Fürsich, K. and Putzky, D. and Wang, Y. and Ilse, S.E. and Christiani, G. and Logvenov, G. and Wochner, P. and Van Aken, P.A. and Goering, E. and Pentcheva, R. and Benckiser, E.
    Physical Review B 104 (2021)
    Oxide heterostructures provide unique opportunities to modify the properties of quantum materials through a targeted manipulation of spin, charge, and orbital states. Here, we use resonant x-ray reflectometry to probe the electronic structure of thin slabs of YVO3 embedded in a superlattice with LaAlO3. We extend the previously established methods of reflectometry analysis to a general form applicable to t2g electron systems and extract quantitative depth-dependent x-ray linear dichroism profiles. Our data reveal an artificial, layered orbital polarization, where the average occupation of xz and yz orbitals in the interface planes next to LaAlO3 is inverted compared to the central part of the YVO3 slab. This phase is stable down to 30 K and the bulklike orbital ordering transitions are absent. We identify the key mechanism for the electronic reconstruction to be a combination of epitaxial strain and spatial confinement by the LaAlO3 layers, in good agreement with predictions from ab initio theory. © 2021 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.104.L121102
  • 2021 • 214 Photoluminescence of Fully Inorganic Colloidal Gold Nanocluster and Their Manipulation Using Surface Charge Effects
    Ziefuss, A.R. and Steenbock, T. and Benner, D. and Plech, A. and Göttlicher, J. and Teubner, M. and Grimm-Lebsanft, B. and Rehbock, C. and Comby-Zerbino, C. and Antoine, R. and Amans, D. and Chakraborty, I. and Bester, G. and Nac...
    Advanced Materials (2021)
    Fully inorganic, colloidal gold nanoclusters (NCs) constitute a new class of nanomaterials that are clearly distinguishable from their commonly studied metal–organic ligand-capped counterparts. As their synthesis by chemical methods is challenging, details about their optical properties remain widely unknown. In this work, laser fragmentation in liquids is performed to produce fully inorganic and size-controlled colloidal gold NCs with monomodal particle size distributions and an fcc-like structure. Results reveal that these NCs exhibit highly pronounced photoluminescence with quantum yields of 2%. The emission behavior of small (2–2.5 nm) and ultrasmall (<1 nm) NCs is significantly different and dominated by either core- or surface-based emission states. It is further verified that emission intensities are a function of the surface charge density, which is easily controllable by the pH of the surrounding medium. This experimentally observed correlation between surface charge and photoluminescence emission intensity is confirmed by density functional theoretical simulations, demonstrating that fully inorganic NCs provide an appropriate material to bridge the gap between experimental and computational studies of NCs. The presented study deepens the understanding of electronic structures in fully inorganic colloidal gold NCs and how to systematically tune their optical properties via surface charge density and particle size. © 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adma.202101549
  • 2021 • 213 Principles of Water Electrolysis and Recent Progress in Cobalt-, Nickel-, and Iron-Based Oxides for the Oxygen Evolution Reaction
    Yu, M. and Budiyanto, E. and Tüysüz, H.
    Angewandte Chemie - International Edition (2021)
    Water electrolysis that results in green hydrogen is the key process towards a circular economy. The supply of sustainable electricity and availability of oxygen evolution reaction (OER) electrocatalysts are the main bottlenecks of the process for large-scale production of green hydrogen. A broad range of OER electrocatalysts have been explored to decrease the overpotential and boost the kinetics of this sluggish half-reaction. Co-, Ni-, and Fe-based catalysts have been considered to be potential candidates to replace noble metals due to their tunable 3d electron configuration and spin state, versatility in terms of crystal and electronic structures, as well as abundance in nature. This Review provides some basic principles of water electrolysis, key aspects of OER, and significant criteria for the development of the catalysts. It provides also some insights on recent advances of Co-, Ni-, and Fe-based oxides and a brief perspective on green hydrogen production and the challenges of water electrolysis. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202103824
  • 2021 • 212 Properties of α-Brass Nanoparticles II: Structure and Composition
    Weinreich, J. and Paleico, M.L. and Behler, J.
    Journal of Physical Chemistry C 125 14897-14909 (2021)
    Nanoparticles have become increasingly interesting for a wide range of applications because in principle it is possible to tailor their properties by controlling size, shape, and composition. One of these applications is heterogeneous catalysis, and a fundamental understanding of the structural details of the nanoparticles is essential for any knowledge-based improvement of reactivity and selectivity. In this work, we investigate the atomic structure of brass nanoparticles containing up to 5000 atoms as a typical example for a binary alloy consisting of Cu and Zn. As systems of this size are too large for electronic structure calculations, in our simulations, we use a recently parameterized machine learning potential providing close to density functional theory accuracy. This potential is employed for a structural characterization as a function of chemical composition by various types of simulations such as Monte Carlo in the semigrand canonical ensemble and simulated annealing molecular dynamics. Our analysis reveals that the distribution of both elements in the nanoparticles is inhomogeneous, and zinc accumulates in the outermost layer, while the first subsurface layer shows an enrichment of copper. Only for high zinc concentrations, alloying can be found in the interior of the nanoparticles, and regular patterns corresponding to crystalline bulk phases of α-brass can then be observed. The surfaces of the investigated clusters exhibit well-ordered single-crystal facets, which can give rise to grain boundaries inside the clusters. The melting temperature of the nanoparticles is found to decrease with increasing zinc-atom fraction, a trend which is well known also for the bulk phase diagram of brass. © 2021 The Authors. Published by American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.1c02314
  • 2021 • 211 Recombination Dynamics in PbS Nanocrystal Quantum Dot Solar Cells Studied through Drift-Diffusion Simulations
    Lin, W.M.M. and Yazdani, N. and Yarema, O. and Yarema, M. and Liu, M. and Sargent, E.H. and Kirchartz, T. and Wood, V.
    ACS Applied Electronic Materials (2021)
    The significant performance increase in nanocrystal (NC)-based solar cells over the last decade is very encouraging. However, many of these gains have been achieved by trial-and-error optimization, and a systematic understanding of what limits the device performance is lacking. In parallel, experimental and computational techniques provide increasing insights into the electronic properties of individual NCs and their assemblies in thin films. Here, we utilize these insights to parameterize drift-diffusion simulations of PbS NC solar cells, which enable us to track the distribution of charge carriers in the device and quantify recombination dynamics, which limit the device performance. We simulate both Schottky- and heterojunction-type devices and, through temperature-dependent measurements in the light and dark, experimentally validate the appropriateness of the parameterization. The results reveal that Schottky-type devices are limited by surface recombination between the PbS and aluminum contact, while heterojunction devices are currently limited by NC dopants and electronic defects in the PbS layer. The simulations highlight a number of opportunities for further performance enhancement, including the reduction of dopants in the nanocrystal active layer, the control over doping and electronic structure in electron- and hole-blocking layers (e.g., ZnO), and the optimization of the interfaces to improve the band alignment and reduce surface recombination. For example, reduction in the percentage of p-type NCs from the current 1-0.01% in the heterojunction device can lead to a 25% percent increase in the power conversion efficiency. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acsaelm.1c00787
  • 2021 • 210 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 • 209 Stabilization of an iridium oxygen evolution catalyst by titanium oxides
    Kasian, O. and Li, T. and Mingers, A.M. and Schweinar, K. and Savan, A. and Ludwig, A. and Mayrhofer, K.
    JPhys Energy 3 (2021)
    The anodic oxygen evolution reaction (OER) has significant importance in many electrochemical technologies. In proton exchange membrane water electrolyzers it plays a pivotal role for electrochemical energy conversion, yet sluggish kinetics and the corrosive environment during operation still compel significant advances in electrode materials to enable a widespread application. Up-To-date Iridium is known as the best catalyst material for the OER in acidic media due to its relatively high activity and long-Term stability. However, scarcity of iridium drives the development of strategies for its efficient utilization. One promising way would be the formation of mixtures in which the noble catalyst element is dispersed in the non-noble matrix of more stable metals or metal oxides. A promising valve metal oxide is TiOx, yet the degree to which performance can be optimized by composition is still unresolved. Thus, using a scanning flow cell connected to an inductively coupled plasma mass spectrometer, we examined the activity and stability for the OER of an oxidized Ir Ti thin film material library covering the composition range from 20 70 at.% of Ir. We find that regardless of the composition the rate of Ir dissolution is observed to be lower than that of thermally prepared IrO2. Moreover, mixtures containing at least 50 at.% of Ir exhibit reactivity comparable to IrO2. Their superior performance is discussed with complementary information obtained from atomic scale and electronic structure analysis using atom probe tomography and x-ray photoelectron spectroscopy. Overall, our data shows that Ir Ti mixtures can be promising OER catalysts with both high activity and high stability. © 2021 JPhys Energy. All right reserved.
    view abstractdoi: 10.1088/2515-7655/abbd34
  • 2021 • 208 Theoretical description of optical and X-ray absorption spectra of MgO including many-body effects
    Begum, V. and Gruner, M.E. and Vorwerk, C. and Draxl, C. and Pentcheva, R.
    Physical Review B 103 (2021)
    Here we report the optical and X-ray absorption spectra of the wide-band-gap oxide MgO using density functional theory and many-body perturbation theory (MBPT). Our comprehensive study of the electronic structure shows that while the band gap is underestimated with the exchange-correlation functional PBEsol (4.58 eV) and the hybrid functional HSE06 (6.58 eV) compared to the experimental value (7.7 eV), it is significantly improved (7.52 eV) and even overcompensated (8.53 eV) when quasiparticle corrections are considered. Inclusion of excitonic effects by solving the Bethe-Salpeter equation (BSE) yields the optical spectrum in excellent agreement with experiment. Excellent agreement is observed also for the O and Mg K-edge absorption spectra, demonstrating the importance of the electron-hole interaction within MBPT. Projection of the electron-hole coupling coefficients from the BSE eigenvectors on the band structure allows us to determine the origin of prominent peaks and identify the orbital character of the relevant contributions. The real-space projection of the lowest energy exciton wave function of the optical spectrum indicates a Wannier-Mott type, whereas the first exciton in the O K edge is more localized. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.195128
  • 2021 • 207 Tunable eg Orbital Occupancy in Heusler Compounds for Oxygen Evolution Reaction**
    Yu, M. and Li, G. and Fu, C. and Liu, E. and Manna, K. and Budiyanto, E. and Yang, Q. and Felser, C. and Tüysüz, H.
    Angewandte Chemie - International Edition 60 5800-5805 (2021)
    Heusler compounds have potential in electrocatalysis because of their mechanical robustness, metallic conductivity, and wide tunability in the electronic structure and element compositions. This study reports the first application of Co2YZ-type Heusler compounds as electrocatalysts for the oxygen evolution reaction (OER). A range of Co2YZ crystals was synthesized through the arc-melting method and the eg orbital filling of Co was precisely regulated by varying Y and Z sites of the compound. A correlation between the eg orbital filling of reactive Co sites and OER activity was found for Co2MnZ compounds (Z=Ti, Al, V, and Ga), whereby higher catalytic current was achieved for eg orbital filling approaching unity. A similar trend of eg orbital filling on the reactivity of cobalt sites was also observed for other Heusler compounds (Co2VZ, Z=Sn and Ga). This work demonstrates proof of concept in the application of Heusler compounds as a new class of OER electrocatalysts, and the influence of the manipulation of the spin orbitals on their catalytic performance. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202013610
  • 2020 • 206 Closing the gap between theory and experiment for lithium manganese oxide spinels using a high-dimensional neural network potential
    Eckhoff, M. and Schönewald, F. and Risch, M. and Volkert, C.A. and Blöchl, P.E. and Behler, J.
    Physical Review B 102 (2020)
    Many positive electrode materials in lithium ion batteries include transition metals, which are difficult to describe by electronic structure methods like density functional theory (DFT) due to the presence of multiple oxidation states. A prominent example is the lithium manganese oxide spinel LixMn2O4 with 0≤x≤2. While DFT, employing the local hybrid functional PBE0r, provides a reliable description, the need for extended computer simulations of large structural models remains a significant challenge. Here, we close this gap by constructing a DFT-based high-dimensional neural network potential (HDNNP) providing accurate energies and forces at a fraction of the computational costs. As different oxidation states and the resulting Jahn-Teller distortions represent a new level of complexity for HDNNPs, the potential is carefully validated by performing x-ray diffraction experiments. We demonstrate that the HDNNP provides atomic level details and is able to predict a series of properties like the lattice parameters and expansion with increasing Li content or temperature, the orthorhombic to cubic transition, the lithium diffusion barrier, and the phonon frequencies. We show that for understanding these properties access to large time and length scales as enabled by the HDNNP is essential to close the gap between theory and experiment. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.174102
  • 2020 • 205 Doublon bottleneck in the ultrafast relaxation dynamics of hot electrons in 1T-TaS2
    Avigo, I. and Queisser, F. and Zhou, P. and Ligges, M. and Rossnagel, K. and Schützhold, R. and Bovensiepen, U.
    Physical Review Research 2 (2020)
    Employing time-resolved photoelectron spectroscopy we analyze the relaxation dynamics of hot electrons in the charge density wave/Mott material 1T-TaS2. At 1.2 eV above the Fermi level we observe a hot electron lifetime of 12±5 fs in the metallic state and of 60±10 fs in the broken symmetry ground state - a direct consequence of the reduced phase space for electron-electron scattering determined by the Mott gap. Boltzmann equation calculations which account for the interaction of hot electrons in a Bloch band with a doublon-holon excitation in the Mott state provide insight into the unoccupied electronic structure in the correlated state. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevResearch.2.022046
  • 2020 • 204 Electron tunneling dynamics between two-dimensional and zero-dimensional quantum systems: Contributions of momentum matching, higher subbands, and phonon-assisted processes
    Korsch, A.R. and Ebler, C. and Nguyen, G.N. and Scholz, S. and Wieck, A.D. and Ludwig, Ar.
    Physical Review B 102 (2020)
    We investigate tunneling dynamics of electrons from an ensemble of self-assembled InAs quantum dots into the subbands of a two-dimensional electron gas (2DEG). LO-phonon-assisted tunneling processes and tunneling into higher subbands of the 2DEG electronic structure cause distinct resonances in the evolution of the tunneling rate as a function of the energy detuning between quantum dot and 2DEG ground state. By devising a semiquantitative model, we identify the momentum mismatch between the quantum dot and 2DEG wave function as the crucial quantity governing the evolution of the tunneling rate. In particular, we demonstrate that this mechanism along with the availability of tunneling into the second 2DEG subband allows for tuning of the tunneling rate by more than two orders of magnitude. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.035413
  • 2020 • 203 Electronic structure based design of thin film metallic glasses with superior fracture toughness
    Evertz, S. and Kirchlechner, I. and Soler, R. and Kirchlechner, C. and Kontis, P. and Bednarcik, J. and Gault, B. and Dehm, G. and Raabe, D. and Schneider, J.M.
    Materials and Design 186 (2020)
    High fracture toughness is crucial for the application of metallic glasses as structural materials to avoid catastrophic failure of the material in a brittle manner. One fingerprint for fracture toughness in metallic glasses is the fraction of hybridized bonds, which is affected by alloying Pd57.4Al23.5Y7.8M11.3 with M = Fe, Ni, Co, Cu, Os, Ir, Pt, and Au. It is shown that experimental fracture toughness data is correlated to the fraction of hybridized bonds which scale with the localized bonds at the Fermi level. Thus, the localized bonds at the Fermi level are utilized quantitatively as a measure for fracture toughness. Based on ab initio calculations, the minimum fraction of hybridized bonds was identified for Pd57.4Al23.5Y7.8Ni11.3. According to the ansatz that the crystal orbital overlap population at the Fermi level scales with fracture toughness, for Pd57.4Al23.5Y7.8Ni11.3 a value of around 95 ± 20 MPa·m0.5 is predicted quantitatively for the first time. Consistent with this prediction, in micro-mechanical beam bending experiments Pd57.4Al23.5Y7.8Ni11.3 thin films show pronounced plasticity and absence of crack growth. © 2018 The Authors
    view abstractdoi: 10.1016/j.matdes.2019.108327
  • 2020 • 202 Electronic structure of isolated molecules
    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 25-34 (2020)
    Phthalocyanine molecules with a 3d transition metal in the center, like MnPc, FePc, CoPc, NiPc and CuPc, have attracted a huge interest in the last decades due to the large number of possible applications. Experimental and theoretical gas phase studies are an important reference to understand the properties of the molecules, as well as how they can be modified and manipulated upon deposition on substrates or in supramolecular conformations. However, in several 3d metal phthalocyanines the electronic structure of the single molecule is still under debate even after several spectroscopical studies and computational works have been performed. This is mostly due to the highly correlated 3d electrons of the metal atoms, which pose a challenge for the theory. In addition, the experiments to determine the electronic structure are often carried out in different conditions (on thick films or in gas phase for example), and this can lead to different results. The following chapter provides an overview of the theoretical and experimental results and debates related to the electronic structure of gas phase MnPc, FePc, CoPc, NiPc and CuPc. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstractdoi: 10.1007/978-981-15-3719-6_4
  • 2020 • 201 Experimental techniques
    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 5-17 (2020)
    Different spectroscopic methods can be used to characterize the electronic structure of a system of interest, i.e. molecular, solid or adsorbate samples. The different techniques give complementary information about the geometric and electronic structure of the system. By Photoelectron Spectroscopy (PES), Auger and resonant photoemission (RPES) the occupied electronic levels can be studied, whereas X-ray Absorption Spectroscopy (XAS) gives information about the unoccupied valence states of the system in presence, however, of a core hole. Magnetic information can be obtained from X-ray Magnetic Circular Dichroism (XMCD). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstractdoi: 10.1007/978-981-15-3719-6_2
  • 2020 • 200 Formation of a 2D Meta-stable Oxide by Differential Oxidation of AgCu Alloys
    Schweinar, K. and Beeg, S. and Hartwig, C. and Rajamathi, C.R. and Kasian, O. and Piccinin, S. and Prieto, M.J. and Tanase, L.C. and Gottlob, D.M. and Schmidt, T. and Raabe, D. and Schlögl, R. and Gault, B. and Jones, T.E. and Gr...
    ACS Applied Materials and Interfaces 12 23595-23605 (2020)
    Metal alloy catalysts can develop complex surface structures when exposed to reactive atmospheres. The structures of the resulting surfaces have intricate relationships with a myriad of factors, such as the affinity of the individual alloying elements to the components of the gas atmosphere and the bond strengths of the multitude of low-energy surface compounds that can be formed. Identifying the atomic structure of such surfaces is a prerequisite for establishing structure-property relationships, as well as for modeling such catalysts in ab initio calculations. Here, we show that an alloy, consisting of an oxophilic metal (Cu) diluted into a noble metal (Ag), forms a meta-stable two-dimensional oxide monolayer, when the alloy is subjected to oxidative reaction conditions. The presence of this oxide is correlated with selectivity in the corresponding test reaction of ethylene epoxidation. In the present study, using a combination of in situ, ex situ, and theoretical methods (NAP-XPS, XPEEM, LEED, and DFT), we determine the structure to be a two-dimensional analogue of Cu2O, resembling a single lattice plane of Cu2O. The overlayer holds a pseudo-epitaxial relationship with the underlying noble metal. Spectroscopic evidence shows that the oxide's electronic structure is qualitatively distinct from its three-dimensional counterpart, and because of weak electronic coupling with the underlying noble metal, it exhibits metallic properties. These findings provide precise details of this peculiar structure and valuable insights into how alloying can enhance catalytic properties. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c03963
  • 2020 • 199 From photoemission microscopy to an “All-in-One” photoemission experiment
    Tusche, C. and Chen, Y.-J. and Plucinski, L. and Schneider, C.M.
    e-Journal of Surface Science and Nanotechnology 18 48-56 (2020)
    Photoelectron spectroscopy is our main tool to explore the electronic structure of novel material systems, the properties of which are often determined by an intricate interplay of competing interactions. Elucidating the role of this interactions requires studies over an extensive range of energy, momentum, length, and time scales. We show that immersion lens-based momentum microscopy with spin-resolution is able to combine these seemingly divergent requirements in a unifying experimental approach. We will discuss applications to different areas in information research, for example, resistive switching and spintronics. The analysis of resistive switching phenomena in oxides requires high lateral resolution and chemical selectivity, as the processes involve local redox processes and oxygen vacancy migration. In spintronics topological phenomena are currently a hot topic, which lead to complex band structures and spin textures in reciprocal space. Spin-resolved momentum microscopy is uniquely suited to address these aspects. © 2020 The Japan Society of Vacuum and Surface Science. All rights reserved.
    view abstractdoi: 10.1380/ejssnt.2020.48
  • 2020 • 198 How photocorrosion can trick you: A detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production
    Kampmann, J. and Betzler, S. and Hajiyani, H. and Häringer, S. and Beetz, M. and Harzer, T. and Kraus, J. and Lotsch, B.V. and Scheu, C. and Pentcheva, R. and Fattakhova-Rohlfing, D. and Bein, T.
    Nanoscale 12 7766-7775 (2020)
    The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper(ii) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (Li0.03Cu0.97O) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor. Therefore, Li doping emerged as a suitable approach to manipulate the electronic structure of copper oxide based photocathodes. As this material class suffers from instability in water under operating conditions, the recorded photocurrents are repeatedly misinterpreted as hydrogen evolution evidence. We investigated the photocorrosion behavior of LixCu1-xO cathodes in detail and give the first mechanistic study of the fundamental physical process. The reduced copper oxide species were localized by electron energy loss spectroscopy mapping. Cu2O grows as distinct crystallites on the surface of LixCu1-xO instead of forming a dense layer. Additionally, there is no obvious Cu2O gradient inside the films, as Cu2O seems to form on all LixCu1-xO nanocrystals exposed to water. The application of a thin Ti0.8Nb0.2Ox coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of LixCu1-xO against decomposition. These devices showed a stable hydrogen evolution for 15 minutes. © The Royal Society of Chemistry .
    view abstractdoi: 10.1039/c9nr10250g
  • 2020 • 197 Hybrid density functional theory benchmark study on lithium manganese oxides
    Eckhoff, M. and Blöchl, P.E. and Behler, J.
    Physical Review B 101 (2020)
    The lithium manganese oxide spinel LixMn2O4, with 0≤x≤2, is an important example for cathode materials in lithium ion batteries. However, an accurate description of LixMn2O4 by first-principles methods like density functional theory is far from trivial due to its complex electronic structure, with a variety of energetically close electronic and magnetic states. It was found that the local density approximation as well as the generalized gradient approximation (GGA) are unable to describe LixMn2O4 correctly. Here, we report an extensive benchmark for different LixMnyOz systems using the hybrid functionals PBE0 and HSE06, as well as the recently introduced local hybrid functional PBE0r. We find that all of these functionals yield energetic, structural, electronic, and magnetic properties in good agreement with experimental data. The notable benefit of the PBE0r functional, which relies on onsite Hartree-Fock exchange only, is a much reduced computational effort that is comparable to GGA functionals. Furthermore, the Hartree-Fock mixing factors in PBE0r are smaller than in PBE0, which improves the results for (lithium) manganese oxides. The investigation of LixMn2O4 shows that two Mn oxidation states, +III and +IV, coexist. The MnIII ions are in the high-spin state and the corresponding MnO6 octahedra are Jahn-Teller distorted. The ratio between MnIII and MnIV and thus the electronic structure changes with the Li content while no major structural changes occur in the range from x=0 to 1. This work demonstrates that the PBE0r functional provides an equally accurate and efficient description of the investigated LixMnyOz systems. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.205113
  • 2020 • 196 In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts
    Beheshti Askari, A. and Al Samarai, M. and Hiraoka, N. and Ishii, H. and Tillmann, L. and Muhler, M. and Debeer, S.
    Nanoscale 12 15185-15192 (2020)
    The promoting effect of cobalt on the catalytic activity of a NiCoO Dry Methane Reforming (DMR) catalyst was studied by a combination of in situ Kβ X-ray Emission Spectroscopy (XES) and Kβ-detected High Energy Resolution Fluorescence Detected X-ray absorption spectroscopy (HERFD XAS). Following the calcination process, Ni XES and Kβ-detected HERFD XAS data revealed that the NiO coordination in the NiCoO catalyst has a higher degree of symmetry and is different than that of pure NiO/γ-Al2O3. Following the reductive activation, it was found that the NiCoO/γ-Al2O3 catalyst required a relatively higher temperature compared to the monometallic NiO/γ-Al2O3 catalyst. This finding suggests that Co is hampering the reduction of Ni in the NiCoO catalyst by modulation of its electronic structure. It has also been previously shown that the addition of Co enhances the DMR activity. Further, the Kβ XES spectrum of the partly reduced catalysts at 450 °C reveals that the Ni sites in the NiCoO catalyst are electronically different from the NiO catalyst. The in situ X-ray spectroscopic study demonstrates that reduced metallic Co and Ni are the primary species present after reduction and are preserved under DMR conditions. However, the NiCo catalyst appears to always be somewhat more oxidized than the Ni-only species, suggesting that the presence of cobalt modulates the Ni electronic structure. The electronic structural modulations resulting from the presence of Co may be the key to the increased activity of the NiCo catalyst relative to the Ni-only catalyst. This study emphasizes the potential of in situ X-ray spectroscopy experiments for probing the electronic structure of catalytic materials during activation and under operating conditions. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0nr01960g
  • 2020 • 195 Ligand Effects on the Electronic Structure of Heteroleptic Antimony-Centered Radicals
    Helling, C. and Cutsail, G.E., III and Weinert, H. and Wölper, C. and Schulz, S.
    Angewandte Chemie - International Edition 59 7561-7568 (2020)
    We report on the structures of three unprecedented heteroleptic Sb-centered radicals [L(Cl)Ga](R)Sb. (2-R, R=B[N(Dip)CH]2 2-B, 2,6-Mes2C6H3 2-C, N(SiMe3)Dip 2-N) stabilized by one electropositive metal fragment [L(Cl)Ga] (L=HC[C(Me)N(Dip)]2, Dip=2,6-i-Pr2C6H3) and one bulky B- (2-B), C- (2-C), or N-based (2-N) substituent. Compounds 2-R are predominantly metal-centered radicals. Their electronic properties are largely influenced by the electronic nature of the ligands R, and significant delocalization of unpaired-spin density onto the ligands was observed in 2-B and 2-N. Cyclic voltammetry (CV) studies showed that 2-B undergoes a quasi-reversible one-electron reduction, which was confirmed by the synthesis of [K([2.2.2]crypt)][L(Cl)GaSbB[N(Dip)CH]2] ([K([2.2.2]crypt)][2-B]) containing the stibanyl anion [2-B]−, which was shown to possess significant Sb−B multiple-bonding character. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.202000586
  • 2020 • 194 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 • 193 Modern quantum chemistry with [open]molcas
    Aquilante, F. and Autschbach, J. and Baiardi, A. and Battaglia, S. and Borin, V.A. and Chibotaru, L.F. and Conti, I. and De Vico, L. and Delcey, M. and Galván, I.F. and Ferré, N. and Freitag, L. and Garavelli, M. and Gong, X. an...
    Journal of Chemical Physics 152 (2020)
    MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0004835
  • 2020 • 192 Photoemission electron microscopy of magneto-ionic effects in La0.7Sr0.3MnO3
    Wilhelm, M. and Giesen, M. and Duchoň, T. and Moors, M. and Mueller, D.N. and Hackl, J. and Baeumer, C. and Hamed, M.H. and Cao, L. and Zhang, H. and Petracic, O. and Glöß, M. and Cramm, S. and Nemšák, S. and Wiemann, C. and ...
    APL Materials 8 (2020)
    Magneto-ionic control of magnetism is a promising route toward the realization of non-volatile memory and memristive devices. Magneto-ionic oxides are particularly interesting for this purpose, exhibiting magnetic switching coupled to resistive switching, with the latter emerging as a perturbation of the oxygen vacancy concentration. Here, we report on electric-field-induced magnetic switching in a La0.7Sr0.3MnO3 (LSMO) thin film. Correlating magnetic and chemical information via photoemission electron microscopy, we show that applying a positive voltage perpendicular to the film surface of LSMO results in the change in the valence of the Mn ions accompanied by a metal-to-insulator transition and a loss of magnetic ordering. Importantly, we demonstrate that the voltage amplitude provides granular control of the phenomena, enabling fine-tuning of the surface electronic structure. Our study provides valuable insight into the switching capabilities of LSMO that can be utilized in magneto-ionic devices. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0022150
  • 2020 • 191 Predicting oxidation and spin states by high-dimensional neural networks: Applications to lithium manganese oxide spinels
    Eckhoff, M. and Lausch, K.N. and Blöchl, P.E. and Behler, J.
