Prof. Dr. Uwe Bovensiepen

Experimental Physics
University of Duisburg-Essen

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  • Effect of lattice excitations on transient near-edge x-ray absorption spectroscopy
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Šipr, O. and Ebert, H. and Sokolowski-Tinten, K. and Pentcheva, R. and Bovensiepen, U. and Eschenlohr, A. and Wende, H.
    Physical Review B 104 (2021)
    Time-dependent and constituent-specific spectral changes in soft near-edge x-ray absorption spectroscopy (XAS) of an metal/insulator heterostructure after laser excitation are analyzed at the O K-edge with picosecond time resolution. The oxygen absorption edge of the insulator features a uniform intensity decrease of the fine structure at elevated phononic temperatures, which can be quantified by a simple simulation and fitting procedure presented here. Combining XAS with ultrafast electron diffraction measurements and ab initio calculations demonstrates that the transient intensity changes in XAS can be assigned to a transient lattice temperature. Thus, the sensitivity of transient near-edge XAS to phonons is demonstrated. © 2021 American Physical Society
    view abstract10.1103/PhysRevB.104.144302
  • Impact of electron solvation on ice structures at the molecular scale
    Bertram, C. and Auburger, P. and Bockstedte, M. and Stähler, J. and Bovensiepen, U. and Morgenstern, K.
    Journal of Physical Chemistry Letters 11 (2020)
    Electron attachment and solvation at ice structures are well-known phenomena. The energy liberated in such events is commonly understood to cause temporary changes at such ice structures, but it may also trigger permanent modifications to a yet unknown extent. We determine the impact of electron solvation on D2O structures adsorbed on Cu(111) with low-Temperature scanning tunneling microscopy, two-photon photoemission, and ab initio theory. Solvated electrons, generated by ultraviolet photons, lead not only to transient but also to permanent structural changes through the rearrangement of individual molecules. The persistent changes occur near sites with a high density of dangling OD groups that facilitate electron solvation. We conclude that energy dissipation during solvation triggers permanent molecular rearrangement via vibrational excitation. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jpclett.9b03723
  • Local and Nonlocal Electron Dynamics of Au/Fe/MgO (001) Heterostructures Analyzed by Time-Resolved Two-Photon Photoemission Spectroscopy
    Beyazit, Y. and Beckord, J. and Zhou, P. and Meyburg, J.P. and Kühne, F. and Diesing, D. and Ligges, M. and Bovensiepen, U.
    Physical Review Letters 125 (2020)
    Employing femtosecond laser pulses in front and back side pumping of Au/Fe/MgO(001) combined with detection in two-photon photoelectron emission spectroscopy, we analyze local relaxation dynamics of excited electrons in buried Fe, injection into Au across the Fe-Au interface, and electron transport across the Au layer at 0.6 to 2.0 eV above the Fermi energy. By analysis as a function of Au film thickness we obtain the electron lifetimes of bulk Au and Fe and distinguish the relaxation in the heterostructure's constituents. We also show that the excited electrons propagate through Au in a superdiffusive regime and conclude further that electron injection across the epitaxial interface proceeds ballistically by electron wave packet propagation. © 2020 American Physical Society.
    view abstract10.1103/PhysRevLett.125.076803
  • Competing Spin Transfer and Dissipation at Co/Cu (001) Interfaces on Femtosecond Timescales
    Chen, J. and Bovensiepen, U. and Eschenlohr, A. and Müller, T. and Elliott, P. and Gross, E.K.U. and Dewhurst, J.K. and Sharma, S.