    Journal of Chemical Physics 153 (2020)
    Lithium ion batteries often contain transition metal oxides such as LixMn2O4 (0 ≤ x ≤ 2). Depending on the Li content, different ratios of MnIII to MnIV ions are present. In combination with electron hopping, the Jahn-Teller distortions of the MnIIIO6 octahedra can give rise to complex phenomena such as structural transitions and conductance. While for small model systems oxidation and spin states can be determined using density functional theory (DFT), the investigation of dynamical phenomena by DFT is too demanding. Previously, we have shown that a high-dimensional neural network potential can extend molecular dynamics (MD) simulations of LixMn2O4 to nanosecond time scales, but these simulations did not provide information about the electronic structure. Here, we extend the use of neural networks to the prediction of atomic oxidation and spin states. The resulting high-dimensional neural network is able to predict the spins of the Mn ions with an error of only 0.03 We find that the Mn eg electrons are correctly conserved and that the number of Jahn-Teller distorted MnIIIO6 octahedra is predicted precisely for different Li loadings. A charge ordering transition is observed between 280 K and 300 K, which matches resistivity measurements. Moreover, the activation energy of the electron hopping conduction above the phase transition is predicted to be 0.18 eV, deviating only 0.02 eV from experiment. This work demonstrates that machine learning is able to provide an accurate representation of both the geometric and the electronic structure dynamics of LixMn2O4 on time and length scales that are not accessible by ab initio MD. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0021452
  • 2020 • 190 Semiclassical Dispersion Corrections Efficiently Improve Multiconfigurational Theory with Short-Range Density-Functional Dynamic Correlation
    Stein, C.J. and Reiher, M.
    Journal of Physical Chemistry A 124 2834-2841 (2020)
    Multiconfigurational wave functions are known to describe the electronic structure across a Born-Oppenheimer surface qualitatively correct. However, for quantitative reaction energies, dynamic correlation originating from the many configurations involving excitations out of the restricted orbital space, the active space, must be considered. Standard procedures involve approximations that eventually limit the ultimate accuracy achievable (most prominently, multireference perturbation theory). At the same time, the computational cost increases dramatically due to the necessity to obtain higher-order reduced density matrices. It is this disproportion that leads us here to propose an MC-srDFT-D hybrid approach of semiclassical dispersion (D) corrections to cover long-range dynamic correlation in a multiconfigurational (MC) wave function theory, which includes short-range (sr) dynamic correlation by density functional theory (DFT) without double counting. We demonstrate that the reliability of this approach is very good (at negligible cost), especially when considering that standard second-order multireference perturbation theory usually overestimates dispersion interactions. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpca.0c02130
  • 2020 • 189 Stepwise Bi-Bi Bond Formation: From a Bi-centered Radical to Bi4Butterfly and Bi8Cuneane-Type Clusters
    Krüger, J. and Wölper, C. and Schulz, S.
    Inorganic Chemistry 59 11142-11151 (2020)
    In contrast to their lighter homologues (P, As, Sb), the synthesis of polybismuthane clusters is still restricted to classical solid-state approaches. We herein report on systematic reduction reactions of different bismuth precursors with Ga(I) and Mg(I) complexes. This study not only yielded the first metal-coordinated tetrabismuthane ([{L1(Cl)Ga}2-μ,η1:1-Bi4] 3, L1 = HC[C(Me)N(2,6-i-Pr2C6H3)]2) and realgar-type bismuth cluster ([(L2Mg)4(μ4,η2:2:2:2-Bi8)] 4, L2 = HC[C(Me)N(2,4,6-Me3C6H2)]2) in addition to the bismuth-centered radical [L1Ga(Cl)]2Bi• 1 and dibismuthene [L1(Cl)GaBi]2 2, but clearly demonstrates the crucial role of the substituents and the oxidation state of the bismuth precursor as well as the specific reduction potential of the main group metal reductants on the product formation. Compounds 3 and 4 were spectroscopically characterized (1H, 13C NMR, IR), and the structures of 1-4 were determined by single-crystal X-ray diffraction. Computational calculations gave deeper insights into the electronic structures of 1′, 3′, and 4′. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.inorgchem.0c01657
  • 2020 • 188 Surface Charges at the CaF2/Water Interface Allow Very Fast Intermolecular Vibrational-Energy Transfer
    Lesnicki, D. and Zhang, Z. and Bonn, M. and Sulpizi, M. and Backus, E.H.G.
    Angewandte Chemie - International Edition 59 13116-13121 (2020)
    We investigate the dynamics of water in contact with solid calcium fluoride, where at low pH, localized charges can develop upon fluorite dissolution. We use 2D surface-specific vibrational spectroscopy to quantify the heterogeneity of the interfacial water (D2O) molecules and provide information about the sub-picosecond vibrational-energy-relaxation dynamics at the buried solid/liquid interface. We find that strongly H-bonded OD groups, with a vibrational frequency below 2500 cm−1, display very rapid spectral diffusion and vibrational relaxation; for weakly H-bonded OD groups, above 2500 cm−1, the dynamics slows down substantially. Atomistic simulations based on electronic-structure theory reveal the molecular origin of energy transport through the local H-bond network. We conclude that strongly oriented H-bonded water molecules in the adsorbed layer, whose orientation is pinned by the localized charge defects, can exchange vibrational energy very rapidly due to the strong collective dipole, compensating for a partially missing solvation shell. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.202004686
  • 2020 • 187 Tailoring Morphology and Electronic Structure of Cobalt Iron Oxide Nanowires for Electrochemical Oxygen Evolution Reaction
    Budiyanto, E. and Yu, M. and Chen, M. and Debeer, S. and Rüdiger, O. and Tüysüz, H.
    ACS Applied Energy Materials 3 8583-8594 (2020)
    The influence of iron on nanocasting of cobalt oxide nanowires and the performance of these materials for the oxygen evolution reaction (OER) are investigated. Pristine Co3O4 and mixed cobalt iron oxide nanowires with a diameter of 7 nm have been synthesized via a nanocasting route by using SBA-15 silica as a template. A small amount of iron added during the synthesis results in a decrease in the nanowires' array length and induces the formation of a bimodal pore size distribution. Raman spectroscopy, X-ray emission, and high-energy resolution X-ray absorption spectroscopies further show that Fe incorporation alters the electronic structure by increasing the average distortion around the cobalt centers and the amount of Co2+ in tetrahedral sites. These affect the OER activity significantly; the overpotential of pristine Co3O4 at 10 mA/cm2 decreases from 398 to 378 mV, and the current density at 1.7 V increases from 107 to 150 mA/cm2 with the addition of iron at the Co/Fe atomic ratio of 32. Furthermore, post-reaction characterization confirmed that both the morphology and electronic structure of nanowires remain intact after a long-term stability test. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.0c01201
  • 2020 • 186 Theoretical methods
    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 19-24 (2020)
    In this chapter, the theoretical methods required for the description of structure, electronic structure and magnetism of magnetic molecules in the gas phase and in the adsorbed configurations will be discussed. The main workhorse of the theoretical methods is the density functional theory that provides a materials-specific description of electronic structure, which is quite sufficient for many of the materials. However, in the present context of magnetic molecules, one needs to go beyond standard approximations in density functional theory. In this regard, some of the crucial characteristics in the electronic structure and magnetism will be discussed such as electron correlation, van der Waals interaction, band gaps, magnetic anisotropy and spin-dipole moments. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstractdoi: 10.1007/978-981-15-3719-6_3
  • 2020 • 185 Tunable coupling by means of oxygen intercalation and removal at the strongly interacting graphene/cobalt interface
    Jugovac, M. and Genuzio, F. and Menteş, T.O. and Locatelli, A. and Zamborlini, G. and Feyer, V. and Schneider, C.M.
    Carbon 163 341-347 (2020)
    It is well known that intercalated species can strongly affect the graphene-substrate interaction. As repeatedly shown by experiment and theory, the intercalation of atomic species may establish a free-standing character in chemisorbed graphene systems. Here, we focus on graphene grown on a strongly interacting support, cobalt, and demonstrate that the film electronic structure and doping can be tuned via the intercalation/removal of interfacial oxygen. Importantly, cathode lens microscopy reveals the main mechanism of oxygen intercalation, and in particular how microscopic openings in the mesh enable oxygen accumulation at the graphene-cobalt interface. Our experiments show that this process can be carefully controlled through temperature, without affecting the film morphology and crystalline quality. The presence of oxygen at the interface induces an upward shift of the graphene π band, moving its crossing above the Fermi level, accompanied by an increased Fermi velocity and reduced momentum width. Control on the graphene coupling to cobalt may enable one to alter the induced spin polarization in graphene's electronic states. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.carbon.2020.03.034
  • 2019 • 184 A review of the intrinsic ductility and toughness of hard transition-metal nitride alloy thin films
    Kindlund, H. and Sangiovanni, D.G. and Petrov, I. and Greene, J.E. and Hultman, L.
    Thin Solid Films 688 (2019)
    Over the past decades, enormous effort has been dedicated to enhancing the hardness of refractory ceramic materials. Typically, however, an increase in hardness is accompanied by an increase in brittleness, which can result in intergranular decohesion when materials are exposed to high stresses. In order to avoid brittle failure, in addition to providing high strength, films should also be ductile, i.e., tough. However, fundamental progress in obtaining hard-yet-ductile ceramics has been slow since most toughening approaches are based on empirical trial-and-error methods focusing on increasing the strength and ductility extrinsically, with a limited focus on understanding thin-film toughness as an inherent physical property of the material. Thus, electronic structure investigations focusing on the origins of ductility vs. brittleness are essential in understanding the physics behind obtaining both high strength and high plastic strain in ceramics films. Here, we review recent progress in experimental validation of density functional theory predictions on toughness enhancement in hard ceramic films, by increasing the valence electron concentration, using examples from the V1-xWxN and V1-xMoxN alloy systems. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2019.137479
  • 2019 • 183 Accelerating spin-space sampling by auxiliary spin dynamics and temperature-dependent spin-cluster expansion
    Wang, N. and Hammerschmidt, T. and Rogal, J. and Drautz, R.
    Physical Review B 99 (2019)
    Atomistic simulations of the thermodynamic properties of magnetic materials rely on an accurate modeling of magnetic interactions and an efficient sampling of the high-dimensional spin space. Recent years have seen significant progress with a clear trend from model systems to material-specific simulations that are usually based on electronic-structure methods. Here we develop a Hamiltonian Monte Carlo framework that makes use of auxiliary spin dynamics and an auxiliary effective model, the temperature-dependent spin-cluster expansion, in order to efficiently sample the spin space. Our method does not require a specific form of the model and is suitable for simulations based on electronic-structure methods. We demonstrate fast warm-up and a reasonably small dynamical critical exponent of our sampler for the classical Heisenberg model. We further present an application of our method to the magnetic phase transition in bcc iron using magnetic bond-order potentials. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.094402
  • 2019 • 182 Anomalously Low Barrier for Water Dimer Diffusion on Cu(111)
    Bertram, C. and Fang, W. and Pedevilla, P. and Michaelides, A. and Morgenstern, K.
    Nano Letters 19 3049-3056 (2019)
    A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b00392
  • 2019 • 181 autoCAS: A Program for Fully Automated Multiconfigurational Calculations
    Stein, C.J. and Reiher, M.
    Journal of Computational Chemistry 40 2216-2226 (2019)
    We present our implementation autoCAS for fully automated multiconfigurational calculations, which we also make available free of charge on our webpages. The graphical user interface of autoCAS connects a general electronic structure program with a density-matrix renormalization group program to carry out our recently introduced automated active space selection protocol for multiconfigurational calculations (Stein and Reiher, J. Chem. Theory Comput., 2016, 12, 1760). Next to this active space selection, autoCAS carries out several steps of multiconfigurational calculations so that only a minimal input is required to start them, comparable to that of a standard Kohn–Sham density-functional theory calculation, so that black-box multiconfigurational calculations become feasible. Furthermore, we introduce a new extension to the selection algorithm that facilitates automated selections for molecules with large valence orbital spaces consisting of several hundred orbitals. © 2019 Wiley Periodicals, Inc. © 2019 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jcc.25869
  • 2019 • 180 Bismuth-Antimony mixed double perovskites Cs2AgBi1-xSbxBr6in solar cells
    Pantaler, M. and Olthof, S. and Meerholz, K. and Lupascu, D.C.
    MRS Advances 4 3545-3552 (2019)
    Reported conversion efficiencies of lead based perovskite solar cells keep increasing steadily. But next to the demand for high efficiency, the need for analogue non-toxic material systems remains. One promising lead free absorber material is the double perovskite Cs2AgBiBr6. Interest in this and other double perovskites has been increasing in the last three years and several solar cells using different device structures have been reported. However, the efficiency of these solar cells is merely in the range of 2%. To further improve solar cell performance we prepared mixed bismuth-antimony double perovskite Cs2AgBi1-xSbxBr6 where different fractions of antimony (x=0.125, 0.25, 0.375, 0.50) are used. This was motivated by reports of lower bandgap values in these mixed system. After the optimization of preparation of these thin films, we have carefully analysed the effects on the structure, composition, electronic structure, as well as optical properties. Finally, we have fabricated Bi-Sb mixed double perovskite solar cells in a mesoscopic device architecture. © Materials Research Society 2019.
    view abstractdoi: 10.1557/adv.2019.404
  • 2019 • 179 Chemical control of the electrical surface properties in donor-doped transition metal oxides
    Andrä, M. and Bluhm, H. and Dittmann, R. and Schneider, C.M. and Waser, R. and Mueller, D.N. and Gunkel, F.
    Physical Review Materials 3 (2019)
    Donor-doped transition metal oxides such as donor-doped strontium titanate (n-SrTiO3) are of fundamental importance for oxide electronic devices as well as for electronic surface and interface engineering. Here we quantitatively analyze the variable band alignment and the resulting space charge layer at the surface of n-SrTiO3, determined by its surface redox chemistry. Synchrotron-based ambient-pressure x-ray photoelectron spectroscopy conducted under applied thermodynamic bias is used to access electronic structure and chemistry of the surface. We find an electron depletion layer driven by cationic surface point defects that are controlled by adjusting the ambient atmosphere (pO2). We correlate the pO2 dependence to a response of the strontium sublattice, namely the precipitation of strontium oxide and the formation of charged strontium vacancies at the surface. We suggest the reversible conversion of surface-terminating strontium oxide into extended strontium oxide clusters as the responsible process by resolving chemical dynamics in situ. As we show, atomic control of these subtle changes in the surface redox chemistry allows us to tailor electrical transport properties along the n-SrTiO3 surface. Our study thereby gives access to engineering electronic band bending in transition metal oxides by the control of the surface chemistry. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.044604
  • 2019 • 178 Combining X-ray Kβ 1,3 , valence-to-core, and X-ray Raman spectroscopy for studying Earth materials at high pressure and temperature: The case of siderite
    Weis, C. and Spiekermann, G. and Sternemann, C. and Harder, M. and Vankó, G. and Cerantola, V. and Sahle, C.J. and Forov, Y. and Sakrowski, R. and Kupenko, I. and Petitgirard, S. and Yavaş, H. and Bressler, C. and Gawelda, W. an...
    Journal of Analytical Atomic Spectrometry 34 384-393 (2019)
    X-ray emission and X-ray Raman scattering spectroscopy are powerful tools to investigate the local electronic and atomic structure of high and low Z elements in situ. Notably, these methods can be applied for in situ spectroscopy at high pressure and high temperature using resistively or laser-heated diamond anvil cells in order to achieve thermodynamic conditions which appear in the Earth's interior. We present a setup for combined X-ray emission and X-ray Raman scattering studies at beamline P01 of PETRA III using a portable wavelength-dispersive von Hamos spectrometer together with the permanently installed multiple-analyzer Johann-type spectrometer. The capabilities of this setup are exemplified by investigating the iron spin crossover of siderite FeCO 3 up to 49.3 GPa by measuring the Fe M 2,3 -edge and the Fe Kβ 1,3 emission line simultaneously. With this setup, the Fe valence-to-core emission can be detected together with the Kβ 1,3 emission line providing complementary information on the sample's electronic structure. By implementing a laser-heating device, we demonstrate the strength of using a von Hamos type spectrometer for spin state mapping at extreme conditions. Finally, we give different examples of low Z elements' absorption edges relevant for application in geoscience that are accessible with the Johann-type XRS spectrometer. With this setup new insights into the spin transition and compression mechanisms of Earth's mantle materials can be obtained of importance for comprehension of the macroscopic physical and chemical properties of the Earth's interior. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8ja00247a
  • 2019 • 177 Dynamics of optical excitations in a Fe/MgO(001) heterostructure from time-dependent density functional theory
    Gruner, M.E. and Pentcheva, R.
    Physical Review B 99 (2019)
    In the framework of real-time time-dependent density functional theory we unravel the layer-resolved dynamics of excited carriers in a (Fe)1/(MgO)3(001) multilayer after an optical excitation with a frequency below the band gap of bulk MgO. Substantial transient changes to the electronic structure, which persist after the duration of the pulse, are mainly observed for in-plane polarized electric fields, corresponding to a laser pulse arriving perpendicular to the interface. While the strongest charge redistribution takes place in the Fe layer, a time-dependent change in the occupation numbers is visible in all layers, mediated by the presence of interface states. The time evolution of the layer-resolved time-dependent occupation numbers indicates a strong orbital dependence with the depletion from in-plane orbitals (e.g., dx2-y2 of Fe) and accumulation in out-of-plane orbitals (d3z2-r2 of Fe and pz of apical oxygen). We also observe a small net charge transfer of less than one percent of an electron away from oxygen towards the Mg sites, even for MgO layers which are not directly in contact with the metallic Fe. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.195104
  • 2019 • 176 Electronic Structure of a Graphene-like Artificial Crystal of NdNiO3
    Arab, A. and Liu, X. and Köksal, O. and Yang, W. and Chandrasena, R.U. and Middey, S. and Kareev, M. and Kumar, S. and Husanu, M.-A. and Yang, Z. and Gu, L. and Strocov, V.N. and Lee, T.-L. and Minár, J. and Pentcheva, R. and Ch...
    Nano Letters 19 8311-8317 (2019)
    Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO3 confined between two 4-unit cell-thick layers of insulating LaAlO3. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b03962
  • 2019 • 175 Epitaxial and contamination-free Co(0001) electrodes on insulating substrates for molecular spintronic devices
    Königshofen, S. and Matthes, F. and Bürgler, D.E. and Schneider, C.M. and Dirksen, E. and Müller, T.J.J.
    Thin Solid Films 680 67-74 (2019)
    The growing field of molecular spintronics is an auspicious route to future concepts of data storage and processing. It has been reported that the hybridization of the electronic structures of non-magnetic organic molecules and ferromagnetic transition-metal (FM) surfaces can form new magnetic units, so-called hybrid molecular magnets, with distinct magnetic properties, which promise molecular spintronic devices with extremely high information density and low energy consumption. The investigation and profound understanding of these device concepts require the formation of clean and epitaxial interfaces between the surface of a FM bottom electrode and molecular thin films. This can only be realized under ultra-high vacuum conditions. In addition, the FM electrodes must be grown on an insulating substrate to electrically separate neighboring devices. Here, we report on procedures to realize an entirely in-situ preparation of mesoscopic test devices featuring structurally and chemically well-defined interfaces. Au(111)-buffered Co(0001) electrodes are deposited by molecular-beam epitaxy onto sapphire or mica substrates using a shadow-mask to define the geometry. The surface quality is subsequently characterized by scanning tunneling microscopy (STM) and other surface science analysis tools. 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI), which serves as an exemplary molecule, is sublimed through another shadow-mask, and the interface formation in the monolayer regime is also studied by STM. Finally, we deposit a Cu top electrode through yet another shadow-mask to complete a mesoscopic (200 × 200 μm2) test device, which reveals in ex-situ transport measurements for the Co/BNTCDI/Cu junction non-metallic behavior and a resistance-area product of 24 MΩ·μm2 at 10 K. © 2019
    view abstractdoi: 10.1016/j.tsf.2019.04.021
  • 2019 • 174 High-Dimensional Neural Network Potentials for Atomistic Simulations
    Hellström, M. and Behler, J.
    ACS Symposium Series 1326 49-59 (2019)
    Machine-learning methods have become increasingly popular for describing potential energy surfaces for molecular and materials simulations, and they are even beginning to challenge the present-day dominance of force fields for this task. This chapter reviews high-dimensional neural network potentials (HDNNPs), which are a general-purpose reactive potential method that can be used for simulations of an arbitrary number of atoms, can describe all types of chemical interactions (e.g., covalent, metallic, and dispersion), and includes the breaking and forming of chemical bonds. Before an HDNNP can be applied, it must be parameterized using electronic structure data, and great care must be taken at the parameterization stage to ensure that all pertinent parts of the potential energy surface are adequately covered. Typically, this is done iteratively through the addition of more training data and refitting of parameters. This chapter illustrates these points through the use of two case studies from our recent work for aqueous NaOH solutions and the ZnO/water interface. © 2019 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/bk-2019-1326.ch003
  • 2019 • 173 High-temperature 2D Fermi surface of SrTiO 3 studied by energy-filtered PEEM
    Mathieu, C. and Gonzalez, S. and Lubin, C. and Copie, O. and Feyer, V. and Schneider, C.M. and Barrett, N.
    Surface and Interface Analysis 51 7-11 (2019)
    Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry-induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO 3 (001) at 140 K. The 2DEG is created by soft X-ray irradiation and can be reversibly controlled by a combination of soft X-rays and oxygen partial pressure. © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.6533
  • 2019 • 172 High-temperature 2D Fermi surface of SrTiO3 studied by energy-filtered PEEM
    Mathieu, C. and Gonzalez, S. and Lubin, C. and Copie, O. and Feyer, V. and Schneider, C.M. and Barrett, N.
    Surface and Interface Analysis 51 7-11 (2019)
    Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry-induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO3(001) at 140 K. The 2DEG is created by soft X-ray irradiation and can be reversibly controlled by a combination of soft X-rays and oxygen partial pressure. © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.6533
  • 2019 • 171 In-Gap States and Band-Like Transport in Memristive Devices
    Baeumer, C. and Funck, C. and Locatelli, A. and Menteş, T.O. and Genuzio, F. and Heisig, T. and Hensling, F. and Raab, N. and Schneider, C.M. and Menzel, S. and Waser, R. and DIttmann, R.
    Nano Letters 19 54-60 (2019)
    Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.8b03023
  • 2019 • 170 Large thermopower anisotropy in PdCo O2 thin films
    Yordanov, P. and Sigle, W. and Kaya, P. and Gruner, M.E. and Pentcheva, R. and Keimer, B. and Habermeier, H.-U.
    Physical Review Materials 3 (2019)
    Motivated by recent theoretical studies predicting a large thermopower anisotropy in the layered delafossite PdCoO2, we have used pulsed laser deposition to synthesize thin films on (0001)-oriented and offcut Al2O3 substrates. By combining transport measurements on films with different offcut angles, tensor rotation relations, and an iterative fit procedure for the transport parameters, we have determined the resistivity and the thermopower along the main crystallographic axes in the temperature range 300-815 K. The data reveal a small positive Seebeck coefficient along the delafossite planes and a large negative Seebeck coefficient perpendicular to the planes, in excellent agreement with density functional calculations in the presence of moderate Coulomb correlations. The methodology introduced here is generally applicable for measurements of the thermoelectric properties of materials with highly anisotropic electronic structures. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.085403
  • 2019 • 169 Mechanical properties of VMoNO as a function of oxygen concentration: Toward development of hard and tough refractory oxynitrides
    Edström, D. and Sangiovanni, D.G. and Landälv, L. and Eklund, P. and Greene, J.E. and Petrov, I. and Hultman, L. and Chirita, V.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 37 (2019)
    Improved toughness is a central goal in the development of wear-resistant refractory ceramic coatings. Extensive theoretical and experimental research has revealed that NaCl-structure VMoN alloys exhibit surprisingly high ductility combined with high hardness and toughness. However, during operation, protective coatings inevitably oxidize, a problem that may compromise material properties and performance. Here, the authors explore the role of oxidation in altering VMoN properties. Density functional theory and theoretical intrinsic hardness models are used to investigate the mechanical behavior of cubic V0.5Mo0.5N1-xOx solid solutions as a function of the oxygen concentration x. Elastic constant and intrinsic hardness calculations show that oxidation does not degrade the mechanical properties of V0.5Mo0.5N. Electronic structure analyses indicate that the presence of oxygen reduces the covalent bond character, which slightly lowers the alloy strength and intrinsic hardness. Nevertheless, the character of metallic d-d states, which are crucial for allowing plastic deformation and enhancing toughness, remains unaffected. Overall, the authors' results suggest that VMoNO oxynitrides, with oxygen concentrations as high as 50%, possess high intrinsic hardness, while still being ductile. © 2019 Author(s).
    view abstractdoi: 10.1116/1.5125302
  • 2019 • 168 Ni-Metalloid (B, Si, P, As, and Te) Alloys as Water Oxidation Electrocatalysts
    Masa, J. and Piontek, S. and Wilde, P. and Antoni, H. and Eckhard, T. and Chen, Y.-T. and Muhler, M. and Apfel, U.-P. and Schuhmann, W.
    Advanced Energy Materials 9 (2019)
    Breakthroughs toward effective water-splitting electrocatalysts for mass hydrogen production will necessitate material design strategies based on unexplored material chemistries. Herein, Ni-metalloid (B, Si, P, As, Te) alloys are reported as an emergent class of highly promising electrocatalysts for the oxygen evolution reaction (OER) and insight is offered into the origin of activity enhancement on the premise of the surface electronic structure, the OER activation energy, influence of the guest metalloid elements on the lattice structure of the host metal (Ni), and surface-oxidized metalloid oxoanions. The metalloids modify the lattice structure of Ni, causing changes in the nearest Ni–Ni interatomic distance (dNi–Ni). The activation energy Ea scales with dNi–Ni indicating an apparent dependence of the OER activity on lattice properties. During the OER, surface Ni atoms are oxidized to nickel oxyhydroxide, which is the active state of the catalyst, meanwhile, the surface metalloids are oxidized to the corresponding oxoanions that affect the interfacial electrode/electrolyte properties and hence the adsorption/desorption interaction energies of the reacting species. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/aenm.201900796
  • 2019 • 167 Oxygen vacancies and hydrogen doping in LaAlO3/SrTiO3 heterostructures: Electronic properties and impact on surface and interface reconstruction
    Piyanzina, I.I. and Eyert, V. and Lysogorskiy, Yu.V. and Tayurskii, D.A. and Kopp, T.
    Journal of Physics Condensed Matter 31 (2019)
    We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in a LaAlO3 slab can suppress the surface conductivity. In contrast, in insulating SrTiO3 slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating LaAlO3/SrTiO3 heterostructure slabs with three LaAlO3 overlayers lead to the emergence of interface conductivity. Calculated defect formation energies reveal strong preference of hydrogen dopant atoms for surface sites for all structures and concentrations considered. Strong decrease of the defect formation energy of hydrogen adatoms with increasing thickness of the LaAlO3 overlayer and crossover from positive to negative values, taken together with the metallic conductivity induced by hydrogen adatoms, seamlessly explains the semiconductor-metal transition observed for these heterostructures as a function of the overlayer thickness. Moreover, we show that the potential drop and concomitant shift of (layer resolved) band edges is suppressed for the metallic configuration. Finally, magnetism with stable local moments, which form atomically thin magnetic layers at the interface, is generated by oxygen vacancies either at the surface or the interface, or by hydrogen atoms buried at the interface. In particular, oxygen vacancies in the TiO2 interface layer cause drastic downshift of the 3d eg states of the Ti atoms neighboring the vacancies, giving rise to strongly localized magnetic moments, which add to the two-dimensional background magnetization. © 2019 IOP Publishing Ltd Printed in the UK.
    view abstractdoi: 10.1088/1361-648X/ab1831
  • 2019 • 166 Phase transitions in titanium with an analytic bond-order potential
    Ferrari, A. and Schröder, M. and Lysogorskiy, Y. and Rogal, J. and Mrovec, M. and Drautz, R.
    Modelling and Simulation in Materials Science and Engineering 27 (2019)
    Titanium is the base material for a number of technologically important alloys for energy conversion and structural applications. Atomic-scale studies of Ti-based metals employing first-principles methods, such as density functional theory, are limited to ensembles of a few hundred atoms. To perform large-scale and/or finite temperature simulations, computationally more efficient interatomic potentials are required. In this work, we coarse grain the tight-binding (TB) approximation to the electronic structure and develop an analytic bond-order potential (BOP) for Ti by fitting to the energies and forces of elementary deformations of simple structures. The BOP predicts the structural properties of the stable and defective phases of Ti with a quality comparable to previous TB parameterizations at a much lower computational cost. The predictive power of the model is demonstrated for simulations of martensitic transformations. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/ab471d
  • 2019 • 165 Role of Boron and Phosphorus in Enhanced Electrocatalytic Oxygen Evolution by Nickel Borides and Nickel Phosphides
    Masa, J. and Andronescu, C. and Antoni, H. and Sinev, I. and Seisel, S. and Elumeeva, K. and Barwe, S. and Marti-Sanchez, S. and Arbiol, J. and Roldan Cuenya, B. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 6 235-240 (2019)
    The modification of nickel with boron or phosphorus leads to significant enhancement of its electrocatalytic activity for the oxygen evolution reaction (OER). However, the precise role of the guest elements, B and P, in enhancing the OER of the host element (Ni) remains unclear. Herein, we present insight into the role of B and P in enhancing electrocatalysis of oxygen evolution by nickel borides and nickel phosphides. The apparent activation energy, Ea*, of electrocatalytic oxygen evolution on Ni2P was 78.4 kJ/mol, on Ni2B 65.4 kJ/mol, and on Ni nanoparticles 94.0 kJ/mol, thus revealing that both B and P affect the intrinsic activity of nickel. XPS data revealed shifts of −0.30 and 0.40 eV in the binding energy of the Ni 2p3/2 peak of Ni2B and Ni2P, respectively, with respect to that of pure Ni at 852.60 eV, thus indicating that B and P induce opposite electronic effects on the surface electronic structure of Ni. The origin of enhanced activity for oxygen evolution cannot, therefore, be attributed to such electronic modification or ligand effect. Severe changes induced on the nickel lattice, specifically, the Ni-Ni atomic order and interatomic distances (strain effect), by the presence of the guest atoms seem to be the dominant factors responsible for enhanced activity of oxygen evolution in nickel borides and nickel phosphides. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201800669
  • 2019 • 164 Role of London Dispersion Interactions in Ga-Substituted Dipnictenes
    Song, L. and Schoening, J. and Wölper, C. and Schulz, S. and Schreiner, P.R.