    Physical Review Letters 122 (2019)
    By combining interface-sensitive nonlinear magneto-optical experiments with femtosecond time resolution and ab initio time-dependent density functional theory, we show that optically excited spin dynamics at Co/Cu(001) interfaces proceeds via spin-dependent charge transfer and back transfer between Co and Cu. This ultrafast spin transfer competes with dissipation of spin angular momentum mediated by spin-orbit coupling already on sub 100 fs timescales. We thereby identify the fundamental microscopic processes during laser-induced spin transfer at a model interface for technologically relevant ferromagnetic heterostructures. © 2019 American Physical Society.
    view abstract10.1103/PhysRevLett.122.067202
  • Decelerated lattice excitation and absence of bulk phonon modes at surfaces: Ultra-fast electron diffraction from Bi(111) surface upon fs-laser excitation
    Tinnemann, V. and Streubühr, C. and Hafke, B. and Witte, T. and Kalus, A. and Hanisch-Blicharski, A. and Ligges, M. and Zhou, P. and Von Der Linde, D. and Bovensiepen, U. and Horn-Von Hoegen, M.
    Structural Dynamics 6 (2019)
    Ultrafast reflection high-energy electron diffraction is employed to follow the lattice excitation of a Bi(111) surface upon irradiation with a femtosecond laser pulse. The thermal motion of the atoms is analyzed through the Debye-Waller effect. While the Bi bulk is heated on time scales of 2 to 4 ps, we observe that the excitation of vibrational motion of the surface atoms occurs much slower with a time constant of 12 ps. This transient nonequilibrium situation is attributed to the weak coupling between bulk and surface phonon modes which hampers the energy flow between the two subsystems. From the absence of a fast component in the transient diffraction intensity, it is in addition concluded that truncated bulk phonon modes are absent at the surface. © 2019 Author(s).
    view abstract10.1063/1.5128275
  • Erratum: Ultrafast Doublon Dynamics in Photoexcited 1T-TaS2 (Physical Review Letters (2018) 120 (166401) DOI: 10.1103/PhysRevLett.120.166401)
    Ligges, M. and Avigo, I. and GoleŽ, D. and Strand, H.U.R. and Beyazit, Y. and Hanff, K. and Diekmann, F. and Stojchevska, L. and Kalläne, M. and Zhou, P. and Rossnagel, K. and Eckstein, M. and Werner, P. and Bovensiepen, U.
    Physical Review Letters 122 (2019)
    In the simulations, time is measured in units of h / J , where J is the hopping parameter. In the conversion from [ eV - 1 ] to [fs], we missed a factor of 2π , which implies that the physical times given in [fs] in the Letter should be divided by 2π . This affects the labels of Fig. 3(a), see Fig. 1 for the corrected version. The comparison of the experimental data and the simulation results is more favorable with the corrected time axis, since the timescales for the dynamics become comparable. The same error occurred in our statements regarding the timescales of electronic hopping, which is h / J ≈ 2fs . The discussions and conclusions on the comparison between theory and experiment remain valid. (Figure Presented). © 2019 American Physical Society.
    view abstract10.1103/PhysRevLett.122.159901
  • Magneto-optical properties of Au upon the injection of hot spin-polarized electrons across Fe/Au(0 0 1) interfaces
    Alekhin, A. and Razdolski, I. and Berritta, M. and Bürstel, D. and Temnov, V. and Diesing, D. and Bovensiepen, U. and Woltersdorf, G. and Oppeneer, P.M. and Melnikov, A.
    Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019)
    We demonstrate a novel method for the excitation of sizable magneto-optical effects in Au by means of the laser-induced injection of hot spin-polarized electrons in Au/Fe/MgO(0 0 1) heterostructures. It is based on the energy- and spin-dependent electron transmittance of Fe/Au interface which acts as a spin filter for non-thermalized electrons optically excited in Fe. We show that after crossing the interface, majority electrons propagate through the Au layer with the velocity on the order of 1 nm fs-1 (close to the Fermi velocity) and the decay length on the order of 100 nm. Featuring ultrafast functionality and requiring no strong external magnetic fields, spin injection results in a distinct magneto-optical response of Au. We develop a formalism based on the phase of the transient complex MOKE response and demonstrate its robustness in a plethora of experimental and theoretical MOKE studies on Au, including our ab initio calculations. Our work introduces a flexible tool to manipulate magneto-optical properties of metals on the femtosecond timescale that holds high potential for active magneto-photonics, plasmonics, and spintronics.
    view abstract10.1088/1361-648X/aafd06
  • Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Pentcheva, R. and Wende, H. and Bovensiepen, U. and Sokolowski-Tinten, K. and Eschenlohr, A.