    Organometallics 38 1640-1647 (2019)
    We report the synthesis and structural characterization of Ga-substituted diarsene [L(EtO)GaAs]2, which is an exemplary case of a Ga-substituted dipnictene of the general type [L(X)Ga]2E2 (L = C[C(Me)N(2,6-i-Pr2-C6H3)]2, X = F, Cl, Br, I, NMe2, OEt; E = As, Sb, Bi). We examined this extended series of compounds computationally and found that attractive London dispersion interactions between the substituents on the N,N′-chelating β-diketiminate ligands as well as pnictogen-π interactions between the group 15 metal center and the ligand are key factors for the stability and the electronic structures of such types of complexes. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.9b00072
  • 2019 • 163 Spin caloric transport from density-functional theory
    Popescu, V. and Kratzer, P. and Entel, P. and Heiliger, C. and Czerner, M. and Tauber, K. and Töpler, F. and Herschbach, C. and Fedorov, D.V. and Gradhand, M. and Mertig, I. and Kováčik, R. and Mavropoulos, P. and Wortmann, D. ...
    Journal of Physics D: Applied Physics 52 (2019)
    Spin caloric transport refers to the coupling of heat with spin transport. Its applications primarily concern the generation of spin currents and control of magnetisation by temperature gradients for information technology, known by the synonym spin caloritronics. Within the framework of ab initio theory, new tools are being developed to provide an additional understanding of these phenomena in realistic materials, accounting for the complexity of the electronic structure without adjustable parameters. Here, we review this progress, summarising the principles of the density-functional-based approaches in the field and presenting a number of application highlights. Our discussion includes the three most frequently employed approaches to the problem, namely the Kubo, Boltzmann, and Landauer-Büttiker methods. These are showcased in specific examples that span, on the one hand, a wide range of materials, such as bulk metallic alloys, nano-structured metallic and tunnel junctions, or magnetic overlayers on heavy metals, and, on the other hand, a wide range of effects, such as the spin-Seebeck, magneto-Seebeck, and spin-Nernst effects, spin disorder, and the thermal spin-transfer and thermal spin-orbit torques. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aae8c5
  • 2019 • 162 Structure and Reactivity of 1,8-Bis(naphthalenediyl)dipnictanes
    Dzialkowski, K. and Gehlhaar, A. and Wölper, C. and Auer, A.A. and Schulz, S.
    Organometallics 38 2927-2942 (2019)
    Syntheses and solid-state structures of diarsane Naph2As2 (Naph = 1,8-naphthalenediyl, 1) and (Naph)5Sb4Cl2 3 are reported and the σ-donor capacity of Naph2E2 (E = As 1, Sb 2) was studied in reactions with (coe)Cr(CO)5 (coe = Z-cyclooctene), yielding [Naph2As2][Cr(CO)5]2 (4) and [Naph2E2][Cr(CO)5] (E = As 5, Sb 6). In contrast, reactions of 1 and 3 with Me2SAuCl proceed with oxidation and formation of elemental gold as well as Naph2(AsCl)2 (7) and [NaphSbCl2]2 8. All complexes were characterized by elemental analyses, heteronuclear (1H, 13C) NMR and FT-IR spectroscopy, as well as single crystal X-ray diffraction. Intermolecular E···πinteractions (E = As, Sb), which were observed in 7 and 8, were quantified by use of density functional theory and local coupled cluster electronic structure theory calculations. These allow to assess the nature and relative importance of covalent and noncovalent interactions and illustrate how dispersion interactions change with the electronic structure of the compounds. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.9b00269
  • 2019 • 161 Surface states in bulk single crystal of topological semimetal Co3Sn2S2 toward water oxidation
    Li, G. and Xu, Q. and Shi, W. and Fu, C. and Jiao, L. and Kamminga, M.E. and Yu, M. and Tüysüz, H. and Kumar, N. and Süß, V. and Saha, R. and Srivastava, A.K. and Wirth, S. and Auffermann, G. and Gooth, J. and Parkin, S. and S...
    Science Advances 5 (2019)
    The band inversion in topological phase matters bring exotic physical properties such as the topologically protected surface states (TSS). They strongly influence the surface electronic structures of the materials and could serve as a good platform to gain insight into the surface reactions. Here we synthesized high-quality bulk single crystals of Co3Sn2S2 that naturally hosts the band structure of a topological semimetal. This guarantees the existence of robust TSS from the Co atoms. Co3Sn2S2 crystals expose their Kagome lattice that constructed by Co atoms and have high electrical conductivity. They serves as catalytic centers for oxygen evolution process (OER), making bonding and electron transfer more efficient due to the partially filled orbital. The bulk single crystal exhibits outstanding OER catalytic performance, although the surface area is much smaller than that of Co-based nanostructured catalysts. Our findings emphasize the importance of tailoring TSS for the rational design of high-activity electrocatalysts. Copyright © 2019 The Authors.
    view abstractdoi: 10.1126/sciadv.aaw9867
  • 2019 • 160 Synthesis and structures of gallaarsenes LGaAsGa(X)L featuring a Ga-As double bond
    Schoening, J. and John, L. and Wölper, C. and Schulz, S.
    Dalton Transactions 48 17729-17734 (2019)
    Three equivalents of LGa {L = HC[C(Me)N(2,6-i-Pr2C6H3)]2} react with AsX3 (X = Cl, Br) by insertion into two As-X bonds, followed by the elimination of LGaX2 and formation of LGaAsGa(Cl)L (1) and LGaAsGa(Br)L (2). According to single crystal X-ray analysis, 1 and 2 each exhibit one Ga-As single bond and one Ga-As double bond. The π-bonding contribution (9.71 kcal mol-11 and 9.44 kcal mol-12) was proved by variable temperature (VT) 1H NMR spectroscopy, while the electronic structure of 1′ was studied by quantum chemical calculations. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9dt03998h
  • 2019 • 159 The Poisson-Boltzmann model for implicit solvation of electrolyte solutions: Quantum chemical implementation and assessment via Sechenov coefficients
    Stein, C.J. and Herbert, J.M. and Head-Gordon, M.
    Journal of Chemical Physics 151 (2019)
    We present the theory and implementation of a Poisson-Boltzmann implicit solvation model for electrolyte solutions. This model can be combined with arbitrary electronic structure methods that provide an accurate charge density of the solute. A hierarchy of approximations for this model includes a linear approximation for weak electrostatic potentials, finite size of the mobile electrolyte ions, and a Stern-layer correction. Recasting the Poisson-Boltzmann equations into Euler-Lagrange equations then significantly simplifies the derivation of the free energy of solvation for these approximate models. The parameters of the model are either fit directly to experimental observables - e.g., the finite ion size - or optimized for agreement with experimental results. Experimental data for this optimization are available in the form of Sechenov coefficients that describe the linear dependence of the salting-out effect of solutes with respect to the electrolyte concentration. In the final part, we rationalize the qualitative disagreement of the finite ion size modification to the Poisson-Boltzmann model with experimental observations by taking into account the electrolyte concentration dependence of the Stern layer. A route toward a revised model that captures the experimental observations while including the finite ion size effects is then outlined. This implementation paves the way for the study of electrochemical and electrocatalytic processes of molecules and cluster models with accurate electronic structure methods. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5131020
  • 2019 • 158 The Role of Non-Metallic and Metalloid Elements on the Electrocatalytic Activity of Cobalt and Nickel Catalysts for the Oxygen Evolution Reaction
    Masa, J. and Schuhmann, W.
    ChemCatChem 11 5842-5854 (2019)
    Compounds and alloys of cobalt and nickel with some nonmetals (N, P, S, Se) and metalloids (C, B, C, As and Te) have emerged as very promising noble metal-free pre-catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, the exact role played by the non-metals and metalloids in promoting the OER is not well understood. A holistic understanding of the origin of the OER activity enhancement in these compounds is vital for their exploitation as models to inspire knowledge-guided design of improved OER catalysts. In this review, we elucidate the factors that govern the activity and stability of OER catalysts derived from MX compounds (M=Co or Ni, and X=nonmetal or metalloid), including the impact of surface electronic structure, M : X stoichiometry, material composition, structure and crystallinity, as well as the role of oxoanions on the properties of the electrochemical double layer and interaction energies of the reaction intermediates. Finally, we outline a few perspectives and research directions towards a deeper understanding of the role of the nonmetal and metalloid elements and design of improved OER catalysts. ©2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.201901151
  • 2019 • 157 Three-dimensional character of the Fermi surface in ultrathin LaTiO3/SrTiO3 heterostructures
    Veit, M.J. and Chan, M.K. and Ramshaw, B.J. and Arras, R. and Pentcheva, R. and Suzuki, Y.
    Physical Review B 99 (2019)
    LaTiO3 films on SrTiO3 single crystal substrates exhibit metallic behavior attributed to the LaTiO3 film, the interface as well as part of the SrTiO3. In the limit of ultrathin LaTiO3 films on SrTiO3, the contribution to the metallicity from strain-induced electronic structure modification of the LaTiO3 film is minimized so that the dominant contribution to metallicity is from the interface and part of the SrTiO3 due to charge transfer of 3d electrons from LaTiO3 to SrTiO3. In such a limit, we observe quantum oscillations whose angular dependence indicates a three-dimensional Fermi surface. Such angular dependence is observed in two sets of quantum oscillations - one low frequency and one high frequency - that we have attributed to an inner and outer Fermi surface associated with a Rashba-like spin split hybridized dxz+yz band. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.115126
  • 2019 • 156 Tuning the magnetic and magnetocaloric properties of austenitic Ni-Mn-(In,Sn)Heuslers
    Cavazzini, G. and Cugini, F. and Gruner, M.E. and Bennati, C. and Righi, L. and Fabbrici, S. and Albertini, F. and Solzi, M.
    Scripta Materialia 170 48-51 (2019)
    In this work, we highlight the occurrence of different physical mechanisms that independently control the saturation magnetization and the ferro-paramagnetic transition temperature of Ni-Mn-based Heusler compounds, opening new possibilities in mastering the functional properties of this wide class of magnetic materials. We present the magnetic, structural and magnetocaloric features of a complete Ni48Mn36In16−xSnx (x = 0–16)series. The observed different trends of the critical temperature and of the saturation magnetization on varying the Sn to In ratio are discussed with the help of first-principles calculations of the electronic structure and magnetic interactions of the compound. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.05.027
  • 2018 • 155 A comparative study on the photocatalytic degradation of organic dyes using hybridized 1T/2H, 1T/3R and 2H MoS2 nano-sheets
    Saber, M.R. and Khabiri, G. and Maarouf, A.A. and Ulbricht, M. and Khalil, A.S.G.
    RSC Advances 8 26364-26370 (2018)
    MoS2 is a very attractive material and has been well studied for potential applications in various areas. However, due to the wide variety of factors affecting the molecular and electronic structure of MoS2, several contradictory reports about the adsorptive and photocatalytic properties of such materials have been published. In most of these reports, the effect of the actual phase of the materials on the properties was neglected. Here, different phases of MoS2 nanosheets (1T/2H, 1T/3R and 2H) have been obtained using the hydrothermal method with different Mo : S molar ratios and different autoclave filling ratios. The obtained materials have been thoroughly characterized using Raman, UV-vis, powder XRD, SEM, TEM and XPS measurements in order to accurately identify the existing phases in each material. A comparative study of the photocatalytic organic dye degradation efficiency under white light irradiation has been conducted using methyl orange to correlate the different activity of each material to the respective phase composition. The results indicate a much higher performance of the 1T/2H phase compared to the 2H and 3R phases. Detailed computational studies of the different phases revealed the emergence of mid-gap states upon introducing 1T sites into the 2H lattice. This leads to the improvement of the photocatalytic activity of 1T/2H compared to the other prepared materials. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8ra05387a
  • 2018 • 154 A Microscopic Interpretation of Pump-Probe Vibrational Spectroscopy Using Ab Initio Molecular Dynamics
    Lesnicki, D. and Sulpizi, M.
    Journal of Physical Chemistry B 122 6604-6609 (2018)
    What happens when extra vibrational energy is added to water? Using nonequilibrium molecular dynamics simulations, also including the full electronic structure, and novel descriptors, based on projected vibrational density of states, we are able to follow the flow of excess vibrational energy from the excited stretching and bending modes. We find that the energy relaxation, mostly mediated by a stretching-stretching coupling in the first solvation shell, is highly heterogeneous and strongly depends on the local environment, where a strong hydrogen bond network can transport energy with a time scale of 200 fs, whereas a weaker network can slow down the transport by a factor 2-3. Copyright © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.8b04159
  • 2018 • 153 Atomic-Scale Explanation of O2 Activation at the Au-TiO2 Interface
    Siemer, N. and Lüken, A. and Zalibera, M. and Frenzel, J. and Muñoz-Santiburcio, D. and Savitsky, A. and Lubitz, W. and Muhler, M. and Marx, D. and Strunk, J.
    Journal of the American Chemical Society 140 18082-18092 (2018)
    By a combination of electron paramagnetic resonance spectroscopy, finite-temperature ab initio simulations, and electronic structure analyses, the activation of molecular dioxygen at the interface of gold nanoparticles and titania in Au/TiO2 catalysts is explained at the atomic scale by tracing processes down to the molecular orbital picture. Direct evidence is provided that excess electrons in TiO2, for example created by photoexcitation of the semiconductor, migrate to the gold particles and from there to oxygen molecules adsorbed at gold/titania perimeter sites. Superoxide species are formed more efficiently in this way than on the bare TiO2 surface. This catalytic effect of the gold nanoparticles is attributed to a weakening of the internal O-O bond, leading to a preferential splitting of the molecule at shorter bond lengths together with a 70% decrease of the dissociation free energy barrier compared to the non-catalyzed case on bare TiO2. The findings are an important step forward in the clarification of the role of gold in (photo)catalytic processes. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/jacs.8b10929
  • 2018 • 152 Comparison of permutationally invariant polynomials, neural networks, and Gaussian approximation potentials in representing water interactions through many-body expansions
    Nguyen, T.T. and Székely, E. and Imbalzano, G. and Behler, J. and Csányi, G. and Ceriotti, M. and Götz, A.W. and Paesani, F.
    Journal of Chemical Physics 148 (2018)
    The accurate representation of multidimensional potential energy surfaces is a necessary requirement for realistic computer simulations of molecular systems. The continued increase in computer power accompanied by advances in correlated electronic structure methods nowadays enables routine calculations of accurate interaction energies for small systems, which can then be used as references for the development of analytical potential energy functions (PEFs) rigorously derived from many-body (MB) expansions. Building on the accuracy of the MB-pol many-body PEF, we investigate here the performance of permutationally invariant polynomials (PIPs), neural networks, and Gaussian approximation potentials (GAPs) in representing water two-body and three-body interaction energies, denoting the resulting potentials PIP-MB-pol, Behler-Parrinello neural network-MB-pol, and GAP-MB-pol, respectively. Our analysis shows that all three analytical representations exhibit similar levels of accuracy in reproducing both two-body and three-body reference data as well as interaction energies of small water clusters obtained from calculations carried out at the coupled cluster level of theory, the current gold standard for chemical accuracy. These results demonstrate the synergy between interatomic potentials formulated in terms of a many-body expansion, such as MB-pol, that are physically sound and transferable, and machine-learning techniques that provide a flexible framework to approximate the short-range interaction energy terms. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5024577
  • 2018 • 151 CVD-grown copper tungstate thin films for solar water splitting
    Peeters, D. and Mendoza Reyes, O. and Mai, L. and Sadlo, A. and Cwik, S. and Rogalla, D. and Becker, H.-W. and Schütz, H.M. and Hirst, J. and Müller, S. and Friedrich, D. and Mitoraj, D. and Nagli, M. and Toroker, M.C. and Eichb...
    Journal of Materials Chemistry A 6 10206-10216 (2018)
    In this paper, a direct chemical vapor deposition (CVD) approach is applied for the first time to synthesize high quality copper oxide (CuO), copper tungstate (CuWO4) and tungsten oxide (WO3) on F:SnO2 (FTO) substrates for photocatalytic water splitting. Variation of process parameters enables us to tune the stoichiometry of the deposits to obtain stoichiometric, W-rich, and Cu-rich deposits. It is found that the presence of Cu in WO3 thin films reduces the bandgap and enhances the absorption properties of the material in the visible range. The photoelectrocatalytic performance of stoichiometric CuWO4 was found to be superior to that of WO3 oxide under frontside illumination when thin films were used. However, detailed photoelectrochemical investigations of both thin and thicker CuWO4 films reveal that the incorporation of copper also decreases the mobility of both electrons and holes, the latter being the performance-limiting factor. These results are in line with our first-principles calculations of the electronic structure of CuWO4. A charge carrier mobility and diffusion length of ∼6× 10-3 cm2 V-1 s-1 and 30 nm were determined by time-resolved microwave conductivity measurements, values comparable to those of undoped bismuth vanadate (BiVO4). Our findings establish new insights into the advantages and limits of CuWO4-based photoanodes, and suggest a possibility of using very thin CuWO4 films on top of highly absorbing semiconductors with optimal electronic properties. © The Royal Society of Chemistry 2018.
    view abstractdoi: 10.1039/c7ta10759e
  • 2018 • 150 Different Breathing Mechanisms in Flexible Pillared-Layered Metal-Organic Frameworks: Impact of the Metal Center
    Schneemann, A. and Vervoorts, P. and Hante, I. and Tu, M. and Wannapaiboon, S. and Sternemann, C. and Paulus, M. and Wieland, D.C.F. and Henke, S. and Fischer, R.A.
    Chemistry of Materials 30 1667-1676 (2018)
    The pillared-layered metal-organic framework compounds M2(BME-bdc)2(dabco) (M2+ = Zn2+, Co2+, Ni2+, Cu2+; BME-bdc2- = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M2+. This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO2 pressure reveal different mechanisms for the np to lp phase transition during CO2 adsorption. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.7b05052
  • 2018 • 149 Elastic properties and plastic deformation of TiC- and VC-based pseudobinary alloys
    Edström, D. and Sangiovanni, D.G. and Hultman, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Acta Materialia 144 376-385 (2018)
    Transition-metal (TM) carbides are an important class of hard, protective coating materials; however, their brittleness often limits potential applications. We use density functional theory to investigate the possibility of improving ductility by forming pseudobinary cubic M1M2C alloys, for which M1 = Ti or V and M2 = W or Mo. The alloying elements are chosen based on previous results showing improved ductility of the corresponding pseudobinary nitride alloys with respect to their parent compounds. While commonly-used empirical criteria do not indicate enhanced ductility in the carbide alloys, calculated stress/strain curves along known slip systems, supported by electronic structure analyses, indicate ductile behavior for VMoC. As VMoC layers are sheared along the 11¯0 direction on {111} planes, the stress initially increases linearly up to a yield point where the accumulated stress is partially dissipated. With further increase in strain, the stress increases again until fracture occurs. A similar mechanical behavior is observed for the corresponding TM nitride VMoN, known to be a ductile ceramic material [1]. Thus, our results show that VMoC is a TM carbide alloy which may be both hard and ductile, i.e. tough. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.10.047
  • 2018 • 148 Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption
    Ur Rahman, A. and Rahman, G. and Kratzer, P.
    Journal of Physics Condensed Matter 30 (2018)
    The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aab8b8
  • 2018 • 147 From Quasicrystals to Crystals with Interpenetrating Icosahedra in Ca-Au-Al: In Situ Variable-Temperature Transformation
    Pham, J. and Meng, F. and Lynn, M.J. and Ma, T. and Kreyssig, A. and Kramer, M.J. and Goldman, A.I. and Miller, G.J.
    Journal of the American Chemical Society 140 1337-1347 (2018)
    The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu4.39Al1.61 to its cubic 2/1 crystalline approximant (CA) Ca13Au56.31(3)Al21.69 (CaAu4.33(1)Al1.67, Pa3 (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ∼570 °C and complete by ∼650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au59.86(2)Al17.14□3.00] and an icosahedral shell of only Al [Al10.5□1.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au-Al nearest neighbor contacts over homoatomic Al-Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. According to electronic structure calculations, a cubic 1/1 CA, "Ca24Au88Al64" (CaAu3.67Al2.67) is proposed. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b10358
  • 2018 • 146 Increased Acid Dissociation at the Quartz/Water Interface
    Parashar, S. and Lesnicki, D. and Sulpizi, M.
    Journal of Physical Chemistry Letters 9 2186-2189 (2018)
    As shown by a quite significant amount of literature, acids at the water surface tend to be "less" acid, meaning that their associated form is favored over the conjugated base. What happens at the solid/liquid interface? In the case of the silica/water interface, we show how the acidity of adsorbed molecules can instead increase. Using a free energy perturbation approach in combination with electronic structure-based molecular dynamics simulations, we show how the acidity of pyruvic acid at the quartz/water interface is increased by almost two units. Such increased acidity is the result of the specific microsolvation at the interface and, in particular, of the stabilization of the deprotonated form by the silanols on the quartz surface and the special interfacial water layer. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.8b00686
  • 2018 • 145 Inherent toughness and fracture mechanisms of refractory transition-metal nitrides via density-functional molecular dynamics
    Sangiovanni, D.G.
    Acta Materialia 151 11-20 (2018)
    Hard refractory transition-metal nitrides possess unique combinations of outstanding mechanical and physical properties, but are typically brittle. Recent experimental results demonstrated that single-crystal NaCl-structure (B1) V0.5Mo0.5N pseudobinary solid solutions are both hard (∼20 GPa) and ductile; that is, they exhibit toughness, which is unusual for ceramics. However, key atomic-scale mechanisms underlying this inherent toughness are unknown. Here, I carry out density-functional ab initio molecular dynamics (AIMD) simulations at room temperature to identify atomistic processes and associated changes in the electronic structure which control strength, plasticity, and fracture in V0.5Mo0.5N, as well as reference B1 TiN, subject to &lt;001&gt; and &lt;110&gt; tensile deformation. AIMD simulations reveal that V0.5Mo0.5N is considerably tougher than TiN owing to its ability to (i) isotropically redistribute mechanical stresses within the elastic regime, (ii) dissipate the accumulated strain energy by activating local structural transformations beyond the yield point. In direct contrast, TiN breaks in brittle manner when applied stresses reach its tensile strength. Charge transfer maps show that the adaptive mechanical response of V0.5Mo0.5N originates from highly populated d-d metallic-states, which allow for counterbalancing the destabilization induced via tensile deformation by enabling formation of new chemical bonds. The high ionic character and electron-localization in TiN precludes the possibility of modifying bonding geometries to accommodate the accumulated stresses, thus suddenly causing material's fracture for relatively low strain values. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.03.038
  • 2018 • 144 New Developments in Spin-Dependent Photoemission∗
    Schneider, C.M.
    e-Journal of Surface Science and Nanotechnology 16 177-185 (2018)
    The electron spin governs many phenomena in modern condensed matter physics, such as magnetism, superconductivity, etc. Often, minute details in the electronic structure determine the physical behavior of a material. Photoelectron emission—being the most established approach to explore electronic structures—is currently entering a new era thanks to a breathtaking development in light sources, spectrometer concepts, and spin detectors. In particular, the evolution in novel highly efficient electron spin polarimeters opens up new experimental opportunities and permits unequaled insights into the electronic structure. This contribution will discuss several examples in this field of spin-dependent interactions and spin-based phenomena. A prominent one refers to the class of topological insulators, where strong spin-orbit coupling (SOC) causes a unique spin-momentum locking around the Dirac cone. Transition metal dichalcogenides consist of quasi-2D layers coupled by v. d. Waals interactions. Here, strong SOC leads to pronounced hybridization effects. We also address fundamental issues in ferromagnetism, e.g., the complex interplay of SOC and exchange interaction, causing characteristic k-, spin- and symmetry-dependent band mixing. Using spin- and time-resolved photoemission we explore ultrafast spin dynamics in ferromagnets driven by strong ultrashort laser pulses. We find the changes in both majority and minority spin states to take place on a 100 fs time scale and to be compatible with band mirroring. In this contribution, we will discuss several new aspects of spin-dependent and spin-resolved photoemission covering both static and dynamic issues of electronic states. © 2018 The Japan Society of Vacuum and Surface Science.
    view abstractdoi: 10.1380/ejssnt.2018.177
  • 2018 • 143 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 • 142 Sp-d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets
    Muckel, F. and Delikanli, S. and Hernández-Martínez, P.L. and Priesner, T. and Lorenz, S. and Ackermann, J. and Sharma, M. and Demir, H.V. and Bacher, G.
    Nano Letters 18 2047-2053 (2018)
    In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin-orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.8b00060
  • 2018 • 141 Sum frequency generation spectra from velocity-velocity correlation functions: New developments and applications
    Rémi, K. and Marialore, S.
    High Performance Computing in Science and Engineering' 17: Transactions of the High Performance Computing Center, Stuttgart (HLRS) 2017 141-156 (2018)
    At the interface, the properties of water can be rather different from those observed in the bulk. In this chapter we present an overview of our computational approach to understand water structure and dynamics at the interface including atomistic and electronic structure details. In particular we show how Density Functional Theory-based molecular dynamics simulations (DFT-MD) of water interfaces can provide a microscopic interpretation of recent experimental results from surface sensitive vibrational Sum Frequency Generation spectroscopy (SFG). In our recent work we developed an expression for the calculation of the SFG spectra of water interfaces which is based on the projection of the atomic velocities on the local normal modes. Our approach permits to obtain the SFG signal from suitable velocity-velocity correlation functions, reducing the computational cost to that of the accumulation of a molecular dynamics trajectory, and therefore cutting the overhead costs associated to the explicit calculation of the dipole moment and polarizability tensor. Our method permits to interpret the peaks in the spectrum in terms of local modes, also including the bending region. The results for the water-air interface, obtained using extensive ab initio molecular dynamics simulations over 400 ns, are discussed in connection to recent phase resolved experimental data. © Springer International Publishing AG 2018.
    view abstractdoi: 10.1007/978-3-319-68394-2_8
  • 2018 • 140 The role of fullerenes in the environmental stability of polymer:fullerene solar cells
    Lee, H.K.H. and Telford, A.M. and Röhr, J.A. and Wyatt, M.F. and Rice, B. and Wu, J. and De Castro Maciel, A. and Tuladhar, S.M. and Speller, E. and McGettrick, J. and Searle, J.R. and Pont, S. and Watson, T. and Kirchartz, T. an...
    Energy and Environmental Science 11 417-428 (2018)
    Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device current-voltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ee02983g
  • 2018 • 139 Why Tin-Doping Enhances the Efficiency of Hematite Photoanodes for Water Splitting—The Full Picture
    Hufnagel, A.G. and Hajiyani, H. and Zhang, S. and Li, T. and Kasian, O. and Gault, B. and Breitbach, B. and Bein, T. and Fattakhova-Rohlfing, D. and Scheu, C. and Pentcheva, R.
    Advanced Functional Materials 28 (2018)
    The beneficial effects of Sn(IV) as a dopant in ultrathin hematite (α-Fe 2 O 3 ) photoanodes for water oxidation are examined. Different Sn concentration profiles are prepared by alternating atomic layer deposition of Fe 2 O 3 and SnO x . Combined data from spectrophotometry and intensity-modulated photocurrent spectroscopy yields the individual process efficiencies for light harvesting, charge separation, and charge transfer. The best performing photoanodes are Sn-doped both on the surface and in the subsurface region and benefit from enhanced charge separation and transfer. Sn-doping throughout the bulk of the hematite photoanode causes segregation at the grain boundaries and hence a lower overall efficiency. Fe 2 O 3 (0001) and terminations, shown to be dominant by microstructural analysis, are investigated by density functional theory (DFT) calculations. The energetics of surface intermediates during the oxygen evolution reaction (OER) reveal that while Sn-doping decreases the overpotential on the (0001) surface, the Fe 2 O 3 orientation shows one of the lowest overpotentials reported for hematite so far. Electronic structure calculations demonstrate that Sn-doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation) and that subsurface Sn-doping introduces a gradient of the band edges that reinforces the band bending at the semiconductor/electrolyte interface and thus boosts charge separation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201804472
  • 2017 • 138 Direct Observation of the Band Gap Transition in Atomically Thin ReS2
    Gehlmann, M. and Aguilera, I. and Bihlmayer, G. and Nemšák, S. and Nagler, P. and Gospodarič, P. and Zamborlini, G. and Eschbach, M. and Feyer, V. and Kronast, F. and Młyńczak, E. and Korn, T. and Plucinski, L. and Schüller,...
    Nano Letters 17 5187-5192 (2017)
    ReS2 is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of this van der Waals compound leads to a highly anisotropic optical, vibrational, and transport behavior. However, the details of the electronic band structure of this fascinating material are still largely unexplored. We present a momentum-resolved study of the electronic structure of monolayer, bilayer, and bulk ReS2 using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS2 are - contrary to assumptions in recent literature - significantly delocalized across the van der Waals gap. Furthermore, we directly observe the evolution of the valence band dispersion as a function of the number of layers, revealing the transition from an indirect band gap in bulk ReS2 to a direct gap in the bilayer and the monolayer. We also find a significantly increased effective hole mass in single-layer crystals. Our results establish bilayer ReS2 as an advantageous building block for two-dimensional devices and van der Waals heterostructures. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b00627
  • 2017 • 137 Electronic properties of LaAlO3/SrTiO3 n-type interfaces: A GGA+U study
    Piyanzina, I.I. and Kopp, T. and Lysogorskiy, Y.V. and Tayurskii, D.A. and Eyert, V.
    Journal of Physics Condensed Matter 29 (2017)
    LaAlO3/SrTiO3 heterostructures as covered by the on-site Coulomb repulsion within the GGA U approach is investigated. Performing a systematic variation of the values of the Coulomb parameters applied to the Ti 3d and La 4f orbitals we put previous suggestions to include a large value for the La 4f states into perspective. Furthermore, our calculations provide deeper insight into the band gap landscape in the space spanned by these Coulomb parameters and the resulting complex interference effects. In addition, we identify important correlations between the local Coulomb interaction within the La 4f shell, the band gap, and the atomic displacements at the interface. In particular, these on-site Coulomb interactions influence buckling within the LaO interface layer, which via its strong coupling to the electrostatic potential in the LAO overlayer causes considerable shifts of the electronic states at the surface and eventually controls the band gap. © 2017 IOP Publishing Ltd. Printed in the UK.
    view abstractdoi: 10.1088/1361-648X/aa57ac
  • 2017 • 136 Electronic structure and magnetism of epitaxial Ni-Mn-Ga(-Co) thin films with partial disorder: A view across the phase transition
    Schleicher, B. and Klar, D. and Ollefs, K. and Diestel, A. and Walecki, D. and Weschke, E. and Schultz, L. and Nielsch, K. and Fähler, S. and Wende, H. and Gruner, M.E.
    Journal of Physics D: Applied Physics 50 (2017)
    The influence of Co-doping in off-stoichiometric Ni-Mn-Ga and Ni-Mn-Ga-Co thin films on the magnetic coupling of the atoms is investigated with x-ray magnetic circular dichroism in both the martensitic as well as austenitic phase, respectively. Additionally, first principles calculations were performed to compare the experimentally obtained absorption spectra with theoretical predictions. Calculated exchange constants and density of states for the different atomic sites underline the large influence of chemical and magnetic order on the magnetocaloric properties of the material. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa8e7c
  • 2017 • 135 Electronic structure and ultrafast dynamics of FeAs-based superconductors by angle- and time-resolved photoemission spectroscopy
    Avigo, I. and Thirupathaiah, S. and Rienks, E.D.L. and Rettig, L. and Charnukha, A. and Ligges, M. and Cortes, R. and Nayak, J. and Jeevan, H.S. and Wolf, T. and Huang, Y. and Wurmehl, S. and Sturza, M.I. and Gegenwart, P. and Gol...
    Physica Status Solidi (B) Basic Research 254 (2017)
    In this article, we review our angle- and time-resolved photoemission studies (ARPES and trARPES) on various ferropnictides. In the ARPES studies, we focus first on the band structure as a function of control parameters. We find near optimally “doped” compounds a Lifshitz transition of hole/electron pocket vanishing type. Second, we investigated the inelastic scattering rates as a function of the control parameter. In contrast to the heavily discussed quantum critical scenario, we find no enhancement of the scattering rate near optimally “doping.” Correlation effects which show up by the non-Fermi-liquid behavior of the scattering rates, together with the Lifshitz transition offer a new explanation for the strange normal state properties and suggests an interpolating superconducting state between BCS and BE condensation. Adding femtosecond time resolution to ARPES provides complementary information on electron and lattice dynamics. We report on the response of the chemical potential by a collective periodic variation coupled to coherent optical phonons in combination with incoherent electron and phonon dynamics described by a three temperature heat bath model. We quantify electron phonon coupling in terms of λ 〈ω〉2 and show that the analysis of the electron excess energy relaxation is a robust approach. The spin density wave ordering leads to a pronounced momentum dependent relaxation dynamics. In the vicinity of kf, hot electrons dissipate their energy by electron–phonon coupling with a characteristic time constant of 200 fs. Electrons at the center of the hole pocket exhibit a four time slower relaxation which is explained by spin-dependent dynamics with its smaller relaxation phase space. This finding has implications beyond the material class of Fe-pnictides because it establishes experimental access to spin-dependent dynamics in materials with spin density waves. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/pssb.201600382
  • 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 First principles modeling of 3d-metal doped three-layer fluorite-structured TiO2 (4,4) nanotube to be used for photocatalytic hydrogen production
    Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Spohr, E. and D'yachkov, P.N.
    Vacuum 146 562-569 (2017)
    We have estimated theoretically the photocatalytic suitability of thinnest single-wall fluorite-structured titania (4,4) nanotube (NT) possessing three layers each (O-Ti-O) and doped by Sc, V, Cr, Mn, Fe, Co, Ni, Cu and Zn atoms substituted for host Ti atoms. For this goal, we have performed large-scale ab initio calculations on TiO2 NTs with three-layer morphology doped by 3d transition metals, using (i) the method of linear combination of atom-centered Gaussian-type orbitals (LCAO) based on the hybrid density functional theory (DFT) incorporating the Hartree-Fock (HF) exchange contribution (DFT+HF) and (ii) the method of linearized augmented cylindrical waves (LACW) with the muffin-tin approximation based on the local density functional approach (LDA). We have compared the ground state electronic structure, particularly the one-electron densities of states (DOSs) from the LCAO and LACW calculations for periodic arrangements of the 3d-metal dopant atoms. The results show clear evidence for a potential photocatalytic application for water splitting in the case of the Sc-doped titania nanotubes only. These NTs show both a reduced band gap of 2.0 eV relative to the pristine NT and an absence of defect-induced levels between the redox potentials of hydrogen and oxygen, so that electron-hole recombination becomes unlikely. Other 3d dopants with higher atomic number, although their band gap also covers the favorable green to orange region of the solar spectrum, are unsuitable because their defect-induced levels are positioned between the redox potential of oxygen and hydrogen, which can be expected to lead to rapid electron-hole recombination. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.vacuum.2017.05.002
  • 2017 • 132 First Principles Neural Network Potentials for Reactive Simulations of Large Molecular and Condensed Systems
    Behler, J.
    Angewandte Chemie - International Edition 56 12828-12840 (2017)
    Modern simulation techniques have reached a level of maturity which allows a wide range of problems in chemistry and materials science to be addressed. Unfortunately, the application of first principles methods with predictive power is still limited to rather small systems, and despite the rapid evolution of computer hardware no fundamental change in this situation can be expected. Consequently, the development of more efficient but equally reliable atomistic potentials to reach an atomic level understanding of complex systems has received considerable attention in recent years. A promising new development has been the introduction of machine learning (ML) methods to describe the atomic interactions. Once trained with electronic structure data, ML potentials can accelerate computer simulations by several orders of magnitude, while preserving quantum mechanical accuracy. This Review considers the methodology of an important class of ML potentials that employs artificial neural networks. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201703114
  • 2017 • 131 Functional electronic inversion layers at ferroelectric domain walls
    Mundy, J.A. and Schaab, J. and Kumagai, Y. and Cano, A. and Stengel, M. and Krug, I.P. and Gottlob, D.M. and Doǧanay, H. and Holtz, M.E. and Held, R. and Yan, Z. and Bourret, E. and Schneider, C.M. and Schlom, D.G. and Muller, D....
    Nature Materials 16 622-627 (2017)
    Ferroelectric domain walls hold great promise as functional two-dimensional materials because of their unusual electronic properties. Particularly intriguing are the so-called charged walls where a polarity mismatch causes local, diverging electrostatic potentials requiring charge compensation and hence a change in the electronic structure. These walls can exhibit significantly enhanced conductivity and serve as a circuit path. The development of all-domain-wall devices, however, also requires walls with controllable output to emulate electronic nano-components such as diodes and transistors. Here we demonstrate electric-field control of the electronic transport at ferroelectric domain walls. We reversibly switch from resistive to conductive behaviour at charged walls in semiconducting ErMnO 3. We relate the transition to the formation - and eventual activation - of an inversion layer that acts as the channel for the charge transport. The findings provide new insight into the domain-wall physics in ferroelectrics and foreshadow the possibility to design elementary digital devices for all-domain-wall circuitry. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
    view abstractdoi: 10.1038/nmat4878
  • 2017 • 130 In-situ tracking the structural and chemical evolution of nanostructured CuCr alloys
    Zhang, Z. and Guo, J. and Dehm, G. and Pippan, R.
    Acta Materialia 138 42-51 (2017)
    We report the thermal stability of supersaturated CuCr nanocrystallines alloys at the atomic resolution using modern spherical aberration-corrected transmission electron microscopy (TEM) via performing in-situ structural and spectroscopy experiments. It is found that CuCr nanocrystallines are not only subjected to a structural change but also undergo a chemical evolution upon annealing. Chemical destabilization of supersaturated CuCr nanocrystallines occurs at a quite low temperature. Heating triggers a rapid separation of Cu and Cr grains at the forced intermixing zone, accompanied by an obvious decrease of average interface width whereas the grain growth is not significant. Elemental profiles and images recorded in real time reveal that the local compositions vary with heating, which in turn permits to derive the concentration of excess vacancy generated by deformation and observe its evolution with temperature, further to analyze the dynamic behavior in nanocrystalline materials. Electronic structure changes at the interface forced intermixing zone are revealed by the fine structure analysis. The study uncovers the interplay between the thermal stability and chemical decomposition process of bulk nanostructured materials in real-time. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.07.039
  • 2017 • 129 Machine learning molecular dynamics for the simulation of infrared spectra
    Gastegger, M. and Behler, J. and Marquetand, P.
    Chemical Science 8 6924-6935 (2017)
    Machine learning has emerged as an invaluable tool in many research areas. In the present work, we harness this power to predict highly accurate molecular infrared spectra with unprecedented computational efficiency. To account for vibrational anharmonic and dynamical effects-typically neglected by conventional quantum chemistry approaches-we base our machine learning strategy on ab initio molecular dynamics simulations. While these simulations are usually extremely time consuming even for small molecules, we overcome these limitations by leveraging the power of a variety of machine learning techniques, not only accelerating simulations by several orders of magnitude, but also greatly extending the size of systems that can be treated. To this end, we develop a molecular dipole moment model based on environment dependent neural network charges and combine it with the neural network potential approach of Behler and Parrinello. Contrary to the prevalent big data philosophy, we are able to obtain very accurate machine learning models for the prediction of infrared spectra based on only a few hundreds of electronic structure reference points. This is made possible through the use of molecular forces during neural network potential training and the introduction of a fully automated sampling scheme. We demonstrate the power of our machine learning approach by applying it to model the infrared spectra of a methanol molecule, n-alkanes containing up to 200 atoms and the protonated alanine tripeptide, which at the same time represents the first application of machine learning techniques to simulate the dynamics of a peptide. In all of these case studies we find an excellent agreement between the infrared spectra predicted via machine learning models and the respective theoretical and experimental spectra. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7sc02267k
  • 2017 • 128 Metallic NiPS3@NiOOH Core-Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction
    Konkena, B. and Masa, J. and Botz, A. J. R. and Sinev, I. and Xia, W. and Kossmann, J. and Drautz, R. and Muhler, M. and Schuhmann, W.
    ACS Catalysis 7 229--237 (2017)
    We report metallic NiPS3@NiOOH core shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS3 nanosheets are obtained by exfoliation of bulk NiPS3 in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS3@NiOOH core shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS3@NiOOH core shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS3 nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS3 nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS3@NiOOH core shell heterostructure, where the presence of P and S modulates the rface electronic structure of Ni in NiPS3, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS3@ NiOOH core shell electrode materials for the electrochemical water oxidation.
    view abstractdoi: 10.1021/acscatal.6b02203
  • 2017 • 127 Optical anisotropy in the electronic nematic phase of FeSe
    Chinotti, M. and Pal, A. and Degiorgi, L. and Böhmer, A.E. and Canfield, P.C.
    Physical Review B 96 (2017)
    At ambient pressure, FeSe undergoes a structural, tetragonal-to-orthorhombic, phase transition at Ts≃90 K without any magnetic ordering on further cooling. FeSe thus provides an arena for examining the nematic phase without the complications following the reconstruction of the Fermi surface due to the antiferromagnetic order within the orthorhombic state. We perform an optical-reflectivity investigation across the structural transition, as a function of uniaxial stress in order to detwin the specimen. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress for T≤Ts, which extends to energy scales of about 0.5 eV. The sign changes of the optical anisotropy between distinct energy intervals suggest a complex evolution of the polarized electronic structure in the nematic phase. The temperature dependence of the optical anisotropy for the fully detwinned specimen is furthermore acting as a proxy for the order parameter of nematicity. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.96.121112
  • 2017 • 126 Promoting Photocatalytic Overall Water Splitting by Controlled Magnesium Incorporation in SrTiO3 Photocatalysts
    Han, K. and Lin, Y.-C. and Yang, C.-M. and Jong, R. and Mul, G. and Mei, B.
    ChemSusChem 10 4510-4516 (2017)
    SrTiO3 is a well-known photocatalyst inducing overall water splitting when exposed to UV irradiation of wavelengths &lt;370 nm. However, the apparent quantum efficiency of SrTiO3 is typically low, even when functionalized with nanoparticles of Pt or Ni@NiO. Here, we introduce a simple solid-state preparation method to control the incorporation of magnesium into the perovskite structure of SrTiO3. After deposition of Pt or Ni@NiO, the photocatalytic water-splitting efficiency of the Mg:SrTiOx composites is up to 20 times higher compared to SrTiO3 containing similar catalytic nanoparticles, and an apparent quantum yield (AQY) of 10 % can be obtained in the wavelength range of 300–400 nm. Detailed characterization of the Mg:SrTiOx composites revealed that Mg is likely substituting the tetravalent Ti ion, leading to a favorable surface–space–charge layer. This originates from tuning of the donor density in the cubic SrTiO3 structure by Mg incorporation and enables high oxygen-evolution rates. Nevertheless, interfacing with an appropriate hydrogen evolution catalyst is mandatory and non-trivial to obtain high-performance in water splitting. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201701794
  • 2017 • 125 Redox Activity of Oxo-Bridged Iridium Dimers in an N,O-Donor Environment: Characterization of Remarkably Stable Ir(IV,V) Complexes
    Sinha, S.B. and Shopov, D.Y. and Sharninghausen, L.S. and Stein, C.J. and Mercado, B.Q. and Balcells, D. and Pedersen, T.B. and Reiher, M. and Brudvig, G.W. and Crabtree, R.H.
    Journal of the American Chemical Society 139 9672-9683 (2017)
    Chemical and electrochemical oxidation or reduction of our recently reported Ir(IV,IV) mono-μ-oxo dimers results in the formation of fully characterized Ir(IV,V) and Ir(III,III) complexes. The Ir(IV,V) dimers are unprecedented and exhibit remarkable stability under ambient conditions. This stability and modest reduction potential of 0.99 V vs NHE is in part attributed to complete charge delocalization across both Ir centers. Trends in crystallographic bond lengths and angles shed light on the structural changes accompanying oxidation and reduction. The similarity of these mono-μ-oxo dimers to our Ir "blue solution" water-oxidation catalyst gives insight into potential reactive intermediates of this structurally elusive catalyst. Additionally, a highly reactive material, proposed to be a Ir(V,V) μ-oxo species, is formed on electrochemical oxidation of the Ir(IV,V) complex in organic solvents at 1.9 V vs NHE. Spectroelectrochemistry shows reversible conversion between the Ir(IV,V) and proposed Ir(V,V) species without any degradation, highlighting the exceptional oxidation resistance of the 2-(2-pyridinyl)-2-propanolate (pyalk) ligand and robustness of these dimers. The Ir(III,III), Ir(IV,IV) and Ir(IV,V) redox states have been computationally studied both with DFT and multiconfigurational calculations. The calculations support the stability of these complexes and provide further insight into their electronic structures. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b04874
  • 2017 • 124 Room temperature 2D electron gas at the (001)-SrTiO3 surface
    Gonzalez, S. and Mathieu, C. and Copie, O. and Feyer, V. and Schneider, C.M. and Barrett, N.
    Applied Physics Letters 111 (2017)
    Functional oxides and phenomena such as a 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS electronics. Non-invasive techniques are required to study the surface chemistry and electronic structure, underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy to chemistry and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. We present results demonstrating a room temperature 2DEG at the (001)-SrTiO3 surface. The 2DEG is switched on by soft X-ray irradiation. © 2017 Author(s).
    view abstractdoi: 10.1063/1.5001222
  • 2017 • 123 Spontaneous Si-C bond cleavage in (TriphosSi)-nickel complexes
    Petuker, A. and Mebs, S. and Schüth, N. and Gerschel, P. and Reback, M.L. and Mallick, B. and Van Gastel, M. and Haumann, M. and Apfel, U.-P.
    Dalton Transactions 46 907-917 (2017)
    Herein, we report on the versatile reactions of CH3C(CH2PPh2)3 as well as CH3Si(CH2PPh2)3 derived Ni-complexes. While Ni[CH3C(CH2PPh2)3] complexes reveal high stability, the Ni[CH3Si(CH2PPh2)3] analogs show rapid decomposition at room temperature and afford the unprecedented pseudo-tetrahedral phosphino methanide complex 5. We provide a detailed electronic structure of 5 from X-ray absorption and emission spectroscopy data analysis in combination with DFT calculations, as well as from comparison with structurally related complexes. A mechanistic study for the formation of complex 5 by reaction with BF4 − is presented, based on a comparison of experimental data with quantum chemical calculations. We also show a simple route towards isolable Ni(i)-complexes on the gram scale. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6dt04048a
  • 2017 • 122 Surface State Tunneling Signatures in the Two-Component Superconductor UPt3
    Lambert, F. and Akbari, A. and Thalmeier, P. and Eremin, I.
    Physical Review Letters 118 (2017)
    Quasiparticle interference (QPI) imaging of Bogoliubov excitations in quasi-two-dimensional unconventional superconductors has become a powerful technique for measuring the superconducting gap and its symmetry. Here, we present the extension of this method to three-dimensional superconductors and analyze the expected QPI spectrum for the two-component heavy-fermion superconductor UPt3 whose gap structure is still controversial. Starting from a 3D electronic structure and the three proposed chiral gap models E1g,u or E2u, we perform a slab calculation that simultaneously gives extended bulk states and topologically protected in-gap dispersionless surface states. We show that the number of Weyl arcs and their hybridization with the line node provides a fingerprint that may finally determine the true nodal structure of the UPt3 superconductor. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.118.087004
  • 2017 • 121 Synthesis, Solid-State Structure, and Bonding Analysis of a Homoleptic Beryllium Azide
    Naglav, D. and Tobey, B. and Lyhs, B. and Römer, B. and Bläser, D. and Wölper, C. and Jansen, G. and Schulz, S.
    Angewandte Chemie - International Edition 56 8559-8563 (2017)
    [Ph4P]2[Be(N3)4] (1) and [PNP]2[Be(N3)4] (2; PNP=Ph3PNPPh3) were synthesized by reacting Be(N3)2 with [Ph4P]N3 and [PNP]N3. Compound 1 represents the first structurally characterized homoleptic beryllium azide. The electronic structure and bonding situation in the tetraazidoberyllate dianion [Be(N3)4]2− were investigated by quantum-chemical calculations (NPA, ELF, LOL). © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201703147
  • 2017 • 120 The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface
    Morbec, J.M. and Kratzer, P.
    Journal of Chemical Physics 146 (2017)
    Using first-principles calculations based on density-functional theory (DFT), we investigated the effects of the van der Waals (vdW) interactions on the structural and electronic properties of anthracene and pentacene adsorbed on the Ag(111) surface. We found that the inclusion of vdW corrections strongly affects the binding of both anthracene/Ag(111) and pentacene/Ag(111), yielding adsorption heights and energies more consistent with the experimental results than standard DFT calculations with generalized gradient approximation (GGA). For anthracene/Ag(111) the effect of the vdW interactions is even more dramatic: we found that “pure” DFT-GGA calculations (without including vdW corrections) result in preference for a tilted configuration, in contrast to the experimental observations of flat-lying adsorption; including vdW corrections, on the other hand, alters the binding geometry of anthracene/Ag(111), favoring the flat configuration. The electronic structure obtained using a self-consistent vdW scheme was found to be nearly indistinguishable from the conventional DFT electronic structure once the correct vdW geometry is employed for these physisorbed systems. Moreover, we show that a vdW correction scheme based on a hybrid functional DFT calculation (HSE) results in an improved description of the highest occupied molecular level of the adsorbed molecules. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4973839
  • 2017 • 119 Ultra-stiff metallic glasses through bond energy density design
    Schnabel, V. and Köhler, M. and Music, D. and Bednarcik, J. and Clegg, W.J. and Raabe, D. and Schneider, J.M.
    Journal of Physics Condensed Matter 29 (2017)
    The elastic properties of crystalline metals scale with their valence electron density. Similar observations have been made for metallic glasses. However, for metallic glasses where covalent bonding predominates, such as metalloid metallic glasses, this relationship appears to break down. At present, the reasons for this are not understood. Using high energy x-ray diffraction analysis of melt spun and thin film metallic glasses combined with density functional theory based molecular dynamics simulations, we show that the physical origin of the ultrahigh stiffness in both metalloid and non-metalloid metallic glasses is best understood in terms of the bond energy density. Using the bond energy density as novel materials design criterion for ultra-stiff metallic glasses, we are able to predict a Co33.0Ta3.5B63.5 short range ordered material by density functional theory based molecular dynamics simulations with a high bond energy density of 0.94 eV Å-3 and a bulk modulus of 263 GPa, which is 17% greater than the stiffest Co-B based metallic glasses reported in literature. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa72cb
  • 2016 • 118 (MeZn)2(μ-η2:η2-N6Ph2): A Powerful Starting Reagent for the Synthesis of Metal Hexazene Complexes
    Stienen, C. and Gondzik, S. and Gehlhaar, A. and Haack, R. and Wölper, C. and Jansen, G. and Schulz, S.
    Organometallics 35 1022-1029 (2016)
    [(MeLDippZn)2(μ-η2:η2-PhN6Ph)] (3), which was synthesized by reaction of MeLDipp 2Zn2 with PhN3, reacts with two equivalents of Me2Zn to give [(MeZn)2(μ-η2:η2-PhN6Ph)] (2). The reaction of 2 with pyridine gave [(MeZn)2(μ-η2:η2-PhN6Ph)(Py)2] (4), while reactions with H-acidic ligands (MeLMesH, MeLPhH) occurred with elimination of methane and formation of [(MeLMesZn)2(μ-η2:η2-PhN6Ph)] (1) and [(MeLPhZn)2(μ-η2:η2-PhN6Ph)] (5). The reaction of 1 with two equivalents of MeLi yielded the heterobimetallic hexazene complex [(MeZn)(μ-η2:η2-PhN6Ph)(Li)], which was found to undergo stepwise reaction with Me2AlCl to give [MeZn(μ-η2:η2-PhN6Ph)AlMe2] and finally [(Me2Al)2(μ-η2:η2-PhN6Ph)(thf)2] (6). Compounds 3-6 were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction. Quantum chemical calculations were performed in order to investigate the electronic structure of 4′ and 6′ in more detail and to identify the absorption bands of the hexazene unit. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.6b00116
  • 2016 • 117 Ab initio calculations of doped TiO2 anatase (101) nanotubes for photocatalytical water splitting applications
    Lisovski, O. and Chesnokov, A. and Piskunov, S. and Bocharov, D. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Materials Science in Semiconductor Processing 42 138-141 (2016)
    TiO2 (titania) is one of the promising materials for photocatalytic applications. In this paper we report on recently obtained theoretical results for N and S doped, as well as N+S co-doped 6-layer (101) anatase nanotube (NT). First principles calculations in our study have been performed using a modified B3LYP hybrid exchange-correlation functional within density functional theory (DFT). Here we discuss the energy of defect formation mechanism and electronic band structure for nanotubes under study. We also report on influence of dopant concentration on the NT's band structure and discuss the defect-defect interactions. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.mssp.2015.09.003
  • 2016 • 116 Ab Initio Molecular Dynamics Simulations of Nitrogen/VN(001) Surface Reactions: Vacancy-Catalyzed N2 Dissociative Chemisorption, N Adatom Migration, and N2 Desorption
    Sangiovanni, D.G. and Mei, A.B. and Hultman, L. and Chirita, V. and Petrov, I. and Greene, J.E.
    Journal of Physical Chemistry C 120 12503-12516 (2016)
    We use density-functional ab initio molecular dynamics to investigate the kinetics of N/VN(001) surface reactions at temperatures ranging from 1600 to 2300 K. N adatoms (Nad) on VN(001) favor epitaxial atop-V positions and diffuse among them by transiting through 4-fold hollow (FFH) sites, at which they are surrounded by two V and two N surface atoms. After several atop-V → FFH → atop-V jumps, isolated N adatoms bond strongly with an underlying N surface (Nsurf) atom. Frequent Nad/Nsurf pair exchange reactions lead to N2 desorption, which results in the formation of an anion surface vacancy. N vacancies rapidly migrate via in-plane 〈110〉 jumps and act as efficient catalysts for the dissociative chemisorption of incident N2 molecules. During exposure of VN(001) to incident atomic N gas atoms, Nad/Nad recombination and desorption is never observed, despite a continuously high N monomer surface coverage. Instead, N2 desorption is always initiated by a N adatom removing a N surface atom or by energetic N gas atoms colliding with Nad or Nsurf atoms. Similarities and differences between N/VN(001) vs. previous N/TiN(001) results, discussed on the basis of temperature-dependent ab initio electronic structures and chemical bonding, provide insights for controlling the reactivity of NaCl-structure transition-metal nitride (001) surfaces via electron-concentration tuning. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.6b02652
  • 2016 • 115 Accessing and probing of the photo-induced hidden state in 1 T -TaS2 with time- and angle-resolved photoemission spectroscopy
    Avigo, I. and Vaskivskyi, I. and Ligges, M. and Kalläne, M. and Rossnagel, K. and Stojchevska, L. and Mihailovia, D. and Bovensiepen, U.
    Proceedings of SPIE - The International Society for Optical Engineering 9931 (2016)
    A previous time-resolved optical study reported on a metastable hidden electronic state in 1T-TaS2, which is only accessible upon photoexcitation and created under non-equilibrium conditions [1]. The properties of such a state are distinct from those of any other state in the equilibrium phase diagram and it is possible to revert to the thermodynamic initial state either by illuminating with picosecond laser pulses or by applying other thermal erase procedures. In this work we show photoinduced switching to a metastable hidden state on the same material, and probe it by means of both static and time-resolved photoemission spectroscopy, thus having direct access to the electronic structure of the system. From our experimental findings and comparison with other studies, we conclude that we obtain partial switching, leading to a hidden state with persisting insulating nature but significant modifications in the electronic structure and CDW ordering. © 2016 SPIE.
    view abstractdoi: 10.1117/12.2239258
  • 2016 • 114 Analysis of Electronic and Structural Properties of Surfaces and Interfaces Based on LaAlO 3 and SrTiO 3
    Piyanzina, I.I. and Lysogorskiy, Y.V. and Varlamova, I.I. and Kiiamov, A.G. and Kopp, T. and Eyert, V. and Nedopekin, O.V. and Tayurskii, D.A.
    Journal of Low Temperature Physics 185 597-602 (2016)
    Recently, it was established that a two-dimensional electron system can arise at the interface between two oxide insulators LaAlO3 and SrTiO3. This paradigmatic example exhibits metallic behaviors and magnetic properties between non-magnetic and insulating oxides. Despite a huge amount of theoretical and experimental work a thorough understanding is yet to be achieved. We analyzed the structural deformations of a LaAlO3 (001) slab induced by hydrogen adatoms and oxygen vacancies at its surface by means of density functional theory. Moreover, we investigated the influence of surface reconstruction on the density of states and determined the change of the local density of states at the Fermi level with increasing distance from the surface for bare LaAlO3 and for a conducting LaAlO3/SrTiO3 interface. In addition, the Al-atom displacements and distortions of the TiO6-octahedra were estimated. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10909-016-1483-2
  • 2016 • 113 Are Mo2BC nanocrystalline coatings damage resistant? Insights from comparative tension experiments
    Djaziri, S. and Gleich, S. and Bolvardi, H. and Kirchlechner, C. and Hans, M. and Scheu, C. and Schneider, J.M. and Dehm, G.