    Physical Review B 100 (2019)
    The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]n as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or nonlocal relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L3 absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, nonthermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L3 edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with nonequilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these nonlocal, nonequilibrium dynamics. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.174301
  • Optical perturbation of the hole pockets in the underdoped high- Tc superconducting cuprates
    Freutel, S. and Rameau, J.D. and Rettig, L. and Avigo, I. and Ligges, M. and Yoshida, Y. and Eisaki, H. and Schneeloch, J. and Zhong, R.D. and Xu, Z.J. and Gu, G.D. and Bovensiepen, U. and Johnson, P.D.
    Physical Review B 99 (2019)
    The high-Tc superconducting cuprates are recognized as doped Mott insulators. Several studies indicate that as a function of doping and temperature, there is a crossover from this regime into a phase characterized as a marginal Fermi liquid. Several calculations of the doped Mott insulating phase indicate that the Fermi surface defines small pockets which at the higher doping levels switch to a full closed Fermi surface, characteristic of a more metallic system. Here we use femtosecond laser-based pump-probe techniques to investigate the structure of the Fermi surface in the underdoped region of Bi2Sr2CaCu2O8+δ and compare it with that associated with the optimally doped material. We confirm the concept of a small pocket in the underdoped system consistent with theoretical predictions in this strongly correlated state. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.081116
  • Toward attosecond pulse synthesis from solids: Spectral shaping, field autocorrelation, and two-color harmonic generation
    Seyen, A. and Kohn, R. and Bovensiepen, U. and Von Der Linde, D. and Tarasevitch, A.
    Physical Review A 99 (2019)
    Harmonic generation from solid surfaces is a promising tool for producing high-energy attosecond pulses. We report shaping of the harmonic spectrum to achieve the bandwidth necessary for attosecond pulse generation. The shaping is demonstrated for lower as well as for higher harmonics using single and two-pulse pumping. The measured harmonic field autocorrelation function exhibits attosecond spikes in good agreement with the harmonic spectrum. Double slit experiments reveal a high spatial coherence of the harmonic beam. © 2019 American Physical Society.
    view abstract10.1103/PhysRevA.99.033836
  • Towards high power broad-band OPCPA at 3000 nm
    Bridger, M. and Naranjo-Montoya, O.A. and Tarasevitch, A. and Bovensiepen, U.
    Optics Express 27 (2019)
    High-energy femtosecond laser pulses in the mid-infrared (MIR) wavelength range are essential for a wide range of applications from strong-field physics to selectively pump and probe low energy excitations in condensed matter and molecular vibrations. Here we report a four stage optical parametric chirped pulse amplifier (OPCPA) which generates ultrashort pulses at a central wavelength of 3000 nm with 430 µJ energy per pulse at a bandwidth of 490 nm. Broadband emission of a Ti:sapphire oscillator seeds both the four stage OPCPA 800 nm and the pump line at 1030 nm. The first stage amplifies the 800 nm pulses in BBO using a non-collinear configuration. The second stage converts the wavelength to 1560 nm using difference frequency generation in BBO in a collinear geometry. The third stage amplifies this frequency non-collinearly in KTA. Finally, the fourth stage generates the 3000 nm radiation in a collinear configuration in LiIO3 due to the broad amplification bandwidth this crystal provides. We compress these pulses to 65 fs by transmission through sapphire. Quantitative calculations of the individual non-linear processes in all stages verify that our OPCPA architecture operates close to optimum efficiency. Low absorption losses suggest that this particular design is very suitable for operation at high average power and multi kHz repetition rates. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
    view abstract10.1364/OE.27.031330
  • Ultrafast electron diffraction from a Bi(111) surface: Impulsive lattice excitation and Debye-Waller analysis at large momentum transfer
    Tinnemann, V. and Streubühr, C. and Hafke, B. and Kalus, A. and Hanisch-Blicharski, A. and Ligges, M. and Zhou, P. and Von Der Linde, D. and Bovensiepen, U. and Horn-Von Hoegen, M.