    Surface and Coatings Technology 289 213-218 (2016)
    Mo2BC nanocrystalline coatings were deposited on Cu substrates to compare their mechanical performance with bench-mark TiAlN, and pure Mo, Al and Al2O3 reference coatings. The Mo2BC coatings were characterized by X-ray diffraction and transmission electron microscopy to analyze the microstructure. In order to study the damage behavior, the coatings were subjected to uniaxial tensile loading and the crack spacing with increasing strain was monitored using optical and scanning electron microscopy. Based on crack density measurements, the Mo2BC coatings were found to be significantly less prone to cracking than the bench-mark TiAlN coatings. The higher resistance to cracking arises from the electronic structure of the Mo2BC nanolaminates, which imparts moderate ductility to the deformation behavior. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.02.010
  • 2016 • 112 Comparing the accuracy of high-dimensional neural network potentials and the systematic molecular fragmentation method: A benchmark study for all-trans alkanes
    Gastegger, M. and Kauffmann, C. and Behler, J. and Marquetand, P.
    Journal of Chemical Physics 144 (2016)
    Many approaches, which have been developed to express the potential energy of large systems, exploit the locality of the atomic interactions. A prominent example is the fragmentation methods in which the quantum chemical calculations are carried out for overlapping small fragments of a given molecule that are then combined in a second step to yield the system's total energy. Here we compare the accuracy of the systematic molecular fragmentation approach with the performance of high-dimensional neural network (HDNN) potentials introduced by Behler and Parrinello. HDNN potentials are similar in spirit to the fragmentation approach in that the total energy is constructed as a sum of environment-dependent atomic energies, which are derived indirectly from electronic structure calculations. As a benchmark set, we use all-trans alkanes containing up to eleven carbon atoms at the coupled cluster level of theory. These molecules have been chosen because they allow to extrapolate reliable reference energies for very long chains, enabling an assessment of the energies obtained by both methods for alkanes including up to 10 000 carbon atoms. We find that both methods predict high-quality energies with the HDNN potentials yielding smaller errors with respect to the coupled cluster reference. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4950815
  • 2016 • 111 Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO3/SrTiO3 Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
    Eiteneer, D. and Pálsson, G.K. and Nemšák, S. and Gray, A.X. and Kaiser, A.M. and Son, J. and LeBeau, J. and Conti, G. and Greer, A.A. and Keqi, A. and Rattanachata, A. and Saw, A.Y. and Bostwick, A. and Rotenberg, E. and Gulli...
    Journal of Electron Spectroscopy and Related Phenomena 211 70-81 (2016)
    LaNiO3 (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO3 (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La0.7Sr0.3MnO3/SrTiO3 superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.elspec.2016.04.008
  • 2016 • 110 Diffusion of solutes in fcc Cobalt investigated by diffusion couples and first principles kinetic Monte Carlo
    Neumeier, S. and Rehman, H.U. and Neuner, J. and Zenk, C.H. and Michel, S. and Schuwalow, S. and Rogal, J. and Drautz, R. and Göken, M.
    Acta Materialia 106 304-312 (2016)
    The interdiffusivity of Al and the transition metal solutes Ti, V, Cr, Mn, Fe, Nb, Mo, Ru, Ta, W, and Re in fcc Co is characterized at 1373 K, 1473 K and 1573 K by binary diffusion couples. The experimental results are complemented by first-principles calculations in combination with kinetic Monte Carlo simulations to investigate the diffusion of Re, W, Mo and Ta in fcc Co. The interdiffusion coefficients of alloying elements in fcc Co are generally smaller than in fcc Ni, but the correlation between interdiffusion coefficients and the atomic number of metal solutes is comparable in Co and Ni. With increasing atomic number and decreasing atomic radii the interdiffusion coefficients of the investigated elements, except for Mn and Fe, decrease strongly. The trends in the diffusivity determined by experiment and simulation are in excellent agreement. Re is the slowest diffusing element in fcc Co among the investigated elements. The electronic structure calculations indicate that this is caused by strong directional bonds between Re and neighboring Co atoms. The overall lower diffusivity of solute atoms in Co as compared to Ni suggests that diffusion controlled processes could be slower in Co-base superalloys. © 2016 Acta Materialia Inc. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.028
  • 2016 • 109 Effects of phase stability, lattice ordering, and electron density on plastic deformation in cubic TiWN pseudobinary transition-metal nitride alloys
    Sangiovanni, D.G. and Hultman, L. and Chirita, V. and Petrov, I. and Greene, J.E.
    Acta Materialia 103 823-835 (2016)
    We carry out density functional theory calculations to compare the energetics of layer glide, as well as stress vs. strain curves, for cubic Ti0.5W0.5N pseudobinary alloys and reference B1-structure TiN. Irrespective of the degree of ordering on the metal sublattice, the hardness and stiffness of Ti0.5W0.5N, as estimated by stress-strain results and resistance to layer glide, are comparable to that of the parent binary TiN, while ductility is considerably enhanced. After an initial elastic response to an applied load, the pseudobinary alloy deforms plastically, thus releasing accumulated mechanical stress. In contrast, stress continues to increase linearly with strain in TiN. Layer glide in Ti0.5W0.5N is promoted by a high valence-electron concentration which enables the formation of strong metallic bonds within the slip direction upon deformation. [111]-oriented Ti0.5W0.5N layers characterized by high local metal-sublattice ordering exhibit low resistance to slip along &lt;110&gt; directions due to energetically favored formation of (111) hexagonal stacking faults. This is consistent with the positive formation energy of &lt;111&gt;-ordered Ti0.5W0.5N with respect to mixing of cubic-B1 TiN and hexagonal WC-structure WN. In the cubic pseudobinary alloy, slip occurs parallel, as well as orthogonal, to the resolved applied stress at the interface between layers with the lowest friction. We suggest that analogous structural metastability (mixing cubic and hexagonal TM nitride binary phases) and electronic (high valence electron concentration) effects are responsible for the enhanced toughness recently demonstrated experimentally for cubic single-crystal pseudobinary V0.5W0.5N and V0.5Mo0.5N epitaxial layers. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.10.039
  • 2016 • 108 From electronic structure to phase diagrams: A bottom-up approach to understand the stability of titanium–transition metal alloys
    Huang, L.-F. and Grabowski, B. and Zhang, J. and Lai, M.-J. and Tasan, C.C. and Sandlöbes, S. and Raabe, D. and Neugebauer, J.
    Acta Materialia 113 311-319 (2016)
    We have computed formation energies for all technologically relevant transition metal solutes in the α, β, and ω phases of Ti, employing ab initio simulations. We analyze and explain their periodic-table trends, and from their differences we derive stabilization energies which provide direct insight into phase stabilization effects of the various solutes with respect to α, β, and ω. This allows us to identify strong β stabilizers in the middle of each electronic d shell in consistency with experimental knowledge. Based on an extension of the stabilization energies to free energies we derive a wide range of Ti-transition metal phase diagrams. A detailed comparison to available experimental martensitic transformation temperatures and to measurements performed in this study shows that, despite some quantitative discrepancies, the qualitative trends can be expected to be correct. An important feature that is displayed by a limited range of the computed phase diagrams is a triple point at which the three phases, α, β, and ω, meet. This insight provides a plausible explanation for the complexity observed in gum metals, a class of Ti alloys with very special materials properties. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.059
  • 2016 • 107 High Thermopower with Metallic Conductivity in p-Type Li-Substituted PbPdO2
    Lamontagne, L.K. and Laurita, G. and Gaultois, M.W. and Knight, M. and Ghadbeigi, L. and Sparks, T.D. and Gruner, M.E. and Pentcheva, R. and Brown, C.M. and Seshadri, R.
    Chemistry of Materials 28 3367-3373 (2016)
    PbPdO2 is a band semiconductor with a band gap arising from the filled d8 nature of square-planar Pd2+. We establish that hole doping through Li substitution for Pd in PbPdO2 results in a p-type metallic oxide with a positive temperature coefficient of resistance for substitution amounts as small as 2 mol % Li for Pd. Furthermore, PbPd1-xLixO2 demonstrates a high Seebeck coefficient and is therefore an oxide thermoelectric material with high thermopower despite the metallic conductivity. Up to 4 mol % Li is found to substitute for Pd as verified by Rietveld refinement of neutron diffraction data. At this maximal Li substitution, the resistivity is driven below the Mott metallic maximum to 3.5 × 10-3 ω cm with a Seebeck coefficient of 115 μV/K at room temperature, which increases to 175 μV/K at 600 K. These electrical properties are almost identical to those of the well-known p-type oxide thermoelectric NaxCoO2. Nonmagnetic Li-substituted PbPdO2 does not possess a correlated, magnetic state with high-spin degeneracy as found in some complex cobalt oxides. This suggests that there are other avenues to achieving high Seebeck coefficients with metallic conductivities in oxide thermoelectrics. The electrical properties coupled with the moderately low lattice thermal conductivities allow for a zT of 0.12 at 600 K, the maximal temperature measured here. The trend suggests yet higher values at elevated temperatures. First-principles calculations of the electronic structure and electrical transport provide insight into the observed properties. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b00447
  • 2016 • 106 Interactions between metal species and nitrogen-functionalized carbon nanotubes
    Xia, W.
    Catalysis Science and Technology 6 630-644 (2016)
    Nitrogen-functionalized carbon nanotubes are promising materials in catalysis due to their versatile surface properties involving nitrogen groups, oxygen groups, surface defects and metal impurities. These factors can be used to tune the dispersion, morphology, crystal structure, electronic structure, mobility/stability and finally the catalytic performance of supported metal nanoparticles. This review focuses on selected examples aiming at understanding the interactions between surface groups, defects, and metal species and their impact on the catalytic properties in electrocatalysis and gas-phase redox catalysis. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5cy01694k
  • 2016 • 105 Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO3/La0.7Sr0.3MnO3
    Krug, I.P. and Doganay, H. and Nickel, F. and Gottlob, D.M. and Schneider, C.M. and Morelli, A. and Preziosi, D. and Lindfors-Vrejoiu, I. and Laskowski, R. and Barrett, N.
    Journal of Applied Physics 120 (2016)
    We employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO3/La0.7Sr0.3MnO3 (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4962007
  • 2016 • 104 On the origin of the improved ruthenium stability in RuO2-IrO2 mixed oxides
    Kasian, O. and Geiger, S. and Stock, P. and Polymeros, G. and Breitbach, B. and Savan, A. and Ludwig, Al. and Cherevko, S. and Mayrhofer, K.J.J.
    Journal of the Electrochemical Society 163 F3099-F3104 (2016)
    High oxygen evolution reaction activity of ruthenium and long term stability of iridium in acidic electrolytes make their mixed oxides attractive candidates for utilization as anodes in water electrolyzers. Indeed, such materials were addressed in numerous previous studies. The application of a scanning flow cell connected to an inductively coupled plasma mass spectrometer allowed us now to examine the stability and activity toward oxygen evolution reaction of such mixed oxides in parallel. The whole composition range of Ir-Ru mixtures has been covered in a thin film material library. In the whole composition range the rate of Ru dissolution is observed to be much higher than that of Ir. Eventually, due to the loss of Ru, the activity of the mixed oxides approaches the value corresponding to pure IrO2. Interestingly, the loss of only a few percent of a monolayer in Ru surface concentration results in a significant drop in activity. Several explanations of this phenomenon are discussed. It is concluded that the herein observed stability of mixed Ir-Ru oxide systems is most likely a result of high corrosion resistance of the iridium component, but not due to an alteration of the material's electronic structure. © 2016 by the Authors.
    view abstractdoi: 10.1149/2.0131611jes
  • 2016 • 103 Perspective: Machine learning potentials for atomistic simulations
    Behler, J.
    Journal of Chemical Physics 145 (2016)
    Nowadays, computer simulations have become a standard tool in essentially all fields of chemistry, condensed matter physics, and materials science. In order to keep up with state-of-the-art experiments and the ever growing complexity of the investigated problems, there is a constantly increasing need for simulations of more realistic, i.e., larger, model systems with improved accuracy. In many cases, the availability of sufficiently efficient interatomic potentials providing reliable energies and forces has become a serious bottleneck for performing these simulations. To address this problem, currently a paradigm change is taking place in the development of interatomic potentials. Since the early days of computer simulations simplified potentials have been derived using physical approximations whenever the direct application of electronic structure methods has been too demanding. Recent advances in machine learning (ML) now offer an alternative approach for the representation of potential-energy surfaces by fitting large data sets from electronic structure calculations. In this perspective, the central ideas underlying these ML potentials, solved problems and remaining challenges are reviewed along with a discussion of their current applicability and limitations. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4966192
  • 2016 • 102 Spectroscopy of low and intermediate Z elements at extreme conditions: in situ studies of Earth materials at pressure and temperature via X-ray Raman scattering
    Sternemann, C. and Wilke, M.
    High Pressure Research 36 275-292 (2016)
    X-ray Raman scattering spectroscopy is an emerging method in the study of low and intermediate Z elements' core-electron excitations at extreme conditions in order to reveal information on local structure and electronic state of matter in situ. We discuss the capabilities of this method to address questions in Earth materials' science and demonstrate its sensitivity to detect changes in the oxidation state, electronic structure, coordination, and spin state. Examples are presented for the study of the oxygen K-, silicon L- and iron M-edges. We assess the application of both temperature and pressure in such investigations exploiting diamond anvil cells in combination with resistive or laser heating which is required to achieve realistic conditions of the Earth's crust, mantle, and core. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/08957959.2016.1198903
  • 2016 • 101 Structure and thermodynamics of nondipolar molecular liquids and solutions from integral equation theory
    Frach, R. and Heil, J. and Kast, S.M.
    Molecular Physics 114 2461-2476 (2016)
    Solvent-induced solute polarisation of nondipolar solvents originates mainly from specific directional interactions and higher electrostatic multipole moments. Popular continuum solvation models such as the polarisable continuum models ignore such interactions and, therefore, cannot adequately model solvation effects on electronic structure in these environments. Important examples of nondipolar solvents that are indistinguishable by continuum methods are benzene and hexafluorobenzene. Both substances have very similar macroscopic properties, while solutes dissolved in either benzene or hexafluorobenzene behave differently due to their inverted electrostatic quadrupole moments and slightly different size. As a first step towards a proper and computationally feasible description of nondipolar molecular solvents, we present here integral equation theory results based on various forms of the reference interaction site model coupled to quantum-chemical calculations for benzene and hexafluorobenzene solutions of small molecules. We analyse solvation structures, also in comparison with molecular dynamics simulations, and show that predictions of transfer Gibbs energies, which define partition constants, benefit substantially from considering the exact, wave function-derived electrostatic field distribution beyond a simple point charge solute model in comparison with experimental data. Moreover, by constructing artificial uncharged and charge-inverted toy models of the solvents, it is possible to dissect the relative importance of dispersion and quadrupolar electrostatic effects on the partitioning equilibria. Such insight can help to design specifically optimised solvents to control solubility and selectivity for a wide range of applications. © 2016 Informa UK Limited, trading as Taylor & Francis Group
    view abstractdoi: 10.1080/00268976.2016.1167266
  • 2016 • 100 The impact of carbon and oxygen in alpha-titanium: Ab initio study of solution enthalpies and grain boundary segregation
    Aksyonov, D.A. and Hickel, T. and Neugebauer, J. and Lipnitskii, A.G.
    Journal of Physics Condensed Matter 28 (2016)
    The solution, grain boundary (GB) segregation, and co-segregation of carbon and oxygen atoms in α-titanium are studied using density functional theory. For five titanium tilt boundaries, including T1, T2, and C1 twin systems, we determine the GB structure, as well as GB energy and excess volume. The segregation energies and volumes of carbon and oxygen are calculated for 23 inequivalent interstitial voids, while for co-segregation 75 configurations are considered. It is obtained that depending on the type of the segregation void both a positive and a negative segregation process is possible. The physical reasons of segregation are explained in terms of the analysis of the void atomic geometry, excess volume and features of the electronic structure at the Fermi level. Although carbon and oxygen show qualitatively similar properties in α-Ti, several distinctions are observed for their segregation behavior and mutual interactions. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/38/385001
  • 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 Toughness enhancement in highly NbN-alloyed Ti-Al-N hard coatings
    Mikula, M. and Plašienka, D. and Sangiovanni, D.G. and Sahul, M. and Roch, T. and Truchlý, M. and Gregor, M. and Čaplovič, L. and Plecenik, A. and Kúš, P.
    Acta Materialia 121 59-67 (2016)
    Obtaining high hardness combined with enhanced toughness represents one of the current challenges in material design of hard ceramic protective coatings. In this work, we combine experimental and ab initio density functional theory (DFT) analysis of the mechanical properties of Ti-Al-Nb-N coatings to validate the results of previous theoretical investigations predicting enhanced toughness in TiAlN-based systems highly alloyed (&gt;25 at. %) with nitrides of pentavalent VB group elements Nb, Ta, and V. As-deposited Ti1-x-yAlxNbyN coatings (y = 0 ÷ 0.61) exhibit single phase cubic sodium chloride (B1) structure identified as TiAl(Nb)N solid solutions. The highest hardness, ∼32.5 ± 2 GPa, and the highest Young's modulus, ∼442 GPa, are obtained in Nb-free Ti0.46Al0.54N exhibiting pronounced 111 growth-orientation. Additions of Nb in the coatings promote texture evolution toward 200. Nanoindentation measurements demonstrate that alloying TiAlN with NbN yields significantly decreased elastic stiffness, from 442 to ∼358 ÷ 389 GPa, while the hardness remains approximately constant (between 28 ± 2 and 31 ± 3 GPa) for all Nb contents. DFT calculations and electronic structure analyses reveal that alloying dramatically reduces shear resistances due to enhanced d-d second-neighbor metallic bonding while retaining strong metal-N bonds which change from being primarily ionic (TiAlN) to more covalent (TiAlNbN) in nature. Overall, Nb substitutions are found to improve ductility of TiAlN-based alloys at the cost of slight losses in hardness, equating to enhanced toughness. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.08.084
  • 2015 • 97 Ab initio simulations on N and S co-doped titania nanotubes for photocatalytic applications
    Chesnokov, A. and Lisovski, O. and Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Physica Scripta 90 (2015)
    In this paper we present the results of quantum chemical modeling for energetically stable anatase (001) TiO<inf>2</inf> nanotubes, undoped, doped, and codoped with N<inf>O</inf> and S<inf>O</inf> atoms. We calculate the electronic structure of one-dimensional (1D) nanotubes and zero-dimensional (0D) atomic fragments cut out from these nanotubes, employing hybrid density functional theory with a partial incorporation of an exact, nonlocal Hartree-Fock exchange within the formalism of the linear combination of atomic orbitals, as implemented in both CRYSTAL and NWChem total energy codes. Structural optimization of 1D nanotubes has been performed using CRYSTAL09 code, while the cut-out 0D fragments have been modelled using the NWChem code. The electronic properties of the studied systems prove that the band structure of the pristine TiO<inf>2</inf> nanotube can be substantially modified by introducing substitutional impurity defects. The N-doped nanotube creates a midgap state that largely has a nitrogen 2p character. The S-doped nanotube has a defect state that almost coincides with the top of the valence bond for the pristine material. For nanotubes codoped with both S and N, we observe a downward shift of the gap state of nitrogen relative to the purely N-doped state by about 0.3 eV. This results in a system with a filled gap state about 0.3 eV below the O<inf>2</inf>/H<inf>2</inf>O oxidation level, making it a very promising candidate for photocatalytic hydrogen generation under visible light, because due to the presence of sulfur, the bottom of the conduction band is only about 2.2 eV above the occupied midgap state, and also, clearly above the standard hydrogen electrode level. © 2015 The Royal Swedish Academy of Sciences.
    view abstractdoi: 10.1088/0031-8949/90/9/094013
  • 2015 • 96 Bond-order potentials: Derivation and parameterization for refractory elements
    Drautz, R. and Hammerschmidt, T. and Čák, M. and Pettifor, D.G.
    Modelling and Simulation in Materials Science and Engineering 23 (2015)
    The bond-order potentials are derived from density functional theory by a systematic coarse graining of the electronic structure. Within their functional form the bond-order potentials comprise covalent bond formation, charge transfer and magnetism. We review the derivation of the bond-order potentials from density functional theory and discuss their application to the simulation of refractory transition metals. We show that the derived functional form of the bond-order potentials ensures the transferability of the potentials to atomic environments that have not been taken into account in the parameterization. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/23/7/074004
  • 2015 • 95 C-, N-, S-, and Fe-Doped TiO2 and SrTiO3 Nanotubes for Visible-Light-Driven Photocatalytic Water Splitting: Prediction from First Principles
    Piskunov, S. and Lisovski, O. and Begens, J. and Bocharov, D. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Journal of Physical Chemistry C 119 18686-18696 (2015)
    The ground state electronic structure and the formation energies of both TiO<inf>2</inf> and SrTiO<inf>3</inf> nanotubes (NTs) containing C<inf>O</inf>, N<inf>O</inf>, S<inf>O</inf>, and Fe<inf>Ti</inf> substitutional impurities are studied using first-principles calculations. We observe that N and S dopants in TiO<inf>2</inf> NTs lead to an enhancement of their visible-light-driven photocatalytic response, thereby increasing their ability to split H<inf>2</inf>O molecules. The differences between the highest occupied and lowest unoccupied impurity levels inside the band gap (HOIL and LUIL, respectively) are reduced in these defective nanotubes down to 2.4 and 2.5 eV for N and S doping, respectively. The band gap of an N<inf>O</inf>+S<inf>O</inf> codoped titania nanotube is narrowed down to 2.2 eV (while preserving the proper disposition of the gap edges relatively to the reduction and oxidation potentials, so that ε<inf>HOIL</inf> < ε<inf>O<inf>2</inf>/H<inf>2</inf>O</inf> < ε<inf>H+/H<inf>2</inf></inf> < ε<inf>LUIL</inf>), thus decreasing the photon energy required for splitting of H<inf>2</inf>O molecule. For C- and Fe-doped TiO<inf>2</inf> NTs, some impurity levels lie in the interval between both redox potentials, which would lead to electron-hole recombination. Our calculations also reveal in sulfur-doped SrTiO<inf>3</inf> NTs a suitable band distribution for the oxygen evolution reaction, although the splitting of water molecules would be hardly possible due to an unsuitable conduction band position for the hydrogen reduction reaction. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b03691
  • 2015 • 94 Constructing high-dimensional neural network potentials: A tutorial review
    Behler, J.
    International Journal of Quantum Chemistry 115 1032-1050 (2015)
    A lot of progress has been made in recent years in the development of atomistic potentials using machine learning (ML) techniques. In contrast to most conventional potentials, which are based on physical approximations and simplifications to derive an analytic functional relation between the atomic configuration and the potential-energy, ML potentials rely on simple but very flexible mathematical terms without a direct physical meaning. Instead, in case of ML potentials the topology of the potential-energy surface is "learned" by adjusting a number of parameters with the aim to reproduce a set of reference electronic structure data as accurately as possible. Due to this bias-free construction, they are applicable to a wide range of systems without changes in their functional form, and a very high accuracy close to the underlying first-principles data can be obtained. Neural network potentials (NNPs), which have first been proposed about two decades ago, are an important class of ML potentials. Although the first NNPs have been restricted to small molecules with only a few degrees of freedom, they are now applicable to high-dimensional systems containing thousands of atoms, which enables addressing a variety of problems in chemistry, physics, and materials science. In this tutorial review, the basic ideas of NNPs are presented with a special focus on developing NNPs for high-dimensional condensed systems. A recipe for the construction of these potentials is given and remaining limitations of the method are discussed. © 2015 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/qua.24890
  • 2015 • 93 Effect of substitution on elastic stability, electronic structure and magnetic property of Ni-Mn based Heusler alloys: An ab initio comparison
    Roy, T. and Gruner, M.E. and Entel, P. and Chakrabarti, A.
    Journal of Alloys and Compounds 632 822-829 (2015)
    First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson's ratio and Young's modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt's and Reuss's method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.255
  • 2015 • 92 Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds
    Steil, D. and Schmitt, O. and Fetzer, R. and Kubota, T. and Naganuma, H. and Oogane, M. and Ando, Y. and Rodan, S. and Blum, C.G.F. and Balke, B. and Wurmehl, S. and Aeschlimann, M. and Cinchetti, M.
    48 (2015)
    Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics. Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 - xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/16/164016
  • 2015 • 91 Large magnetocaloric effects in magnetic intermetallics: First-principles and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Ogura, M. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Grünebohm, A. and Arróyave, R. and Uebayashi, K. and Singh, N. and Talapatra, A. and Duong, T. and Acet, M. and Çakir, A.
    MATEC Web of Conferences 33 (2015)
    We have performed ab initio electronic structure calculations and Monte Carlo simulations of frustrated ferroic materials where complex magnetic configurations and chemical disorder lead to rich phase diagrams. With lowering of temperature, we find a ferromagnetic phase which transforms to an antiferromagnetic phase at the magnetostructural (martensitic) phase transition and to a cluster spin glass at still lower temperatures. The Heusler alloys Ni-(Co)-Mn-(Cr)-(Ga, Al, In, Sn, Sb) are of particular interest because of their large inverse magnetocaloric effect associated with the magnetostructural transition and the influence of Co/Cr doping. Besides spin glass features, strain glass behavior has been observed in Ni-Co-Mn-In. The numerical simulations allow a complete characterization of the frustrated ferroic materials including the Fe-Rh-Pd alloys. © Owned by the authors, published by EDP Sciences, 2015.
    view abstractdoi: 10.1051/matecconf/20153302001
  • 2015 • 90 MeL2Zn2(μ-1,6-Ph2-N6)-a building block for new hexazene complexes
    Gondzik, S. and Wölper, C. and Haack, R. and Jansen, G. and Schulz, S.
    Dalton Transactions 44 15703-15711 (2015)
    The zinc hexazene complex MeL<inf>2</inf>Zn<inf>2</inf>(μ-1,6-Ph<inf>2</inf>-N<inf>6</inf>) 1 (MeL = HC[C(Me)N(2,4,6-Me<inf>3</inf>C<inf>6</inf>H<inf>2</inf>)]<inf>2</inf>) is a suitable hexazene transfer reagent in reactions with main group metal and transition metal complexes containing M-Me units. The reactions proceed with elimination of MeLZnMe and the resulting complexes were characterized by NMR and IR spectroscopy and single crystal X-ray diffraction (5, 8). Quantum chemical calculations were performed to investigate the electronic structure of 5′ and 8′ in more detail and to identify the absorption bands of the hexazene unit. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5dt02423d
  • 2015 • 89 Metal-to-Insulator Transition in Au Chains on Si(111)-5×2-Au by Band Filling: Infrared Plasmonic Signal and Ab Initio Band Structure Calculation
    Hötzel, F. and Seino, K. and Chandola, S. and Speiser, E. and Esser, N. and Bechstedt, F. and Pucci, A.
    Journal of Physical Chemistry Letters 6 3615-3620 (2015)
    The Si(111)-5×2-Au surface is increasingly of interest because it is one of the rare atomic chain systems with quasi-one-dimensional properties. For the deposition of 0.7 monolayers of Au, these chains are metallic. Upon the evaporation of an additional submonolayer amount of gold, the surface becomes insulating but keeps the 5×2 symmetry. This metal-to-insulator transition was in situ monitored based on the infrared plasmonic signal change with coverage. The phase transition is theoretically explained by total-energy and band-structure calculations. Accordingly, it can be understood in terms of the occupation of the originally half-filled one-dimensional band at the Fermi level. By annealing the system, the additional gold is removed from the surface and the plasmonic signal is recovered, which underlines the stability of the metallic structure. So, recent results on the infrared plasmonic signals of the Si(111)-5 × 2-Au surface are supported. The understanding of potential one-dimensional electrical interconnects is improved. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.5b01530
  • 2015 • 88 Non-collinear magnetism with analytic Bond-Order Potentials
    Ford, M.E. and Pettifor, D.G. and Drautz, R.
    Journal of Physics Condensed Matter 27 (2015)
    The theory of analytic Bond-Order Potentials as applied to non-collinear magnetic structures of transition metals is extended to take into account explicit rotations of Hamiltonian and local moment matrix elements between locally and globally defined spin-coordinate systems. Expressions for the gradients of the energy with respect to the Hamiltonian matrix elements, the interatomic forces and the magnetic torques are derived. The method is applied to simulations of the rotation of magnetic moments in α iron, as well as α and β manganese, based on d-valent orthogonal tight-binding parametrizations of the electronic structure. A new weighted-average terminator is introduced to improve the convergence of the Bond-Order Potential energies and torques with respect to tight-binding reference values, although the general behavior is qualitatively correct for low-moment expansions. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/8/086002
  • 2015 • 87 On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction
    Masa, J. and Xia, W. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 54 10102-10120 (2015)
    The notion of metal-free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal-free catalysts are prepared using metal-containing precursors. It is highly contested that metal residues in nitrogen-doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal-free catalysts is proposed and the differences between NC and M-N<inf>x</inf>/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M-N<inf>x</inf>/C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201500569
  • 2015 • 86 Propanoate grafting on (H,OH)-Si(0 0 1)-2×1
    Bournel, F. and Gallet, J.-J. and Köhler, U. and Ellakhmissi, B.B. and Kubsky, S. and Carniato, S. and Rochet, F.
    Journal of Physics Condensed Matter 27 (2015)
    We have examined the reactivity of water-covered Si(0 0 1)-2?×1, (H,OH)-Si(0 0 1)-2?×1, with propanoic (C2H5COOH) acid at room temperature. Using a combination of spectroscopic techniques probing the electronic structure (XPS, NEXAFS) and the vibrational spectrum (HREELS), we have proved that the acid is chemisorbed on the surface as a propanoate. Once the molecule is chemisorbed, the strong perturbation of the electronic structure of the hydroxyls, and of their vibrational spectrum, suggests that the molecule makes hydrogen bonds with the surrounding hydroxyls. As we find evidence that surface hydroxyls are involved in the adsorption reaction, we discuss how a concerted or a radical-mediated reaction (involving the surface silicon dangling bonds) could lead to water elimination and formation of the ester. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/5/054005
  • 2015 • 85 Spin-resolved low-energy and hard x-ray photoelectron spectroscopy of off-stoichiometric Co2MnSi Heusler thin films exhibiting a record TMR
    Fetzer, R. and Ouardi, S. and Honda, Y. and Liu, H.-X. and Chadov, S. and Balke, B. and Ueda, S. and Suzuki, M. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M. and Fecher, G.H. and Felser, C.
    48 (2015)
    Half-metallic Co<inf>2</inf>MnSi-based Heusler compounds have attracted attention because they yield very high tunnelling magnetoresistance (TMR) ratios. Record TMR ratios of 1995% (at 4.2 K) are obtained from off-stoichiometric Co<inf>2</inf>MnSi-based magnetic tunnel junctions. This work reports on a combination of band structure calculations and spin-resolved and photon-polarisation-dependent photoelectron spectroscopy for off-stoichiometric Heusler thin films with the composition Co<inf>2</inf>Mn<inf>1.30</inf>Si<inf>0.84</inf>. Co and Mn are probed by magnetic dichroism in angle-resolved photoelectron spectroscopy at the 2p core level. In contrast to the delocalised Co 3d states, a pronounced localisation of the Mn 3d states is deduced from the corresponding 2p core level spectra. The valence states are investigated by linear dichroism using both hard x-ray and very-low-photon-energy excitation. When a very low photon energy is used for excitation, the valence bands exhibit a spin polarisation of about 30% at the Fermi energy. First principles calculations reveal that the low spin polarisation might be caused by a spin-flip process in the photoelectron final states. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/16/164002
  • 2015 • 84 Structural stability of Fe-based topologically close-packed phases
    Ladines, A.N. and Hammerschmidt, T. and Drautz, R.
    Intermetallics 59 59-67 (2015)
    Precipitates of topologically close-packed (TCP) phases play an important role in hardening mechanisms of high-performance steels. We analyze the influence of atomic size, electron count, magnetism and external stress on TCP phase stability in Fe-based binary transition metal alloys. Our density-functional theory calculations of structural stability are complemented by an analysis with an empirical structure map for TCP phases. The structural stability and lattice parameters of the Fe-Nb/Mo/V compounds are in good agreement with experiment. The average magnetic moments follow the Slater-Pauling relation to the average number of valence-electrons and can be rationalized in terms of the electronic density of states. The stabilizing effect of the magnetic energy, estimated by additional non-magnetic calculations, increases as the magnetic moment increases with band filling for the binary systems of Fe and early transition metals. For the case of Fe2Nb, we demonstrate that the influence of magnetism and external stress is sufficiently large to alter the energetic ordering of the closely competing Laves phases C14, C15 and C36. We find that the A15 phase is not stabilized by atomic-size differences, while the stability of C14 is increasing with increasing difference in atomic size. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.12.009
  • 2015 • 83 Synthesis, Solid-State Structures, and Computational Studies of Half-Sandwich Cp∗BeX (X = Cl, Br, I) Compounds
    Naglav, D. and Tobey, B. and Neumann, A. and Bläser, D. and Wölper, C. and Schulz, S.
    Organometallics 34 3072-3078 (2015)
    The solid-state structures of half-sandwich beryllium complexes Cp∗BeX (X = Cl (1), Br (2), I (3)) are described. All attempts to synthesize Cp∗BeF (4) by reaction of 1-3 with several fluorinating reagents such as AgF<inf>2</inf>, XeF<inf>2</inf>, 4-FC<inf>6</inf>H<inf>4</inf>IF<inf>2</inf>, AgF, NaF, and Me<inf>4</inf>NF failed. However, the reaction of 3 and AgF in resulted in the unexpected synthesis of the known beryllocene Cp∗<inf>2</inf>Be (5). Quantum-chemical calculations were performed to investigate the electronic structures of 1-4. (Chemical Equation Presented). © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.5b00389
  • 2015 • 82 The geometric and electronic structure of TCNQ and TCNQ+Mn on Ag(0 0 1) and Cu(0 0 1) surfaces
    Feyer, V. and Graus, M. and Nigge, P. and Zamborlini, G. and Acres, R.G. and Schöll, A. and Reinert, F. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 204 125-131 (2015)
    Copper and silver surfaces can be used as model systems to study structure formation and interfacial bonding upon adsorption of organic molecules. We have investigated the geometric and electronic structure of ordered monolayers of TCNQ on Cu(0 0 1) and Ag(0 0 1) and of TCNQ+Mn on Ag(0 0 1) surfaces by LEED and photoelectron momentum microscopy. While TCNQ forms an incommensurable superstructure on Cu(0 0 1), two coverage-dependant, commensurable superstructures are established on Ag(0 0 1). Subsequent adsorption of Mn on top of TCNQ/Ag(0 0 1) results in the formation of a long-range ordered mixed metal-organic superstructure, which is also commensurable with the Ag(0 0 1) substrate. The photoelectron spectroscopy (PES) data shows a filling of the TCNQ LUMO by charge transfer from the substrate for all investigated interfaces and the coadsorption of Mn leads to an energy shift of the TCNQ HOMO and LUMO of 230 meV with respect to TCNQ/Ag(0 0 1). The characteristic angle-dependent intensity pattern of the TCNQ LUMO in PES was utilized to investigate the azimuthal orientation of the molecules in the respective unit cells. The angle-resolved PES data was further analyzed to identify lateral band dispersion effects in the adsorbate layers, but no significant dispersion was observed. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elspec.2015.02.010
  • 2014 • 81 A chemically inert Rashba split interface electronic structure of C 60, FeOEP and PTCDA on BiAg2/Ag(111) substrates
    Cottin, M.C. and Lobo-Checa, J. and Schaffert, J. and Bobisch, C.A. and Möller, R. and Ortega, J.E. and Walter, A.L.
    New Journal of Physics 16 (2014)
    The fields of organic electronics and spintronics have the potential to revolutionize the electronics industry. Finding the right materials that can retain their electrical and spin properties when combined is a technological and fundamental challenge. We carry out the study of three archetypal organic molecules in intimate contact with the BiAg2 surface alloy. We show that the BiAg2 alloy is an especially suited substrate due to its inertness as support for molecular films, exhibiting an almost complete absence of substrate-molecular interactions. This is inferred from the persistence of a completely unaltered giant spin-orbit split surface state of the BiAg 2 substrate, and from the absence of significant metallic screening of charged molecular levels in the organic layer. Spin-orbit split states in BiAg2 turn out to be far more robust to organic overlayers than previously thought. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/16/4/045002
  • 2014 • 80 A generalized plane-wave formulation of k · p formalism and continuum-elasticity approach to elastic and electronic properties of semiconductor nanostructures
    Marquardt, O. and Boeck, S. and Freysoldt, C. and Hickel, T. and Schulz, S. and Neugebauer, J. and O'Reilly, E.P.
    Computational Materials Science 95 280-287 (2014)
    We present a generalized and flexible plane-wave based implementation of the multiband k·p formalism to study the electronic properties of semiconductor nanostructures. All ingredients of the modeling process, namely the Hamiltonian, the nanostructure's geometry and the required material parameters, are defined in human-readable input files that can be easily generated and modified. The generalized k·p model can contain an arbitrary number of directly treated bands as well as strain, piezoelectric, and external potentials. All calculations can be performed for arbitrary crystal structures. The nanostructure is described in terms of a real-space composition map that may contain an arbitrary number of base compounds and alloys. We demonstrate the applicability and flexibility of our implementation for the example of (111)-oriented, site-controlled InGaAs quantum dots, where a rotated eight-band k·p Hamiltonian is employed. As a second example, a 14-band k·p model that captures the bulk inversion asymmetry of the zinc-blende lattice is applied for the case of a pyramidal (0 0 1)-oriented InAs/GaAs quantum dot. Here we show that the explicit treatment of 14 bands removes the well known shortcoming of eight-band k·p models for (0 0 1)-oriented zinc-blende quantum dots which leads to artificially degenerate p-like electron states. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2014.06.047
  • 2014 • 79 Ab initio based study of finite-temperature structural, elastic and thermodynamic properties of FeTi
    Zhu, L.-F. and Friák, M. and Udyansky, A. and Ma, D. and Schlieter, A. and Kühn, U. and Eckert, J. and Neugebauer, J.
    Intermetallics 45 11-17 (2014)
    We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.09.008
  • 2014 • 78 Adsorption geometry and electronic structure of iron phthalocyanine on Ag surfaces: A LEED and photoelectron momentum mapping study
    Feyer, V. and Graus, M. and Nigge, P. and Wießner, M. and Acres, R.G. and Wiemann, C. and Schneider, C.M. and Schöll, A. and Reinert, F.
    Surface Science 621 64-68 (2014)
    We present a comprehensive study of the adsorption behavior of iron phthalocyanine on the low-index crystal faces of silver. By combining measurements of the reciprocal space by means of photoelectron momentum mapping and low energy electron diffraction, the real space adsorption geometries are reconstructed. At monolayer coverage ordered superstructures exist on all studied surfaces containing one molecule in the unit cell in case of Ag(100) and Ag(111), and two molecules per unit cell for Ag(110). The azimuthal tilt angle of the molecules against the high symmetry directions of the substrate is derived from the photoelectron momentum maps. A comparative analysis of the momentum patterns on the substrates with different symmetry indicates that both constituents of the twofold degenerate FePc lowest unoccupied molecular orbital are occupied by charge transfer from the substrate at the interface. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.10.020
  • 2014 • 77 Bulk sensitive hard x-ray photoemission electron microscopy
    Patt, M. and Wiemann, C. and Weber, N. and Escher, M. and Gloskovskii, A. and Drube, W. and Merkel, M. and Schneider, C.M.
    Review of Scientific Instruments 85 (2014)
    Hard x-ray photoelectron spectroscopy (HAXPES) has now matured into a well-established technique as a bulk sensitive probe of the electronic structure due to the larger escape depth of the highly energetic electrons. In order to enable HAXPES studies with high lateral resolution, we have set up a dedicated energy-filtered hard x-ray photoemission electron microscope (HAXPEEM) working with electron kinetic energies up to 10 keV. It is based on the NanoESCA design and also preserves the performance of the instrument in the low and medium energy range. In this way, spectromicroscopy can be performed from threshold to hard x-ray photoemission. The high potential of the HAXPEEM approach for the investigation of buried layers and structures has been shown already on a layered and structured SrTiO3 sample. Here, we present results of experiments with test structures to elaborate the imaging and spectroscopic performance of the instrument and show the capabilities of the method to image bulk properties. Additionally, we introduce a method to determine the effective attenuation length of photoelectrons in a direct photoemission experiment. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4902141
  • 2014 • 76 Confirmation of intrinsic electron gap states at nonpolar GaN(1-100) surfaces combining photoelectron and surface optical spectroscopy
    Himmerlich, M. and Eisenhardt, A. and Shokhovets, S. and Krischok, S. and Speiser, E. and Neumann, M.D. and Navarro-Quezada, A. and Esser, N.
    Applied Physics Letters 104 (2014)
    The electronic structure of GaN(1-100) surfaces is investigated in-situ by photoelectron spectroscopy (PES) and reflection anisotropy spectroscopy (RAS). Occupied surface states 3.1eV below the Fermi energy are observed by PES, accompanied by surface optical transitions found in RAS around 3.3eV, i.e., below the bulk band gap. These results indicate that the GaN(1-100) surface band gap is smaller than the bulk one due to the existence of intra-gap states, in agreement with density functional theory calculations. Furthermore, the experiments demonstrate that RAS can be applied for optical surface studies of anisotropic crystals. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4873376
  • 2014 • 75 Electronic and structural differences between wurtzite and zinc blende inas nanowire surfaces: Experiment and theory
    Hjort, M. and Lehmann, S. and Knutsson, J. and Zakharov, A.A. and Du, Y.A. and Sakong, S. and Timm, R. and Nylund, G. and Lundgren, E. and Kratzer, P. and Dick, K.A. and Mikkelsen, A.
    ACS Nano 8 12346-12355 (2014)
    We determine the detailed differences in geometry and band structure between wurtzite (Wz) and zinc blende (Zb) InAs nanowire (NW) surfaces using scanning tunneling microscopy/spectroscopy and photoemission electron microscopy. By establishing unreconstructed and defect-free surface facets for both Wz and Zb, we can reliably measure differences between valence and conduction band edges, the local vacuum levels, and geometric relaxations to the few-millielectronvolt and few-picometer levels, respectively. Surface and bulk density functional theory calculations agree well with the experimental findings and are used to interpret the results, allowing us to obtain information on both surface and bulk electronic structure. We can thus exclude several previously proposed explanations for the observed differences in conductivity of Wz-Zb NW devices. Instead, fundamental structural differences at the atomic scale and nanoscale that we observed between NW surface facets can explain the device behavior. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn504795v
  • 2014 • 74 First-principles calculations for point defects in solids
    Freysoldt, C. and Grabowski, B. and Hickel, T. and Neugebauer, J. and Kresse, G. and Janotti, A. and Van De Walle, C.G.
    Reviews of Modern Physics 86 253-305 (2014)
    Point defects and impurities strongly affect the physical properties of materials and have a decisive impact on their performance in applications. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of defects. The theoretical modeling of point defects in crystalline materials by means of electronic-structure calculations, with an emphasis on approaches based on density functional theory (DFT), is reviewed. A general thermodynamic formalism is laid down to investigate the physical properties of point defects independent of the materials class (semiconductors, insulators, and metals), indicating how the relevant thermodynamic quantities, such as formation energy, entropy, and excess volume, can be obtained from electronic structure calculations. Practical aspects such as the supercell approach and efficient strategies to extrapolate to the isolated-defect or dilute limit are discussed. Recent advances in tractable approximations to the exchange-correlation functional (DFT+U, hybrid functionals) and approaches beyond DFT are highlighted. These advances have largely removed the long-standing uncertainty of defect formation energies in semiconductors and insulators due to the failure of standard DFT to reproduce band gaps. Two case studies illustrate how such calculations provide new insight into the physics and role of point defects in real materials. © 2014 American Physical Society.
    view abstractdoi: 10.1103/RevModPhys.86.253
  • 2014 • 73 First-principles studies on graphene-supported transition metal clusters
    Sahoo, S. and Gruner, M.E. and Khanna, S.N. and Entel, P.
    Journal of Chemical Physics 141 (2014)
    Theoretical studies on the structure, stability, and magnetic properties of icosahedral TM13 (TM = Fe, Co, Ni) clusters, deposited on pristine (defect free) and defective graphene sheet as well as graphene flakes, have been carried out within a gradient corrected density functional framework. The defects considered in our study include a carbon vacancy for the graphene sheet and a five-membered and a seven-membered ring structures for graphene flakes (finite graphene chunks). It is observed that the presence of defect in the substrate has a profound influence on the electronic structure and magnetic properties of graphene-transition metal complexes, thereby increasing the binding strength of the TM cluster on to the graphene substrate. Among TM 13 clusters, Co13 is absorbed relatively more strongly on pristine and defective graphene as compared to Fe13 and Ni 13 clusters. The adsorbed clusters show reduced magnetic moment compared to the free clusters. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4893328
  • 2014 • 72 Hydrogen evolution from metal-surface hydroxyl interaction
    Fujimori, Y. and Kaden, W.E. and Brown, M.A. and Roldan Cuenya, B. and Sterrer, M. and Freund, H.-J.
    Journal of Physical Chemistry C 118 17717-17723 (2014)
    The redox interaction between hydroxyl groups on oxide surfaces and metal atoms and clusters deposited thereon, according to which metals get oxidized and hydrogen released, is an effective route to tune both the morphological (particle size and shape) and electronic (oxidation state) properties of oxide-supported metals. While the oxidation state of the metals can straightforwardly be probed by X-ray based methods (e.g., XPS), hydrogen is much more difficult to capture, in particular in highly reactive systems where the redox interaction takes place directly during the nucleation of the metals at room temperature. In the present study, the interaction of Pd with a hydroxylated MgO(001) surface was studied using a combination of vibrational spectroscopy, electronic structure studies including Auger parameter analysis, and thermal desorption experiments. The results provide clear experimental evidence for the redox nature of the interaction by showing a direct correlation between metal oxidation and hydrogen evolution at slightly elevated temperature (390 K). Moreover, a second hydrogen evolution pathway opens up at 500 K, which involves hydroxyl groups on the MgO support and carbon monoxide adsorbed on the Pd particles (water-gas shift reaction). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp504655e
  • 2014 • 71 Negatively charged ions on Mg(0001) surfaces: Appearance and origin of attractive adsorbate-adsorbate interactions
    Cheng, S.-T. and Todorova, M. and Freysoldt, C. and Neugebauer, J.
    Physical Review Letters 113 (2014)
    Adsorption of electronegative elements on a metal surface usually leads to an increase in the work function and decrease in the binding energy as the adsorbate coverage rises. Using density-functional theory calculations, we show that Cl adsorbed on a Mg(0001) surface complies with these expectations, but adsorption of {N,O,F} causes a decrease in the work function and an increase in the binding energy. Analyzing the electronic structure, we show that the presence of a highly polarizable electron spill-out in front of Mg(0001) causes this unusual adsorption behavior and is responsible for the appearance of a hitherto unknown net-attractive lateral electrostatic interaction between same charged adsorbates. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.113.136102
  • 2014 • 70 Next generation interatomic potentials for condensed systems
    Handley, C.M. and Behler, J.
    European Physical Journal B 87 (2014)
    The computer simulation of condensed systems is a challenging task. While electronic structure methods like density-functional theory (DFT) usually provide a good compromise between accuracy and efficiency, they are computationally very demanding and thus applicable only to systems containing up to a few hundred atoms. Unfortunately, many interesting problems require simulations to be performed on much larger systems involving thousands of atoms or more. Consequently, more efficient methods are urgently needed, and a lot of effort has been spent on the development of a large variety of potentials enabling simulations with significantly extended time and length scales. Most commonly, these potentials are based on physically motivated functional forms and thus perform very well for the applications they have been designed for. On the other hand, they are often highly system-specific and thus cannot easily be transferred from one system to another. Moreover, their numerical accuracy is restricted by the intrinsic limitations of the imposed functional forms. In recent years, several novel types of potentials have emerged, which are not based on physical considerations. Instead, they aim to reproduce a set of reference electronic structure data as accurately as possible by using very general and flexible functional forms. In this review we will survey a number of these methods. While they differ in the choice of the employed mathematical functions, they all have in common that they provide high-quality potential-energy surfaces, while the efficiency is comparable to conventional empirical potentials. It has been demonstrated that in many cases these potentials now offer a very interesting new approach to study complex systems with hitherto unreached accuracy. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2014.
    view abstractdoi: 10.1140/epjb/e2014-50070-0
  • 2014 • 69 Potential-induced degradation in solar cells: Electronic structure and diffusion mechanism of sodium in stacking faults of silicon
    Ziebarth, B. and Mrovec, M. and Elsässer, C. and Gumbsch, P.
    Journal of Applied Physics 116 (2014)
    Sodium decorated stacking faults (SFs) were recently identified as the primary cause of potential-induced degradation in silicon (Si) solar-cells due to local electrical short-circuiting of the p-n junctions. In the present study, we investigate these defects by first principles calculations based on density functional theory in order to elucidate their structural, thermodynamic, and electronic properties. Our calculations show that the presence of sodium (Na) atoms leads to a substantial elongation of the Si-Si bonds across the SF, and the coverage and continuity of the Na layer strongly affect the diffusion behavior of Na within the SF. An analysis of the electronic structure reveals that the presence of Na in the SF gives rise to partially occupied defect levels within the Si band gap that participate in electrical conduction along the SF. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4894007
  • 2014 • 68 Pressure-induced changes on the electronic structure and electron topology in the direct FCC → SH transformation of silicon
    Tse, J.S. and Hanfland, M. and Flacau, R. and Desgreniers, S. and Li, Z. and Mende, K. and Gilmore, K. and Nyrow, A. and Moretti Sala, M. and Sternemann, C.
    Journal of Physical Chemistry C 118 1161-1166 (2014)
    X-ray diffraction experiments at 80 K show that when silicon is compressed under hydrostatic conditions the intermediate high-pressure phases are bypassed leading to a direct transformation to the simple hexagonal structure at 17 GPa. A maximum entropy analysis of the diffraction patterns reveals dramatic alterations in the valence electron distribution from tetrahedral covalent bonding to localization in the interstitial sites and along the one-dimensional silicon atom chain running along adjacent hexagonal layers. Changes in the orbital character of the unoccupied states are confirmed using X-ray Raman scattering spectroscopy and theoretical Bethe-Salpeter equation calculations. This is the first direct observation indicating that the silicon valence electrons in 3s and 3p orbitals are transferred to the 3d orbitals at high density which proves that electrons of compressed elemental solids migrate from their native bonding configuration to interstitial regions. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp408666q
  • 2014 • 67 Representing potential energy surfaces by high-dimensional neural network potentials
    Behler, J.
    Journal of Physics Condensed Matter 26 (2014)
    The development of interatomic potentials employing artificial neural networks has seen tremendous progress in recent years. While until recently the applicability of neural network potentials (NNPs) has been restricted to low-dimensional systems, this limitation has now been overcome and high-dimensional NNPs can be used in large-scale molecular dynamics simulations of thousands of atoms. NNPs are constructed by adjusting a set of parameters using data from electronic structure calculations, and in many cases energies and forces can be obtained with very high accuracy. Therefore, NNP-based simulation results are often very close to those gained by a direct application of first-principles methods. In this review, the basic methodology of high-dimensional NNPs will be presented with a special focus on the scope and the remaining limitations of this approach. The development of NNPs requires substantial computational effort as typically thousands of reference calculations are required. Still, if the problem to be studied involves very large systems or long simulation times this overhead is regained quickly. Further, the method is still limited to systems containing about three or four chemical elements due to the rapidly increasing complexity of the configuration space, although many atoms of each species can be present. Due to the ability of NNPs to describe even extremely complex atomic configurations with excellent accuracy irrespective of the nature of the atomic interactions, they represent a general and therefore widely applicable technique, e.g. for addressing problems in materials science, for investigating properties of interfaces, and for studying solvation processes. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/18/183001
  • 2014 • 66 Site occupation of Nb atoms in ternary Ni-Ti-Nb shape memory alloys
    Shi, H. and Frenzel, J. and Martinez, G.T. and Van Rompaey, S. and Bakulin, A. and Kulkova, S. and Van Aert, S. and Schryvers, D.
    Acta Materialia 74 85-95 (2014)
    Nb occupancy in the austenite B2-NiTi matrix and Ti2Ni phase in Ni-Ti-Nb shape memory alloys was investigated by aberration-corrected scanning transmission electron microscopy and precession electron diffraction. In both cases, Nb atoms were found to prefer to occupy the Ti rather than Ni sites. A projector augmented wave method within density functional theory was used to calculate the atomic and electronic structures of the austenitic B2-NiTi matrix phase and the Ti2Ni precipitates both with and without addition of Nb. The obtained formation energies and analysis of structural and electronic characteristics explain the preference for Ti sites for Nb over Ni sites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.03.062
  • 2014 • 65 Size dependence of the dispersion relation for the interface state between NaCl(100) and Ag(111)
    Heidorn, S.-C. and Sabellek, A. and Morgenstern, K.
    Nano Letters 14 13-17 (2014)
    This study investigates the interface state electron dispersion relation between NaCl(100) islands and Ag(111) dependent upon NaCl island size. Both onset energy and effective mass are size dependent. However, these dependencies are relevant at different island sizes. We trace back this effective mass dependency to a misfit-induced strain based on atomically resolved images. Our results open up new avenues for the development of nanodevices by tuning the effective electron mass via strain of the insulating component. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nl403121t
  • 2014 • 64 Structural and electronic properties of micellar Au nanoparticles: Size and ligand effects
    Behafarid, F. and Matos, J. and Hong, S. and Zhang, L. and Rahman, T.S. and Roldan Cuenya, B.
    ACS Nano 8 6671-6681 (2014)
    Gaining experimental insight into the intrinsic properties of nanoparticles (NPs) represents a scientific challenge due to the difficulty of deconvoluting these properties from various environmental effects such as the presence of adsorbates or a support. A synergistic combination of experimental and theoretical tools, including X-ray absorption fine-structure spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and density functional theory was used in this study to investigate the structure and electronic properties of small (∼1-4 nm) Au NPs synthesized by an inverse micelle encapsulation method. Metallic Au NPs encapsulated by polystyrene 2-vinylpiridine (PS-P2VP) were studied in the solution phase (dispersed in toluene) as well as after deposition on γ-Al2O3. Our experimental data revealed a size-dependent contraction of the interatomic distances of the ligand-protected NPs with decreasing NP size. These findings are in good agreement with the results from DFT calculations of unsupported Au NPs surrounded by P2VP, as well as those obtained for pure (ligand-free) Au clusters of analogous sizes. A comparison of the experimental and theoretical results supports the conclusion that the P2VP ligands employed to stabilize the gold NPs do not lead to strong distortions in the average interatomic spacing. The changes in the electronic structure of the Au-P2VP NPs were found to originate mainly from finite size effects and not from charge transfer between the NPs and their environment (e.g., Au-ligand interactions). In addition, the isolated ligand-protected experimental NPs only display a weak interaction with the support, making them an ideal model system for the investigation of size-dependent physical and chemical properties of structurally well-defined nanomaterials. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn406568b
  • 2014 • 63 Surface dynamics of the intermetallic catalyst Pd2Ga, Part i - Structural stability in UHV and different gas atmospheres
    Wowsnick, G. and Teschner, D. and Kasatkin, I. and Girgsdies, F. and Armbrüster, M. and Zhang, A. and Grin, Y. and Schlögl, R. and Behrens, M.
    Journal of Catalysis 309 209-220 (2014)
    The structural and electronic properties of unsupported Pd2Ga were investigated after different pre-treatments. Pd2Ga provides with respect to elemental Pd a significantly modified electronic structure with its d-band center being shifted away from the Fermi level. It was found that the electronic structure of the surface depends strongly on its pre-treatment and on the chemical environment. We report a detailed bulk and surface characterization of the intermetallic compound by means of XRD, DTA/TG/MS, SEM, XPS, and HR-TEM. At moderate temperatures, the bulk of Pd2Ga is chemically resistant against H2 or O2 atmosphere and against mechanical load. Contrariwise its surface is highly sensitive against even traces of oxidizing agents, leading quickly to a disparity between bulk and surface structure and composition. The reversibility of this dynamic effect depends on the degree of decomposition and on the sample history. An almost pure intermetallic surface can only be achieved in highly reducing atmospheres. © 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2013.09.019
  • 2014 • 62 Surface dynamics of the intermetallic catalyst Pd2Ga, Part II - Reactivity and stability in liquid-phase hydrogenation of phenylacetylene
    Wowsnick, G. and Teschner, D. and Armbrüster, M. and Kasatkin, I. and Girgsdies, F. and Grin, Y. and Schlögl, R. and Behrens, M.
    Journal of Catalysis 309 221-230 (2014)
    The catalytic properties of unsupported Pd2Ga for the liquid-phase hydrogenation of phenylacetylene are investigated after different pre-treatments with focus on the stability of the catalyst during reaction. The surface of as-prepared Pd2Ga consists mainly on Pd and oxidized Ga species. Under the conditions of the liquid-phase hydrogenation of phenylacetylene, the intermetallic surface cannot be reformed in situ by reduction of Ga oxide. After a reductive pre-treatment in 5% H2/Ar at 400 C, an almost clean Pd2Ga surface can be obtained. Its hydrogenation activity is significantly lowered compared to elemental Pd, which is due to the intrinsic adsorption properties of the intermetallic surface. However, residues of H2O or O2 lead to oxidation of this surface. Excluding these impurities, the decomposition can be suppressed. In this case, the bulk material of Pd2Ga gets cracked during phenylacetylene hydrogenation. The controlled modification of the crystal and the electronic structure of Pd by formation of the intermetallic compound Pd2Ga are accompanied with a decreased stability. © 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2013.09.018
  • 2014 • 61 Ultrafast modulation of the chemical potential in BaFe2As2 by coherent phonons
    Yang, L.X. and Rohde, G. and Rohwer, T. and Stange, A. and Hanff, K. and Sohrt, C. and Rettig, L. and Cortés, R. and Chen, F. and Feng, D.L. and Wolf, T. and Kamble, B. and Eremin, I. and Popmintchev, T. and Murnane, M.M. and Kap...