    Structural Dynamics 6 (2019)
    The lattice response of a Bi(111) surface upon impulsive femtosecond laser excitation is studied with time-resolved reflection high-energy electron diffraction. We employ a Debye-Waller analysis at large momentum transfer of 9.3 Å -1 ≤ Δ k ≤ 21.8 Å -1 in order to study the lattice excitation dynamics of the Bi surface under conditions of weak optical excitation up to 2 mJ/cm 2 incident pump fluence. The observed time constants τ int of decay of diffraction spot intensity depend on the momentum transfer Δk and range from 5 to 12 ps. This large variation of τ int is caused by the nonlinearity of the exponential function in the Debye-Waller factor and has to be taken into account for an intensity drop ΔI > 0.2. An analysis of more than 20 diffraction spots with a large variation in Δk gave a consistent value for the time constant τ T of vibrational excitation of the surface lattice of 12 ± 1 ps independent on the excitation density. We found no evidence for a deviation from an isotropic Debye-Waller effect and conclude that the primary laser excitation leads to thermal lattice excitation, i.e., heating of the Bi surface. © 2019 Author(s).
    view abstract10.1063/1.5093637
  • Excitation and relaxation dynamics of the photo-perturbed correlated electron system 1T-TaS2
    Avigo, I. and Zhou, P. and Kalläne, M. and Rossnagel, K. and Bovensiepen, U. and Ligges, M.
    Applied Sciences (Switzerland) 9 (2018)
    We investigate the perturbation and subsequent recovery of the correlated electronic ground state of the Mott insulator 1T-TaS2 by means of femtosecond time-resolved photoemission spectroscopy in normal emission geometry. Upon an increase of near-infrared excitation strength, a considerable collapse of the occupied Hubbard band is observed, which reflects a quench of short-range correlations. It is furthermore found that these excitations are directly linked to the lifting of the periodic lattice distortion which provides the localization centers for the formation of the insulating Mott state. We discuss the observed dynamics in a localized real-space picture. © 2018 by the authors.
    view abstract10.3390/app9010044
  • Microscopic Insight into Electron-Induced Dissociation of Aromatic Molecules on Ice
    Auburger, P. and Kemeny, I. and Bertram, C. and Ligges, M. and Bockstedte, M. and Bovensiepen, U. and Morgenstern, K.
    Physical Review Letters 121 (2018)
    We use scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations to investigate the electron-induced dissociation of halogenated benzene molecules adsorbed on ice. Dissociation of halobenzene is triggered by delocalized excess electrons attaching to the π∗ orbitals of the halobenzenes from where they are transferred to σ∗ orbitals. The latter orbitals provide a dissociative potential surface. Adsorption on ice sufficiently lowers the energy barrier for the transfer between the orbitals to facilitate dissociation of bromo- and chloro- but not of flourobenzene at cryogenic temperatures. Our results shed light on the influence of environmentally important ice particles on the reactivity of halogenated aromatic molecules. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.206001
  • Nonequilibrium electron and lattice dynamics of strongly correlated Bi2Sr2CaCu2O8+d single crystals
    Konstantinova, T. and Rameau, J.D. and Reid, A.H. and Abdurazakov, O. and Wu, L. and Li, R. and Shen, X. and Gu, G. and Huang, Y. and Rettig, L. and Avigo, I. and Ligges, M. and Freericks, J.K. and Kemper, A.F. and Dürr, H.A. and Bovensiepen, U. and Johnson, P.D. and Wang, X. and Zhu, Y.
    Science Advances 4 (2018)
    The interplay between the electronic and lattice degrees of freedom in nonequilibrium states of strongly correlated systems has been debated for decades. Although progress has been made in establishing a hierarchy of electronic interactions with the use of time-resolved techniques, the role of the phonons often remains in dispute, a situation highlighting the need for tools that directly probe the lattice. We present the first combined megaelectron volt ultrafast electron diffraction and time- and angle-resolved photoemission spectroscopy study of optimally doped Bi2Sr2CaCu2O8+d. Quantitative analysis of the lattice and electron subsystems’ dynamics provides a unified picture of nonequilibrium electron-phonon interactions in the cuprates beyond the N-temperature model. The work provides new insights on the specific phonon branches involved in the nonequilibrium heat dissipation from the high-energy Cu–O bond stretching “hot” phonons to the lowest-energy acoustic phonons with correlated atomic motion along the <110> crystal directions and their characteristic time scales. It reveals a highly nonthermal phonon population during the first several picoseconds after the photoexcitation. The approach, taking advantage of the distinct nature of electrons and photons as probes, is applicable for studying energy relaxation in other strongly correlated electron systems. Copyright © 2018 The Authors.
    view abstract10.1126/sciadv.aap7427
  • Special issue on ultrafast magnetism
    Eschenlohr, A. and Bovensiepen, U.
    Journal of Physics Condensed Matter 30 (2018)
    view abstract10.1088/1361-648X/aa9e69
  • Ultrafast Doublon Dynamics in Photoexcited 1T - TaS2
    Ligges, M. and Avigo, I. and GoleŽ, D. and Strand, H.U.R. and Beyazit, Y. and Hanff, K. and Diekmann, F. and Stojchevska, L. and Kalläne, M. and Zhou, P. and Rossnagel, K. and Eckstein, M. and Werner, P. and Bovensiepen, U.
    Physical Review Letters 120 (2018)
    Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T-TaS2, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping /J≈14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.120.166401
  • Analysis of the time-resolved magneto-optical Kerr effect for ultrafast magnetization dynamics in ferromagnetic thin films
    Razdolski, I. and Alekhin, A. and Martens, U. and Bü Rstel, D. and Diesing, D. and Münzenberg, M. and Bovensiepen, U. and Melnikov, A.
    Journal of Physics Condensed Matter 29 (2017)
    We discuss fundamental aspects of laser-induced ultrafast demagnetization probed by the time-resolved magneto-optical Kerr effect (MOKE). Studying thin Fe films on MgO substrate in the absence of electronic transport, we demonstrate how to disentangle pump-induced variations of magnetization and magneto-optical coefficients. We provide a mathematical formalism for retrieving genuine laser-induced magnetization dynamics and discuss its applicability in real experimental situations. We further stress the importance of temporal resolution achieved in the experiments and argue that measurements of both time-resolved MOKE rotation and ellipticity are needed for the correct assessment of magnetization dynamics on sub-picosecond timescales. The framework developed here sheds light onto the details of the time-resolved MOKE technique and contributes to the understanding of the interplay between ultrafast laser-induced optical and magnetic effects. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aa63c6
  • 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 Golden, M.S. and Yang, L.X. and Rossnagel, K. and Bauer, M. and Büchner, B. and Vojta, M. and Wolf, M. and Felser, C. and Fink, J. and Bovensiepen, U.
    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 abstract10.1002/pssb.201600382
  • Femtosecond Spin Current Pulses Generated by the Nonthermal Spin-Dependent Seebeck Effect and Interacting with Ferromagnets in Spin Valves
    Alekhin, A. and Razdolski, I. and Ilin, N. and Meyburg, J.P. and Diesing, D. and Roddatis, V. and Rungger, I. and Stamenova, M. and Sanvito, S. and Bovensiepen, U. and Melnikov, A.
    Physical Review Letters 119 (2017)
    Using the sensitivity of optical second harmonic generation to currents, we demonstrate the generation of 250-fs long spin current pulses in Fe/Au/Fe/MgO(001) spin valves. The temporal profile of these pulses indicates ballistic transport of hot electrons across a sub-100 nm Au layer. The pulse duration is primarily determined by the thermalization time of laser-excited hot carriers in Fe. Considering the calculated spin-dependent Fe/Au interface transmittance we conclude that a nonthermal spin-dependent Seebeck effect is responsible for the generation of ultrashort spin current pulses. The demonstrated rotation of spin polarization of hot electrons upon interaction with noncollinear magnetization at Au/Fe interfaces holds high potential for future spintronic devices. © 2017 American Physical Society.
    view abstract10.1103/PhysRevLett.119.017202
  • Multiple particle-hole pair creation in the harmonically driven Fermi-Hubbard model
    Ten Brinke, N. and Ligges, M. and Bovensiepen, U. and Schützhold, R.