    Physical Review Letters 112 (2014)
    Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in BaFe2As2 around the high-symmetry points Γ and M. A global oscillation of the Fermi level at the frequency of the A1g(As) phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the A1g(As) phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.112.207001
  • 2014 • 60 Unoccupied electronic structure and relaxation dynamics of Pb/Si(1 1 1)
    Sandhofer, M. and Sklyadneva, I.Yu. and Sharma, V. and Trontl, V.M. and Zhou, P. and Ligges, M. and Heid, R. and Bohnen, K.-P. and Chulkov, E.V. and Bovensiepen, U.
    Journal of Electron Spectroscopy and Related Phenomena 195 278-284 (2014)
    The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved two-photon photoemission in the over(Γ, -) → over(M, -) direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6pz states with the Si substrate. The experimentally obtained dispersion E(k||) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E(k||) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6pz states with the Si band. However, the momentum range at which mixing of 6pz and 6px,y derived subbands is found to occur in the presence of the Si substrate is closer to over(Γ, -) than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a px,y mediated interaction of the pz states with the Si conduction band. © 2014.
    view abstractdoi: 10.1016/j.elspec.2014.04.006
  • 2013 • 59 A full-dimensional neural network potential-energy surface for water clusters up to the hexamer
    Morawietz, T. and Behler, J.
    Zeitschrift fur Physikalische Chemie 227 1559-1581 (2013)
    Water clusters have attracted a lot of attention as prototype systems to study hydrogen bonded molecular aggregates but also to gain deeper insights into the properties of liquid water, the solvent of life. All these studies depend on an accurate description of the atomic interactions and countless potentials have been proposed in the literature in the past decades to represent the potential-energy surface (PES) of water. Many of these potentials employ drastic approximations like rigid water monomers and fixed point charges, while on the other hand also several attempts have been made to derive very accurate PESs by fitting data obtained in high-level electronic structure calculations. In recent years artificial neural networks (NNs) have been established as a powerful tool to construct high-dimensional PESs of a variety of systems, but to date no full-dimensional NN PES for water has been reported. Here, we present NN potentials for water clusters containing two to six water molecules trained to density functional theory (DFT) data employing two different exchange-correlation functionals, PBE and RPBE. In contrast to other potentials fitted to first principles data, these NN potentials are not based on a truncated many-body expansion of the energy but consider the interactions between all water molecules explicitly. For both functionals an excellent agreement with the underlying DFT calculations has been found with binding energy errors of only about 1%.© by Oldenbourg Wissenschaftsverlag, München.
    view abstractdoi: 10.1524/zpch.2013.0384
  • 2013 • 58 Coherent excitations and electron-phonon coupling in Ba/EuFe2As2 compounds investigated by femtosecond time- and angle-resolved photoemission spectroscopy
    Avigo, I. and Cortés, R. and Rettig, L. and Thirupathaiah, S. and Jeevan, H.S. and Gegenwart, P. and Wolf, T. and Ligges, M. and Wolf, M. and Fink, J. and Bovensiepen, U.
    Journal of Physics Condensed Matter 25 (2013)
    We employed femtosecond time- and angle-resolved photoelectron spectroscopy to analyze the response of the electronic structure of the 122 Fe-pnictide parent compounds Ba/EuFe2As2 and optimally doped BaFe1.85Co0.15As2 near the Γ point to optical excitation by an infrared femtosecond laser pulse. We identify pronounced changes of the electron population within several 100 meV above and below the Fermi level, which we explain as a combination of (i) coherent lattice vibrations, (ii) a hot electron and hole distribution, and (iii) transient modifications of the chemical potential. The responses of the three different materials are very similar. In the coherent response we identify three modes at 5.6, 3.3, and 2.6 THz. While the highest frequency mode is safely assigned to the A1g mode, the other two modes require a discussion in comparison to the literature. Employing a transient three temperature model we deduce from the transient evolution of the electron distribution a rather weak, momentum-averaged electron-phonon coupling quantified by values for λω2 between 30 and 70 meV2. The chemical potential is found to present pronounced transient changes reaching a maximum of 15 meV about 0.6 ps after optical excitation and is modulated by the coherent phonons. This change in the chemical potential is particularly strong in a multiband system like the 122 Fe-pnictide compounds investigated here due to the pronounced variation of the electron density of states close to the equilibrium chemical potential. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/9/094003
  • 2013 • 57 Comparison of density functionals for nitrogen impurities in ZnO
    Sakong, S. and Gutjahr, J. and Kratzer, P.
    Journal of Chemical Physics 138 (2013)
    Hybrid functionals and empirical correction schemes are compared to conventional semi-local density functional theory (DFT) calculations in order to assess the predictive power of these methods concerning the formation energy and the charge transfer level of impurities in the wide-gap semiconductor ZnO. While the generalized gradient approximation fails to describe the electronic structure of the N impurity in ZnO correctly, methods that widen the band gap of ZnO by introducing additional non-local potentials yield the formation energy and charge transfer level of the impurity in reasonable agreement with hybrid functional calculations. Summarizing the results obtained with different methods, we corroborate earlier findings that the formation of substitutional N impurities at the oxygen site in ZnO from N atoms is most likely slightly endothermic under oxygen-rich preparation conditions, and introduces a deep level more than 1 eV above the valence band edge of ZnO. Moreover, the comparison of methods elucidates subtle differences in the predicted electronic structure, e.g., concerning the orientation of unoccupied orbitals in the crystal field and the stability of the charged triplet state of the N impurity. Further experimental or theoretical analysis of these features could provide useful tests for validating the performance of DFT methods in their application to defects in wide-gap materials. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4810862
  • 2013 • 56 Effect of Sn surface states on the photocatalytic activity of anatase TiO2
    Oropeza, F.E. and Mei, B. and Sinev, I. and Becerikli, A.E. and Muhler, M. and Strunk, J.
    Applied Catalysis B: Environmental 140-141 51-59 (2013)
    The influence of surface Sn-doping on the photocatalytic properties of anatase TiO2 has been investigated in samples prepared by a grafting route using Sn(IV) tert-butoxide as Sn precursor. The grafting procedure leads to the formation of isolated Sn(IV) sites on the surface of anatase TiO2 powders as gauged by structural characterisation based on XRD, Raman spectroscopy and XAS. Studies of the surface reduction based on TPR experiments and XPS provide the conditions for a selective reduction of surface Sn(IV) to the divalent oxidation state. Electronic structure characterisation based on valence band XPS and DRS shows that there is a slight widening of the band gap upon Sn(IV)-grafting, but Sn(II) related states emerge at the top of the main valence band upon reduction at temperatures up to 350°C, and this induces visible light absorption. Grafting of TiO2 with Sn(IV) increases the formation rate of OH radicals on the surface of the material. Reduction of the Sn(IV)-grafted TiO2 to form surface Sn(II) brings about substantial increase of the photocatalytic efficiency for the methylene blue degradation under irradiation with λ≥320nm compared with Sn(IV)-grafted and pure anatase TiO2. This observation is explained based on a surface hole trapping by the Sn(II)-related surface states which lie above the top of the main valence band and can therefore act as trapping sites for holes produced under photoexcitation. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2013.03.043
  • 2013 • 55 Electronic structure, surface morphology, and topologically protected surface states of Sb2Te3 thin films grown on Si(111)
    Plucinski, L. and Herdt, A. and Fahrendorf, S. and Bihlmayer, G. and Mussler, G. and Döring, S. and Kampmeier, J. and Matthes, F. and Bürgler, D.E. and Grützmacher, D. and Blügel, S. and Schneider, C.M.
    Journal of Applied Physics 113 (2013)
    We have performed a combined spectroscopy and microscopy study on surfaces of Sb2Te3/Si(111) thin films exposed to air and annealed under ultra-high vacuum conditions. Scanning tunneling microscopy images, with atomic resolution present in most areas of such processed surfaces, show a significant amount of impurities and defects. Scanning tunneling spectroscopy reveals the bulk band gap of ∼ 170 meV centered ∼ 65 meV above the Fermi level. This intrinsic p-type doping behavior is confirmed by high-resolution angle-resolved photoemission spectra, which show the dispersions of the lower Dirac cone and the spectral weight of the bulk valence bands crossing the Fermi level. Spin-polarized photoemission revealed up to ∼15% in-plane spin polarization for photoelectrons related to the topologically protected Dirac cone states near the Fermi level, and up to ∼40% for several states at higher binding energies. The results are interpreted using ab initio electronic structure simulations and confirm the robustness of the time-reversal symmetry protected topological surface states in Sb2 Te3 in the presence of impurities and defects. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4789353
  • 2013 • 54 Environmental tight-binding modeling of nickel and cobalt clusters
    McEniry, E.J. and Drautz, R. and Madsen, G.K.H.
    Journal of Physics Condensed Matter 25 (2013)
    Tight-binding models derived from density functional theory potentially provide a systematic approach to the development of accurate and transferable models of multicomponent systems. We introduce a systematic methodology for environmental tight binding in which both the overlap and environmental contributions to the electronic structure are included. The parameters of the model are determined directly from ab initio considerations, thus providing a formal conceptual link to density functional approaches. In order to test the validity of the approach, the model is applied to small clusters of Ni and Co, whose electronic structure is largely determined by the interplay of tightly bound d-valent states and the disperse s-states. We numerically illustrate that it is essential to include environmental contributions in the tight-binding approach in order to reliably reproduce the electronic structure of such clusters. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/11/115502
  • 2013 • 53 Hidden surface states at non-polar GaN (101̄0) facets: Intrinsic pinning of nanowires
    Lymperakis, L. and Weidlich, P.H. and Eisele, H. and Schnedler, M. and Nys, J.-P. and Grandidier, B. and Stiévenard, D. and Dunin-Borkowski, R.E. and Neugebauer, J. and Ebert, Ph.
    Applied Physics Letters 103 (2013)
    We investigate the electronic structure of the GaN (10 1 ̄ 0) prototype surface for GaN nanowire sidewalls. We find a paradoxical situation that a surface state at all k points in the bandgap cannot be probed by conventional scanning tunneling microscopy, due to a dispersion characterized by a steep minimum with low density of states (DOS) and an extremely flat maximum with high DOS. Based on an analysis of the decay behavior into the vacuum, we identify experimentally the surface state minimum 0.6 ± 0.2 eV below the bulk conduction band in the gap. Hence, GaN nanowires with clean (10 1 ̄ 0) sidewall facets are intrinsically pinned. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4823723
  • 2013 • 52 High-throughput study of the structural stability and thermoelectric properties of transition metal silicides
    Opahle, I. and Parma, A. and McEniry, E.J. and Drautz, R. and Madsen, G.K.H.
    New Journal of Physics 15 (2013)
    The phase stability, electronic structure and transport properties of binary 3d, 4d and 5d transition metal silicides are investigated using high-throughput density functional calculations. An overall good agreement is found between the calculated 0 K phase diagrams and experiment. We introduce descriptors for the phase-stability and thermoelectric properties and hereby identify several candidates with potential for thermoelectric applications. This includes known thermoelectrics like Mn4Si7, β-FeSi2, Ru2Si3 and CrSi2 as well as new potentially meta-stable materials like Rh3Si5, Fe2Si3 and an orthorhombic CrSi2 phase. Analysis of the electronic structure shows that the gap formation in most of the semiconducting transition metal silicides can be understood with simple hybridization models. The transport properties of the Mn4Si 7, Ru2Ge3 and Ir3Si5 structure types and the orthorhombic CrSi2 phase are discussed. The calculated transport properties are in good agreement with available experimental data. It is shown that a better thermoelectric performance may be achieved upon optimal doping. Finally, the high-throughput data are analysed and rationalized using a simple tight-binding model. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/15/10/105010
  • 2013 • 51 Imaging the dynamics of individually adsorbed molecules
    Schaffert, J. and Cottin, M.C. and Sonntag, A. and Karacuban, H. and Bobisch, C.A. and Lorente, N. and Gauyacq, J.-P. and Möller, R.
    Nature Materials 12 223-227 (2013)
    Although noise is observed in many experiments, it is rarely used as a source of information. However, valuable information can be extracted from noisy signals. The motion of particles on a surface induced, for example, by thermal activation or by the interaction with the tip of a scanning tunnelling microscope may lead to fluctuations or switching of the tunnelling current. The analysis of these processes gives insight into dynamics on a single atomic or molecular level. Unfortunately, scanning tunnelling microscopy (STM) is not a useful tool to study dynamics in detail, as it is an intrinsically slow technique. Here, we show that this problem can be solved by providing a full real-time characterization of random telegraph noise in the current signal. The hopping rate, the noise amplitude and the relative occupation of the involved states are measured as a function of the tunnelling parameters, providing spatially resolved maps. In contrast to standard STM, our technique gives access to transiently populated states revealing an electron-driven hindered rotation between the equilibrium and two metastable positions of an individually adsorbed molecule. The new approach yields a complete characterization of copper phthalocyanine molecules on Cu(111), ranging from dynamical processes on surfaces to the underlying electronic structure on the single-molecule level. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nmat3527
  • 2013 • 50 Interplay between Coulomb interaction and quantum-confined Stark-effect in polar and nonpolar wurtzite InN/GaN quantum dots
    Barthel, S. and Schuh, K. and Marquardt, O. and Hickel, T. and Neugebauer, J. and Jahnke, F. and Czycholl, G.
    European Physical Journal B 86 (2013)
    In this paper we systematically analyze the electronic structures of polar and nonpolar wurtzite-InN/GaN quantum dots and their modification due to the quantum-confined Stark effect caused by intrinsic fields. This is achieved by combining continuum elasticity theory with an effective-bond orbital model to describe the elastic and single-particle electronic properties in these nitride systems. Based on these results, a many-body treatment is used to determine optical absorption spectra. The efficiency of optical transitions depends on the interplay between the Coulomb interaction and the quantum-confined Stark effect. We introduce an effective confinement potential which represents the electronic structure under the influence of the intrinsic polarization fields and calculate the needed strength of Coulomb interaction to diminish the separation of electrons and holes. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1140/epjb/e2013-40542-0
  • 2013 • 49 Interplay between forward and backward scattering of spin-orbit split surface states of Bi(111)
    Cottin, M.C. and Bobisch, C.A. and Schaffert, J. and Jnawali, G. and Bihlmayer, G. and Möller, R.
    Nano Letters 13 2717-2722 (2013)
    The electronic structure at the surface of Bi(111) enables us to study the effect of defects scattering into multiple channels. By performing scanning tunneling spectroscopy near step edges, we analyze the resulting oscillations in the local density of electronic states (LDOS) as function of position. At a given energy, forward and backward scattering not only occur simultaneously but may contribute to the same scattering vector Δk. If the scattering phase of both processes differs by π and the amplitudes are almost equal, the oscillations cancel out. A sharp dip in the magnitude of the Fourier transform of the LDOS marks the crossover between forward and backward scattering channels. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nl400878r
  • 2013 • 48 Polarization sensitive surface band structure of doped BaTiO 3(001)
    Rault, J.E. and Dionot, J. and Mathieu, C. and Feyer, V. and Schneider, C. M. and Geneste, G. and Barrett, N.
    Physical Review Letters 111 (2013)
    We present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO 3(001) single crystal. The n-type doping of the BaTiO3 is controlled by in situ vacuum and oxygen annealing, providing experimental evidence of a surface paraelectric-ferroelectric transition below a critical doping level. Real space imaging of photoemission threshold, core level and valence band spectra show contrast due to domain polarization. Reciprocal space imaging of the electronic structure using linearly polarized light provides unambiguous evidence for the presence of both in- and out-of-plane polarization with two- and fourfold symmetry, respectively. The results agree well with first principles calculations. © 2013 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.111.127602
  • 2013 • 47 Revelation of the crucial interactions in spin-hybrid systems by means of X-ray absorption spectroscopy
    Wende, H.
    Journal of Electron Spectroscopy and Related Phenomena 189 171-177 (2013)
    Spin-hybrid systems consisting of magnetic molecules on surfaces are studied by means of X-ray absorption spectroscopy. The relevant magnetic interactions of paramagnetic molecules on ferromagnetic surfaces are analyzed utilizing the element specificity of the X-ray magnetic circular dichroism revealing the magnetic coupling for these systems. By the help of X-ray natural dichroism the orientation of the molecules on the surfaces and the electronic structure is probed. The structural properties are correlated with the magnetic characteristics to achieve a more complete understanding of the spin-hybrid systems. The possibility to tailor the magnetic coupling for these systems is demonstrated by modifying the interface of the molecules and the substrate by using an intermediate layer of atomic oxygen. As an outlook it will be discussed how a spin crossover behavior of the molecules might be achieved in specific spin-hybrid systems by employing graphene. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elspec.2013.04.010
  • 2013 • 46 Spectral properties of a molecular wire in the kondo regime
    Greuling, A. and Temirov, R. and Lechtenberg, B. and Anders, F.B. and Rohlfing, M. and Tautz, F.S.
    Physica Status Solidi (B) Basic Research 250 2386-2393 (2013)
    Before transport data can be understood quantitatively, a few prerequisites have to be fulfilled: the geometric and the electronic structures of the metal/molecule contacts have to be known, and electron correlation effects have to be taken into account. Here we discuss experimental and theoretical approaches to tackle these challenges. On the theoretical side, density-functional theory (including van der Waals-corrections for structural optimization) is combined with many-body perturbation theory and numerical renormalization group theory in order to include all relevant correlation effects. We had already discussed such features in a previous study [Phys. Rev. B 84, 125413 (2011)], but some remaining differences between our calculated spectra and our experimental data from a scanning-tunnelling microscope remained unexplained. Here we show that the explicit incorporation of van der Waals interaction in the calculations, that had been negleted before, yields improved geometric structure and leads to much better agreement with our measured spectra. This clearly demonstrates the significant sensitivity of electronic transport to structural details.PTCDA molecule in a junction between a silver surface and an STM tip. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201349238
  • 2013 • 45 Spin-dependent electronic structure of the Co/Al(OP)3 interface
    Müller, S. and Steil, S. and Droghetti, A. and Großmann, N. and Meded, V. and Magri, A. and Schäfer, B. and Fuhr, O. and Sanvito, S. and Ruben, M. and Cinchetti, M. and Aeschlimann, M.
    15 (2013)
    We have studied the spin-dependent electronic properties of the interface formed between epitaxial Co thin films deposited on Cu(001) and the experimental molecule tris-(9-oxidophenalenone)-aluminum(III) (Al(OP) 3), created as a variation of the prototypical organic semiconductor Alq3 to tailor the spin filtering properties by modifying chemisorption with cobalt. The interfaces have been grown under ultra-high vacuum conditions by progressive deposition of 0.5-5 nm Al(OP)3 on the freshly prepared cobalt substrate. For every growth step we have monitored the energy level alignment at the interface as well as the spin polarization of the occupied manifold by spin-resolved photoemission spectroscopy. We identify two hybrid interface states in the energy window of 2 eV below the Fermi energy. The first is at 0.9 eV below EF and shows an 8% higher spin polarization than Co, while the second is at 1.6 eV below EF and shows a spin polarization reduced by 4%. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/15/11/113054
  • 2013 • 44 The electronic structure of spintronic materials as seen by spin-polarized angle-resolved photoemission
    Plucinski, L. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 189 137-145 (2013)
    The key quantity in spintronic devices is the spin polarization of the current flowing through the various device components, which in turn is closely determined by the components' electronic structure. Modern spin- and angle-resolved photoemission spectroscopy (spin-ARPES) can map the details of the spin-polarized electronic structure in many novel material systems - both magnetic and nonmagnetic. In order to separate close-lying electronic states, however, an improvement in energy and angular resolution as well as information depth is still mandatory. We review several types of modern photoemission spectrometers capable of spin analysis and discuss the application of the technique for several physical systems including ferromagnetic thin films and topological insulators. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.elspec.2013.05.001
  • 2013 • 43 Theoretical prediction of improved figure-of-merit in Si/Ge quantum dot superlattices
    Fiedler, G. and Kratzer, P.
    New Journal of Physics 15 (2013)
    A detailed theoretical model for thermoelectric transport perpendicular to the multilayers of a Si-Ge heterostructure is presented. The electronic structure of a three-dimensional superlattice, consisting of a regular array of Ge quantum dots in each layer, capped by Si layers, is calculated using an atomistic tight-binding approach. The Seebeck coefficient, the electric conductivity and the contribution of the electrons to the thermal conductivity for n-doped samples are worked out within Boltzmann transport theory. Using experimental literature data for the lattice thermal conductivity, we determine the temperature dependence of the figure of merit ZT. A nonlinear increase of ZT with temperature is found, with ZT > 2 at T = 1000 K in highly doped samples. Moreover, we find an enhanced thermoelectric power factor already at room temperature and below, which is due to highly mobile electrons in strain-induced conductive channels. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/15/12/125010
  • 2013 • 42 X-ray Raman scattering: An exciting tool for the study of matter at conditions of the Earth's interior
    Sternemann, C. and Sahle, C.J. and Mende, K. and Schmidt, C. and Nyrow, A. and Simonelli, L. and Sala, M.M. and Tolan, M. and Wilke, M.
    Journal of Physics: Conference Series 425 (2013)
    The study of minerals and melts at in situ conditions is highly relevant to understand the physical and chemical properties of the Earth's crust and mantle. Here, X-ray Raman scattering provides a valuable tool to investigate the local atomic and electronic structure of Earth materials consisting predominantly of low Z elements at high pressures and temperatures. The capabilities of X-ray Raman scattering to investigate silicate minerals, glasses, and melts are discussed and the application of the method to in situ studies of silicate melts using a hydrothermal diamond anvil cell is demonstrated.
    view abstractdoi: 10.1088/1742-6596/425/20/202011
  • 2012 • 41 A neural network potential-energy surface for the water dimer based on environment-dependent atomic energies and charges
    Morawietz, T. and Sharma, V. and Behler, J.
    Journal of Chemical Physics 136 (2012)
    Understanding the unique properties of water still represents a significant challenge for theory and experiment. Computer simulations by molecular dynamics require a reliable description of the atomic interactions, and in recent decades countless water potentials have been reported in the literature. Still, most of these potentials contain significant approximations, for instance a frozen internal structure of the individual water monomers. Artificial neural networks (NNs) offer a promising way for the construction of very accurate potential-energy surfaces taking all degrees of freedom explicitly into account. These potentials are based on electronic structure calculations for representative configurations, which are then interpolated to a continuous energy surface that can be evaluated many orders of magnitude faster. We present a full-dimensional NN potential for the water dimer as a first step towards the construction of a NN potential for liquid water. This many-body potential is based on environment-dependent atomic energy contributions, and long-range electrostatic interactions are incorporated employing environment-dependent atomic charges. We show that the potential and derived properties like vibrational frequencies are in excellent agreement with the underlying reference density-functional theory calculations. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.3682557
  • 2012 • 40 Basic properties of magnetic shape-memory materials from first-principles calculations
    Entel, P. and Dannenberg, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Comtesse, D. and Elmers, H.-J. and Kallmayer, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 2891-2900 (2012)
    The mutual influence of phase transformations, magnetism, and electronic properties of magnetic-shape memory Heusler materials is a basic issue of electronic structure calculations based on density functional theory. In this article, we show that these calculations can be pursued to finite temperatures, which allows to derive on a first-principles basis the temperature versus composition phase diagram of the pseudo-binary Ni-Mn-(Ga, In, Sn, Sb) system. The free energy calculations show that the phonon contribution stabilizes the body-centered-cubic (bcc)-like austenite structure at elevated temperatures, whereas magnetism favors the lowtemperature martensite phase with body-centered-tetragonal (bct) or rather face-centeredtetragonal (fct) structure. The calculations also allow to make predictions of magnetostructural and magnetic field induced properties of other (new) magnetic Heusler alloys not based on NiMn such as Co-Ni-(Ga-Zn) and Fe-Co-Ni-(Ga-Zn) intermetallic compounds. © The Minerals, Metals & Materials Society and ASM International 2011.
    view abstractdoi: 10.1007/s11661-011-0832-7
  • 2012 • 39 Bond order potentials for fracture, wear, and plasticity
    Pastewka, L. and Mrovec, M. and Moseler, M. and Gumbsch, P.
    MRS Bulletin 37 493-503 (2012)
    Coulson's bond order is a chemically intuitive quantity that measures the difference in the occupation of bonding and anti-bonding orbitals. Both empirical and rigorously derived bond order expressions have evolved in the course of time and proven very useful for atomistic modeling of materials. The latest generation of empirical formulations has recently been augmented by screening-function approaches. Using friction and wear of diamond and diamond-like carbon as examples, we demonstrate that such a screened bond order scheme allows for a faithful description of dynamical bond-breaking processes in materials far from equilibrium. The rigorous bond order expansions are obtained by systematic coarse-graining of the tight binding approximation and form a bridge between the electronic structure and the atomistic modeling hierarchies. They have enabled bottom-up derivations of bond order potentials for covalently bonded semiconductors, transition metals, and multicomponent intermetallics. The recently developed magnetic bond order potential gives a correct description of both directional covalent bonds and magnetic interactions in iron and is able to correctly predict the stability of bulk Fe polymorphs as well as the intricate properties of dislocation cores. The bond order schemes hence represent a family of reliable and powerful models that can be applied in large-scale simulations of complex processes involving fracture, wear, and plasticity. © 2012 Materials Research Society.
    view abstractdoi: 10.1557/mrs.2012.94
  • 2012 • 38 Bulk electronic structure of the dilute magnetic semiconductor Ga 1-x Mn x As through hard X-ray angle-resolved photoemission
    Gray, A.X. and Minár, J. and Ueda, S. and Stone, P.R. and Yamashita, Y. and Fujii, J. and Braun, J. and Plucinski, L. and Schneider, C.M. and Panaccione, G. and Ebert, H. and Dubon, O.D. and Kobayashi, K. and Fadley, C.S.
    Nature Materials 11 957-962 (2012)
    A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga 0.97 Mn 0.03 As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga 0.97 Mn 0.03 As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga 1-x Mn x As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nmat3450
  • 2012 • 37 Charge transfer dynamics in molecular solids and adsorbates driven by local and non-local excitations
    Föhlisch, A. and Vijayalakshmi, S. and Pietzsch, A. and Nagasono, M. and Wurth, W. and Kirchmann, P.S. and Loukakos, P.A. and Bovensiepen, U. and Wolf, M. and Tchaplyguine, M. and Hennies, F.
    Surface Science 606 881-885 (2012)
    Charge transfer pathways and charge transfer times in molecular films and in adsorbate layers depend both on the details of the electronic structure as well as on the degree of the initial localization of the propagating excited electronic state. For C 6F 6 molecular adsorbate films on the Cu(111) surface we determined the interplay between excited state localization and charge transfer pathways. In particular we selectively prepared a free-particle-like LUMO band excitation and compared it to a molecularly localized core-excited C1s → π * C 6F 6 LUMO state using time-resolved two-photon photoemission (tr-2PPE) and core-hole-clock (CHC) spectroscopy, respectively. For the molecularly localized core-excited C1s → π * C 6F 6 LUMO state, we separate the intramolecular dynamics from the charge transfer dynamics to the metal substrate by taking the intramolecular dynamics of the free C 6F 6 molecule into account. Our analysis yields a generally applicable description of charge transfer within molecular adsorbates and to the substrate. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2011.12.014
  • 2012 • 36 Construction of high-dimensional neural network potentials using environment-dependent atom pairs
    Jose, K.V.J. and Artrith, N. and Behler, J.
    Journal of Chemical Physics 136 (2012)
    An accurate determination of the potential energy is the crucial step in computer simulations of chemical processes, but using electronic structure methods on-the-fly in molecular dynamics (MD) is computationally too demanding for many systems. Constructing more efficient interatomic potentials becomes intricate with increasing dimensionality of the potential-energy surface (PES), and for numerous systems the accuracy that can be achieved is still not satisfying and far from the reliability of first-principles calculations. Feed-forward neural networks (NNs) have a very flexible functional form, and in recent years they have been shown to be an accurate tool to construct efficient PESs. High-dimensional NN potentials based on environment-dependent atomic energy contributions have been presented for a number of materials. Still, these potentials may be improved by a more detailed structural description, e.g., in form of atom pairs, which directly reflect the atomic interactions and take the chemical environment into account. We present an implementation of an NN method based on atom pairs, and its accuracy and performance are compared to the atom-based NN approach using two very different systems, the methanol molecule and metallic copper. We find that both types of NN potentials provide an excellent description of both PESs, with the pair-based method yielding a slightly higher accuracy making it a competitive alternative for addressing complex systems in MD simulations. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4712397
  • 2012 • 35 Depth-selective electronic and magnetic properties of a Co2MnSi tunnel magneto-resistance electrode at a MgO tunnel barrier
    Krumme, B. and Ebke, D. and Weis, C. and Makarov, S.I. and Warland, A. and Hütten, A. and Wende, H.