    Physical Review B 95 (2017)
    We study the Fermi-Hubbard model in the strongly correlated Mott phase under the influence of a harmonically oscillating hopping rate J(t)=J0+ΔJcos(ωt). Apart from the well-known fundamental resonance, where the frequency ω of this oscillation equals (or a little exceeds) the Mott gap, we also find higher-order resonances where multiple particle-hole pairs are created when ω is near an integer multiple of the gap. These findings should be relevant for experimental realizations such as ultracold fermionic atoms in optical lattices or pump-probe experiments using laser pulses incident on correlated electrons in solid-state materials. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.95.195123
  • Nanoscale interface confinement of ultrafast spin transfer torque driving non-uniform spin dynamics
    Razdolski, I. and Alekhin, A. and Ilin, N. and Meyburg, J.P. and Roddatis, V. and DIesing, D. and Bovensiepen, U. and Melnikov, A.
    Nature Communications 8 (2017)
    Spintronics had a widespread impact over the past decades due to transferring information by spin rather than electric currents. Its further development requires miniaturization and reduction of characteristic timescales of spin dynamics combining the sub-nanometre spatial and femtosecond temporal ranges. These demands shift the focus of interest towards the fundamental open question of the interaction of femtosecond spin current (SC) pulses with a ferromagnet (FM). The spatio-temporal properties of the impulsive spin transfer torque exerted by ultrashort SC pulses on the FM open the time domain for probing non-uniform magnetization dynamics. Here we employ laser-generated ultrashort SC pulses for driving ultrafast spin dynamics in FM and analysing its transient local source. Transverse spins injected into FM excite inhomogeneous high-frequency spin dynamics up to 0.6 THz, indicating that the perturbation of the FM magnetization is confined to 2 nm. © The Author(s) 2017.
    view abstract10.1038/ncomms15007
  • Optically excited structural transition in atomic wires on surfaces at the quantum limit
    Frigge, T. and Hafke, B. and Witte, T. and Krenzer, B. and Streubühr, C. and Samad Syed, A. and Mikšić Trontl, V. and Avigo, I. and Zhou, P. and Ligges, M. and Von Der Linde, D. and Bovensiepen, U. and Horn-Von Hoegen, M. and Wippermann, S. and Lücke, A. and Sanna, S. and Gerstmann, U. and Schmidt, W.G.
    Nature 544 (2017)
    Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
    view abstract10.1038/nature21432
  • Ultrafast inhomogeneous magnetization dynamics analyzed by interface-sensitive nonlinear magneto-optics
    Chen, J. and Wieczorek, J. and Eschenlohr, A. and Xiao, S. and Tarasevitch, A. and Bovensiepen, U.
    Applied Physics Letters 110 (2017)
    We analyze laser-induced ultrafast, spatially inhomogeneous magnetization dynamics of epitaxial Co/Cu(001) films in a 0.4-10 nm thickness range with time-resolved magnetization-induced second harmonic generation, which probes femtosecond spin dynamics at the vacuum/Co and Co/Cu interfaces. The interference of these two contributions makes the overall signal particularly sensitive to differences in the transient magnetization redistribution between the two interfaces, i.e., ultrafast magnetization profiles in the ferromagnetic film. We conclude that the magnetization dynamics within the first several hundred femtoseconds is characteristically dependent on the Co film thickness. In films up to 3 nm thickness, we find a stronger demagnetization at the film surface compared to the Cu/Co interface, which we explain by a spin current from Co into the Cu substrate with an effective mean free path of about 3 nm. For film thicknesses larger than 3 nm, the transient magnetization profile over the Co film reverses its sign since spins can be transferred into the substrate only from the interface near region. Our work emphasizes that spatial inhomogeneities in the dynamic magnetic response to femtosecond laser excitation allow conclusions on the underlying microscopic processes. © 2017 Author(s).
    view abstract10.1063/1.4977767
  • 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 abstract10.1117/12.2239258
  • Analyzing ultrafast laser-induced demagnetization in Co/Cu(001) via the depth sensitivity of the time-resolved transversal magneto-optical Kerr effect
    Eschenlohr, A. and Wieczorek, J. and Chen, J. and Weidtmann, B. and Rösner, M. and Bergeard, N. and Tarasevitch, A. and Wehling, T.O. and Bovensiepen, U.