    Applied Physics Letters 101 (2012)
    We investigated the electronic structure as well as the magnetic properties of a Co2MnSi film on MgO(100) element-specifically at the interface to a MgO tunnel barrier by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The electronic structure of the Co atoms as a function of the capping layer thickness remained unchanged, whereas the XA spectra of Mn indicate an increase of the unoccupied d states. The experimental findings are consistent with the interfacial structure proposed in the work by B. Hülsen [Phys. Rev. Lett. 103, 046802 (2009)], where a MnSi layer is present at the interface to the MgO with oxygen atoms at top positions in the first MgO layer. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4769180
  • 2012 • 34 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 • 33 Electronic and magnetic trends in martensitically transforming Fe-Pd alloys
    Gruner, M.E. and Entel, P. and Minár, J. and Polesya, S. and Mankovsky, S. and Ebert, H.
    Journal of Magnetism and Magnetic Materials 324 3524-3529 (2012)
    We discuss the compositional trends in the electronic structure of Fe-Pd alloys in the composition range relevant for martensitic transformations and magnetic shape-memory applications. The results are obtained within the framework of density functional theory based on the Korringa-Kohn-Rostoker Greens function approach in combination with the coherent potential approximation for the description of disorder. For the body centered cubic structure, we observe with increasing Pd content the gradual disappearance of the pseudogap like structure in the center of the minority spin d-band, while on the face centered cubic side a small peak passes the Fermi level around 80 at. % Fe, which has been related previously to a band-Jahn-Teller instability. In addition, we investigate the structural variation of magnetic properties of the magnetic shape-memory composition Fe 70Pd 30 based on magnetic exchange parameters obtained from our ab initio calculations being mapped onto a classical Heisenberg model. This allows us to estimate the magnetic transition temperature within a mean field approach and Monte Carlo simulations taking into account the induced nature of the Pd atoms, leading to a close, semi-quantitative agreement with experiment in the latter case. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2012.02.081
  • 2012 • 32 Electronic structure changes across the metamagnetic transition in FeRh via hard X-ray photoemission
    Gray, A.X. and Cooke, D.W. and Krüger, P. and Bordel, C. and Kaiser, A.M. and Moyerman, S. and Fullerton, E.E. and Ueda, S. and Yamashita, Y. and Gloskovskii, A. and Schneider, C. M. and Drube, W. and Kobayashi, K. and Hellman, F...
    Physical Review Letters 108 (2012)
    Stoichiometric FeRh undergoes a temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) transition at ∼350K. In this Letter, changes in the electronic structure accompanying this transition are investigated in epitaxial FeRh thin films via bulk-sensitive valence-band and core-level hard x-ray photoelectron spectroscopy with a photon energy of 5.95keV. Clear differences between the AFM and FM states are observed across the entire valence-band spectrum and these are well reproduced using density-functional theory. Changes in the 2p core levels of Fe are also observed and interpreted using Anderson impurity model calculations. These results indicate that significant electronic structure changes over the entire valence-band region are involved in this AFM-FM transition. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.108.257208
  • 2012 • 31 Elementary relaxation processes investigated by femtosecond photoelectron spectroscopy of two-dimensional materials
    Bovensiepen, U. and Kirchmann, P.S.
    Laser and Photonics Reviews 6 589-606 (2012)
    Elementary scattering processes in solid matter occur on ultrafast timescales and photoelectron spectroscopy in the time domain represents an excellent tool for their analysis. Conventional photoemission accesses binding energies of electronic states and their momentum dispersion. The use of femtosecond laser pulses in pump-probe experiments allows obtaining direct insights to the energy and momentum dependence of ultrafast dynamics. This article introduces the elementary interaction processes and emphasizes recent work performed in this rapidly developing field. Decay processes in the low excitation limit are addressed, where electrons decay according to their interaction with carriers in equilibrium. Here, hot electron relaxation in epitaxial metallic film is reviewed. In the limit of an intense optical excitation, scattering of the excited electrons among each other establishes a non-equilibrium state. Results on charge-density wave materials and the effect of coherent nuclear motion on the electronic structure, which can break low symmetry ground states, are discussed. Figure reprinted with permission from [71]. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/lpor.201000035
  • 2012 • 30 First-principles study of the thermodynamic and elastic properties of eutectic Fe-Ti alloys
    Zhu, L.-F. and Friák, M. and Dick, A. and Grabowski, B. and Hickel, T. and Liot, F. and Holec, D. and Schlieter, A. and Kühn, U. and Eckert, J. and Ebrahimi, Z. and Emmerich, H. and Neugebauer, J.
    Acta Materialia 60 1594-1602 (2012)
    Ti-Fe alloys covering a broad range of Ti concentrations are studied using quantum-mechanical calculations. Employing density functional theory, we correctly reproduce selected key features of the experimental Fe-Ti phase diagram. Analyzing the electronic structure of the stable phases in detail provides an explanation for the thermodynamic stability in terms of the strong correlation between the composition and density of states at the Fermi energy (DOS(EF)). Based on this insight, we extend our study on both single-crystalline and polycrystalline elasticity of various Fe-Ti alloys by computing the compositional dependence of homogenized elastic constants. These quantities and their compositional dependence provide a direct explanation for the origin of the ductility and softness of the β-Ti(Fe) phase. Specifically, we find that this phase has an Fe concentration close to a threshold value connected with the onset of mechanical instability. By interlinking thermodynamic and mechanical stabilities we explain the softness and ductility of the β-Ti(Fe) in terms of a reduced mechanical stability that is connected with an increased DOS(EF) in the β-Ti(Fe). © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.11.046
  • 2012 • 29 Reconstructions and electronic structure of (112̄2) and (112̄2 ) semipolar AlN surfaces
    Kalesaki, E. and Lymperakis, L. and Kioseoglou, J. and Neugebauer, J. and Karakostas, T. and Komninou, P.
    Journal of Applied Physics 112 (2012)
    he energetics, atomic geometry, and electronic structure of semipolar (112̄2) and (112̄2 ) AlN surfaces are investigated employing first principles calculations. For metal-rich growth conditions, metallic reconstructions are favoured on both polarity surfaces. For N rich to moderate Al rich conditions, the (112̄2) planes promote semiconducting reconstructions having 2 × 2 or c(2 × 2) periodicity. In contrast, under the particular range of the Al chemical potential the (112̄2 ) surfaces stabilize reconstructions with excess metal and it is only at the extreme N rich limit that the semiconducting c(2 × 2) N adatom structure prevails. The present study reveals that the reconstructed (112̄2) surfaces do not contain steps in contrast to (112̄2 ) where surface steps are inherent for N rich to moderate metal rich growth conditions and may result in intrinsic step-flow growth and/or growth of parasitic semipolar orientations. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4743007
  • 2012 • 28 Termination control of electronic phases in oxide thin films and interfaces: LaAlO 3/SrTiO 3(001)
    Pentcheva, R. and Arras, R. and Otte, K. and Ruiz, V.G. and Pickett, W.E.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370 4904-4926 (2012)
    A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO 3 and SrTiO 3 such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO 3 films on SrTiO 3(001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects. A further aspect that is addressed is how the electronic behaviour and functionality can be tuned by an SrTiO 3 capping layer, adsorbates and metallic contacts. Lastly, we discuss recent reports on the coexistence of magnetism and superconductivity in this system for what they might imply about the electronic structure of this system. © 2012 The Royal Society.
    view abstractdoi: 10.1098/rsta.2012.0202
  • 2012 • 27 The Electronic Structure of Solids
    Bovensiepen, U. and Biermann, S. and Perfetti, L.
    Dynamics at Solid State Surfaces and Interfaces 2 1-25 (2012)
    doi: 10.1002/9783527646463.ch1
  • 2012 • 26 Toughness enhancement in TiAlN-based quarternary alloys
    Sangiovanni, D.G. and Chirita, V. and Hultman, L.
    Thin Solid Films 520 4080-4088 (2012)
    Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl) 1 - xM xN thin films in the B1 structure, with 0.06 ≤ x ≤ 0.75, obtained by alloying TiAlN with M = V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti 0.5Al 0.5 N. For (TiAl) 1 - xW xN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t 2g metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2012.01.030
  • 2011 • 25 Atom-centered symmetry functions for constructing high-dimensional neural network potentials
    Behler, J.
    Journal of Chemical Physics 134 (2011)
    Neural networks offer an unbiased and numerically very accurate approach to represent high-dimensional ab initio potential-energy surfaces. Once constructed, neural network potentials can provide the energies and forces many orders of magnitude faster than electronic structure calculations, and thus enable molecular dynamics simulations of large systems. However, Cartesian coordinates are not a good choice to represent the atomic positions, and a transformation to symmetry functions is required. Using simple benchmark systems, the properties of several types of symmetry functions suitable for the construction of high-dimensional neural network potential-energy surfaces are discussed in detail. The symmetry functions are general and can be applied to all types of systems such as molecules, crystalline and amorphous solids, and liquids. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3553717
  • 2011 • 24 Electronic structure of 1/6〈202̄3〉 partial dislocations in wurtzite GaN
    Kioseoglou, J. and Kalesaki, E. and Lymperakis, L. and Neugebauer, J. and Komninou, Ph. and Karakostas, Th.
    Journal of Applied Physics 109 (2011)
    The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {112̄0} (a-plane) and {11̄00} (m-plane) grown GaN. Their importance is established by recent experimental results, which correlate the partial dislocations (PDs) bounding I1 BSFs to the luminescence characteristics of GaN. PDs are also found to play a critical role in the alleviation of misfit strain in hetero-epitaxially grown nonpolar and semipolar films. In the present study, the energetics and the electronic structure of twelve edge and mixed 1/6〈202̄3〉 PD configurations are investigated by first principles calculations. The specific PD cores of the dislocation loop bounding the I1 BSF are identified for III-rich and N-rich growth conditions. The core structures of PDs induce multiple shallow and deep states, attributed to the low coordinated core atoms, indicating that the cores are electrically active. In contrast to edge type threading dislocations no strain induced states are found. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3569856
  • 2011 • 23 Electronic structure of EuO spin filter tunnel contacts directly on silicon
    Caspers, C. and Müller, M. and Gray, A.X. and Kaiser, A.M. and Gloskovskii, A. and Fadley, C.S. and Drube, W. and Schneider, C.M.
    Physica Status Solidi - Rapid Research Letters 5 441-443 (2011)
    We present an electronic structure study of a magnetic oxide/ semiconductor model system, EuO on silicon, which is dedicated for efficient spin injection and spin detection in silicon-based spintronics devices. A combined electronic structure analysis of Eu core levels and valence bands using hard X-ray photoemission spectroscopy was performed to quantify the nearly ideal stoichiometry of EuO "spin filter" tunnel barriers directly on silicon, and the absence of silicon oxide at the EuO/Si interface. These results provide evidence for the successful integration of a magnetic oxide tunnel barrier with silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices. Hard X-ray photoemission spectroscopy of an Al/EuO/Si heterostructure probing the buried EuO and EuO/Si interface. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Hard X-ray photoemission spectroscopy reveals the nearly ideal stoichiometry of EuO spin filter tunnel barriers grown directly on silicon, and the absence of silicon oxide formation at the EuO/Si interface. These results demonstrate the successful integration of a magnetic oxide tunnel barrier with silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201105403
  • 2011 • 22 Element-specific magnetic hysteresis of individual 18 nm Fe nanocubes
    Kronast, F. and Friedenberger, N. and Ollefs, K. and Gliga, S. and Tati-Bismaths, L. and Thies, R. and Ney, A. and Weber, R. and Hassel, C. and Römer, F.M. and Trunova, A.V. and Wirtz, C. and Hertel, R. and Dürr, H.A. and Farle, M.
    Nano Letters 11 1710-1715 (2011)
    Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl200242c
  • 2011 • 21 Growth process, characterization, and modeling of electronic properties of coupled InAsSbP nanostructures
    Marquardt, O. and Hickel, T. and Neugebauer, J. and Gambaryan, K.M. and Aroutiounian, V.M.
    Journal of Applied Physics 110 (2011)
    Quaternary III-V InAsSbP quantum dots (QDs) have been grown in the form of cooperative InAsSb/InAsP structures using a modified version of the liquid phase epitaxy. High resolution scanning electron microscopy, atomic force microscopy, and Fourier-transform infrared spectrometry were used to investigate these so-called nano-camomiles, mainly consisting of a central InAsSb QD surrounded by six InAsP-QDs, that shall be referred to as leaves in the following. The observed QDs average density ranges from 0.8 to 2 × 109 cm -2, with heights and widths dimensions from 2 to 20 nm and 5 to 45 nm, respectively. The average density of the leaves is equal to (6-10) × 109 cm-2 with dimensions of approx. 5 to 40 nm in width and depth. To achieve a first basic understanding of the electronic properties, we have modeled these novel nanostructures using second-order continuum elasticity theory and an eight-band k p model to calculate the electronic structure. Our calculations found a clear localization of hole states in the central InAsSb dot. The localization of electron states, however, was found to be weak and might thus be easily influenced by external electric fields or strain. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3624621
  • 2011 • 20 Probing bulk electronic structure with hard X-ray angle-resolved photoemission
    Gray, A.X. and Papp, C. and Ueda, S. and Balke, B. and Yamashita, Y. and Plucinski, L. and Minár, J. and Braun, J. and Ylvisaker, E.R. and Schneider, C.M. and Pickett, W.E. and Ebert, H. and Kobayashi, K. and Fadley, C.S.
    Nature Materials 10 759-764 (2011)
    Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nmat3089
  • 2011 • 19 Spatial variation of the surface state onset close to three types of surface steps on Ag(111) studied by scanning tunnelling spectroscopy
    Heidorn, S. and Morgenstern, K.
    New Journal of Physics 13 (2011)
    A regular step, a dislocation slip step and a step formed by the emergence of a split edge dislocation (SED) to the surface influence the local density of states close to the onset of the surface state as investigated by scanning tunnelling spectroscopy at low temperature.The onset of the surface state shifts close to the regular step and the dislocation slip step by approximately 15 meV towards the Fermi energy.Additional maxima above the onset are only observed if a second step leads to confinement.In both cases, the conductivity decreases close to the step.However, an increase in conductance above the surface state onset is observed close to the SED step.Furthermore, a variety of additional states are discernable.Thus, different types of steps lead to markedly different changes in the local electronic structure on surfaces.© IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/13/3/033034
  • 2011 • 18 Strain-induced effects on the electronic structure and N K-edge ELNES of wurtzite A1N and AlxGa1-xN
    Petrov, M. and Holec, D. and Lymperakis, L. and Neugebauer, J. and Humphreys, C.J.
    Journal of Physics: Conference Series 326 (2011)
    Analysis of the electron energy loss near edge structure (ELNES) provides an experimental tool to probe the density of unoccupied states. Here we present a first principles study on the projected density of states (PDOS) of AlN, GaN, and AlxGa1-xN alloys in order to investigate the impact of strain on the N K-edge ELNES. Uni-axial and bi-axial strain, volume conserving, and bi-axial stress deformation modes are calculated for the whole compositional range from AlN to GaN. Our results show that only the strain along the c-axis has a pronounced impact on the PDOS. Furthermore, we find that bi-axial stress in the basal plane, which is present in pseudomorphic polar heteroepitaxial layers, does not significantly influence the N K-edge spectra. However, strain-induced changes may appear for different deformation modes and/or specimen geometries.
    view abstractdoi: 10.1088/1742-6596/326/1/012016
  • 2011 • 17 Supertoughening in B1 transition metal nitride alloys by increased valence electron concentration
    Sangiovanni, D.G. and Hultman, L. and Chirita, V.
    Acta Materialia 59 2121-2134 (2011)
    We use density functional theory calculations to explore the effects of alloying cubic TiN and VN with transition metals M = Nb, Ta, Mo or W at 50% concentrations. The ternary systems obtained are predicted to be supertough, as they are shown to be harder and significantly more ductile compared with reference binary systems. The primary electronic mechanism of this supertoughening effect is shown in a comprehensive electronic structure analysis of these compounds to be the increased valence electron concentration intrinsic to these ternary systems. Our investigations reveal the complex nature of chemical bonding in these compounds, which ultimately explains the observed selective response to stress. The findings presented in this paper thus offer a design route for the synthesis of supertough transition metal nitride alloys via valence electron concentration tuning. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.12.013
  • 2011 • 16 Synthesis, structural properties, and catalytic behavior of Cu-BTC and mixed-linker Cu-BTC-PyDC in the oxidation of benzene derivatives
    Marx, S. and Kleist, W. and Baiker, A.
    Journal of Catalysis 281 76-87 (2011)
    Mixed-linker metal-organic frameworks based on the Cu-BTC structure have been synthesized in which the benzene-1,3,5-tricarboxylate (BTC) linkers have been partially replaced by pyridine-3,5-dicarboxylate (PyDC). X-ray-based techniques (powder XRD and XAS), thermal analysis, and infrared spectroscopy proved that a desired amount of PyDC (up to 50%) can be incorporated without changing significantly the crystal structure. The pyridine unit can be seen as a defect site in the local coordination environment of the dimeric copper units, which is significantly altering their electronic structure and the catalytic properties. Both Cu-BTC and the mixed Cu-BTC-PyDCs catalyze the demanding direct hydroxylation of toluene both in acetonitrile and in neat substrate. Different selectivity toward the desired ortho- and para-cresol and other oxidation products (benzaldehyde, benzyl alcohol, methylbenzoquinone) was observed for Cu-BTC and the Cu-BTC-PyDCs, respectively. Leaching tests and comparison with homogeneously dissolved Cu catalysts indicate mainly a heterogeneous reaction pathway. © 2011 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2011.04.004
  • 2011 • 15 Textured growth of the high moment material Gd(0 0 0 1)/Cr(0 0 1)/Fe(0 0 1)
    Stromberg, F. and Antoniak, C. and Von Hörsten, U. and Keune, W. and Sanyal, B. and Eriksson, O. and Wende, H.
    Journal of Physics D: Applied Physics 44 (2011)
    By magnetic coupling of Fe and Gd via Cr interlayers, the large local moment of Gd can be combined with the high Curie temperature of Fe. The textured growth of a Gd film is studied here by preparing trilayer systems of Fe/Cr/Gd on MgO(1 0 0) substrates by molecular-beam epitaxy (MBE). The thickness of the Cr interlayer was varied between 3 and 5 monolayers. The structural quality of the samples was confirmed by in situ RHEED and ex situ XRD measurements. Epitaxial Cr(0 0 1)/Fe(0 0 1) growth was observed, as expected. By use of 57Fe-CEMS (conversion electron Mössbauer spectroscopy) in combination with the 57Fe tracer layer method the Fe/Cr interface could be examined on an atomic scale and well separated Fe/Gd layers for all Cr thicknesses were confirmed. The unusual Gd/Cr crystallographic relationship of Gd(0 0 0 1)∥Cr(0 0 1), with domains of the hexagonal Gd basal planes randomly oriented in the sample plane and not in registry with the underlying Cr(0 0 1) lattice, was found from combined RHEED and x-ray measurements. Annealing of the samples resulted in a remarkable improvement of the crystalline structure of the Gd layers. On the other hand, the appearance of a single line in the CEM spectrum leads to the conclusion that during annealing a small amount of Fe diffuses into the Cr layer. The electronic structure and magnetism of this system are investigated by first-principles theory. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/26/265004
  • 2011 • 14 The frontier orbitals of a push-pull azobenzene adsorbed on a metal surface in different bonding geometries investigated by scanning tunneling spectroscopy and spectroscopy mapping
    Henzl, J. and Morgenstern, K.
    Journal of Chemical Physics 135 (2011)
    The electronic structure of 4-anilino-4′-nitroazobenzene superstructures formed on Au(111) at 250 K is investigated by low temperature scanning tunneling microscopy, scanning tunneling spectroscopy, and dI/dV mapping at 5 K. Changes in the dI/dV maps of this push-pull molecule reflect the spatial distribution of the frontier orbitals on the molecular scale. Spectra of the trans- and the cis-isomer differ between themselves and in different parts of supramolecular assemblies. The relative importance of these differences is discussed. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3631340
  • 2011 • 13 The object-oriented DFT program library S/PHI/nX
    Boeck, S. and Freysoldt, C. and Dick, A. and Ismer, L. and Neugebauer, J.
    Computer Physics Communications 182 543-554 (2011)
    In order to simplify the development and implementation process of quantum mechanical algorithms, we developed a set of object-oriented C++ libraries which can exploit modern computer architectures. The libraries are characterized as follows: (i) State-of-the-art computer science techniques have been applied or developed in this work to provide language elements to express algebraic expressions efficiently on modern computer platforms. (ii) Quantum mechanical algorithms are crucial in the field of materials research. The new libraries support the Dirac notation to implement such algorithms in the native language of physicists. (iii) The libraries are completed by elements to express equations of motions efficiently which is required for implementing structural algorithms such as molecular dynamics. Based on these libraries we introduce the DFT program package S/PHI/nX. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cpc.2010.09.016
  • 2011 • 12 Visualization and functions of surface defects on carbon nanotubes created by catalytic etching
    Xia, W. and Yin, X. and Kundu, S. and Sánchez, M. and Birkner, A. and Wöll, C. and Muhler, M.
    Carbon 49 299-305 (2011)
    Surface defects were created on carbon nanotubes (CNTs) by catalytic steam gasification or catalytic etching with iron as catalysts. The structure and morphology of the etched CNTs were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The electronic structure of the etched CNTs was investigated by ultraviolet photoelectron spectroscopy (UPS). The etched CNTs were treated by nitric acid to obtain oxygen-containing functional groups. The amount and the thermal stability of these groups were studied by temperature-resolved X-ray photoelectron spectroscopy (XPS). Temperature-programmed desorption with ammonia as a probe molecule (NH 3-TPD) was employed to investigate the interaction of the surface defects with foreign molecules in gas phase. TEM and STM studies disclosed the presence of surface defects especially edge planes on the etched CNTs. Etching of CNTs led to a less pronounced p-π band than the as-is CNTs, as evidenced by UPS studies. The XPS and NH 3-TPD studies demonstrated that the defects on the CNTs enhanced the reactivity of the exposed surfaces allowing obtaining a higher degree of oxygen functionalization and more active adsorption sites. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2010.09.025
  • 2010 • 11 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 • 10 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 • 9 Electronic structure of Co-phthalocyanine calculated by GGA+U and hybrid functional methods
    Bhattacharjee, S. and Brena, B. and Banerjee, R. and Wende, H. and Eriksson, O. and Sanyal, B.
    Chemical Physics 377 96-99 (2010)
    Electronic structure calculations have been performed for the Co-phthalocyanine molecule using density functional theory (DFT) within the framework of Generalized Gradient Approximation (GGA). The electronic correlation in Co 3d orbitals is treated in terms of the GGA+U method in the framework of the Hubbard model. We find that for U = 6 eV, the calculated structural parameters as well as the spectral features are in good agreement with the experimental findings. From our calculation both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are dominated by the pyrrole carbon, with a HOMO-LUMO gap of about 1.4 eV. The GGA+U results obtained with U = 6 eV compare reasonably well with the calculations performed using Gaussian basis set and hybrid functionals in terms of ground state geometry, spin state and spectral features. The calculated valence band photoemission spectrum is in quite good agreement with the recently published experimental results. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.chemphys.2010.08.020
  • 2010 • 8 Electronic structure of self-assembled InGaAs/GaAs quantum rings studied by capacitance-voltage spectroscopy
    Lei, W. and Notthoff, C. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Applied Physics Letters 96 (2010)
    Self-assembled InGaAs quantum rings, embedded in a GaAs matrix, were investigated using magneto-capacitance-voltage spectroscopy. The magnetic-field dispersion of the charging energies exhibits characteristic features for both the first and second electron, which can be attributed to a ground state transition from l=0 into l=-1, and a ground state transition from l=-1 into l=-2, respectively. Furthermore, using a combination of capacitance-voltage spectroscopy and one-dimensional numerical simulations, the conduction band structure of these InGaAs quantum rings was determined. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3293445
  • 2010 • 7 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 • 6 Pd-Ga intermetallic compounds as highly selective semihydrogenation catalysts
    Armbrüster, M. and Kovnir, K. and Behrens, M. and Teschner, D. and Grin, Y. and Schlögl, R.
    Journal of the American Chemical Society 132 14745-14747 (2010)
    The intermetallic compounds Pd3Ga7, PdGa, and Pd 2Ga are found to be highly selective semihydrogenation catalysts for acetylene outperforming established systems. The stability of the crystal and electronic structure under reaction conditions allows the direct relation of structural and catalytic properties and a knowledge-based development of new intermetallic catalyst systems. In the crystal structure of PdGa palladium is exclusively surrounded by gallium atoms. The alteration of the Pd coordination in PdGa leads to a strong modification of the electronic structure around the Fermi level in comparison to elemental Pd. Electronic modification and isolation of active sites causes the excellent catalytic semihydrogenation properties. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja106568t
  • 2010 • 5 Quasiparticle interference in the spin-density wave phase of iron-based superconductors
    Knolle, J. and Eremin, I. and Akbari, A. and Moessner, R.
    Physical Review Letters 104 (2010)
    We propose an explanation for the electronic nematic state observed recently in parent iron-based superconductors. We argue that the quasi-one-dimensional nanostructure identified in the quasiparticle interference (QPI) is a consequence of the interplay of the magnetic (π, 0) spin-density wave (SDW) order with the underlying electronic structure. We show that the evolution of the QPI peaks largely reflects quasiparticle scattering between bands involved in the SDW formation. Because of the ellipticity of the electron pocket and the fact that only one of the electron pockets is involved in the SDW, the resulting QPI has a pronounced one-dimensional structure. We further predict that the QPI crosses over to two dimensionality on an energy scale, set by the SDW gap. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.104.257001
  • 2010 • 4 Shape-dependent catalytic properties of Pt nanoparticles
    Mostafa, S. and Behafarid, F. and Croy, J.R. and Ono, L.K. and Li, L. and Yang, J.C. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 132 15714-15719 (2010)
    Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al2O3. Nanoparticles with analogous average size distributions (∼0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja106679z
  • 2010 • 3 Studies regarding the homogeneity range of the zirconium phosphide telluride Zr2+δPTe2
    Tschulik, K. and Hoffmann, S. and Fokwa, B.P.T. and Gilleßen, M. and Schmidt, P.
    Solid State Sciences 12 2030-2035 (2010)
    The phosphide tellurides Zr2+δPTe2 (0 ≤ δ ≤ 1) can be synthesized from the elements in a solid state reaction or by thermal decomposition of Z. Zr2PTe2 decomposes under release of Te2(g) + P4(g) forming the homogeneity range Zr2+δPTe2. The growth of single crystals of Zr 2+δPTe2 succeeded by chemical vapour transport using iodine as transport agent from 830 °C in direction of higher temperatures up to 900 °C. Zr2+δPTe2 crystallizes in the rhombohedral space group R3m (no. 166) with lattice parameters a = 383(1)...386(1) pm and c = 2935(4)...2970(4) pm for δ = 0...1, respectively. Single crystal data have been determined for Zr 2.40(2)PTe2 with lattice parameters a = 385.24(4) pm and c = 2967.8(4) pm. The electronic structure and chemical bonding in Zr 2+δPTe2 was investigated by the linear muffin-tin orbital (LMTO) method. Both Zr2PTe2 and Zr 3PTe2 show non-vanishing DOS values at the Fermi level (EF) indicating metallic character. According to COHP bonding analyses, mainly the heteroatomic Zr-P and Zr-Te bonds are responsible for the structural stability of Zr3PTe2. The new Zr2-Te bond, which is not present in Zr2PTe2, is stronger than Zr1-Te and is thought to be responsible for the stability of phases having Zr in excess.
    view abstractdoi: 10.1016/j.solidstatesciences.2010.08.022
  • 2010 • 2 Synthesis and x-ray crystal structures of tetranuclear zincamidinate complexes
    Gutschank, B. and Schulz, S. and Westphal, U. and Bläser, D. and Boese, R.
    Organometallics 29 2093-2097 (2010)
    Polynuclear amidinate zinc halide complexes of the general type{C[C(Ni-Pr)2ZnX]4} [X = Cl, 2; Br, 3; I, 4] were prepared in high yields via methyl/halide exchange reaction of {C[C(Ni-Pr) 2ZnMe]4} (1a) with AlX3. 2-4 were characterized by elemental analysis, multinuclear NMR, and IR spectroscopy and single-crystal X-ray diffraction. Computational calculations of halide-substituted complexes {C[C(Ni-Pr)2ZnX]4} [X = F-I] were performed to clarify the influence of the halide atom on the structural parameters of the complexes and to elucidate their electronic structure and bonding situation. The capability of these halide-substituted complexes to serve as suitable starting reagents for further salt elimination reactions was proven by reaction of 2 with LiR (R = Me, n-Bu) and EtMgBr, which yielded the corresponding Zn-alkyl species {C[C(Ni-Pr)2ZnR]4} [R = Me, 1a; n-Bu, 5; Et, 6]. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/om100065w
  • 2009 • 1 Similarities between structural distortions underpressure and chemical doping in superconducting BaFe 2 As 2
    Kimber, S.A.J. and Kreyssig, A. and Zhang, Y.-Z. and Jeschke, H.O. and Valentí, R. and Yokaichiya, F. and Colombier, E. and Yan, J. and Hansen, T.C. and Chatterji, T. and Mcqueeney, R.J. and Canfield, P.C. and Goldman, A.I. and A...
    Nature Materials 8 471-475 (2009)
    The discovery of a new family of high-T C materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe 2 As 2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe 2 As 2. © 2009 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nmat2443