    Proceedings of SPIE - The International Society for Optical Engineering 9746 (2016)
    Ultrafast demagnetization after femtosecond laser excitation of thin ferromagnetic films has been shown to occur due to a combination of spin-flip scattering in the film and spin transport to a conducting substrate or adjacent layer. Here we demonstrate that the inherent depth sensitivity of the transversal magneto-optical Kerr effect can be employed to derive conclusions on a transient spatial profile in the magnetization in the direction normal to the sample surface. This magnetization profile is qualitatively different for demagnetization caused by spin flips and spin transport. With the help of simulations based on simple phenomenological models we show that spin transport to the substrate in Co/Cu(001) films dominates the demagnetization before the thermalization of the electronic system, i.e. at times < 100 fs, while after approximately 200 fs mainly spin-flip scattering determines the magnetization profile, in agreement with our earlier findings employing the longitudinal magneto-optical Kerr effect. © 2016 SPIE.
    view abstract10.1117/12.2211055
  • Doping dependence and electron-boson coupling in the ultrafast relaxation of hot electron populations in Ba(Fe1-xCox)2As2
    Avigo, I. and Thirupathaiah, S. and Ligges, M. and Wolf, T. and Fink, J. and Bovensiepen, U.
    New Journal of Physics 18 (2016)
    Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in Ba(Fe1-xCox)2As2 in a range of 0 ≤ x < 0.15 at temperatures slightly above the Nel temperature. By analyzing the time-dependent photoemission intensity of the pump laser excited population as a function of energy, we found that the relaxation times at 0 < E - EF < 0.2 eV are doping dependent and about 100 fs shorter at optimal doping than for overdoped and parent compounds. Analysis of the relaxation rates also reveals the presence of a pump fluence dependent step in the relaxation time at E - EF = 200 meV whichwe explain by coupling of the excited electronic system to a boson of this energy.Wecompare our results with static ARPES and transport measurements and find disagreement and agreement concerning the doping dependence, respectively. We discuss the effect of the electron-boson coupling on the energy dependent relaxation and assign the origin of the boson to a magnetic excitation.
    view abstract10.1088/1367-2630/18/9/093028
  • Energy dissipation from a correlated system driven out of equilibrium
    Rameau, J.D. and Freutel, S. and Kemper, A.F. and Sentef, M.A. and Freericks, J.K. and Avigo, I. and Ligges, M. and Rettig, L. and Yoshida, Y. and Eisaki, H. and Schneeloch, J. and Zhong, R.D. and Xu, Z.J. and Gu, G.D. and Johnson, P.D. and Bovensiepen, U.
    Nature Communications 7 (2016)
    In complex materials various interactions have important roles in determining electronic properties. Angle-resolved photoelectron spectroscopy (ARPES) is used to study these processes by resolving the complex single-particle self-energy and quantifying how quantum interactions modify bare electronic states. However, ambiguities in the measurement of the real part of the self-energy and an intrinsic inability to disentangle various contributions to the imaginary part of the self-energy can leave the implications of such measurements open to debate. Here we employ a combined theoretical and experimental treatment of femtosecond time-resolved ARPES (tr-ARPES) show how population dynamics measured using tr-ARPES can be used to separate electron-boson interactions from electron-electron interactions. We demonstrate a quantitative analysis of a well-defined electron-boson interaction in the unoccupied spectrum of the cuprate Bi 2 Sr 2 CaCu 2 O 8+x characterized by an excited population decay time that maps directly to a discrete component of the equilibrium self-energy not readily isolated by static ARPES experiments.
    view abstract10.1038/ncomms13761
  • Persistent order due to transiently enhanced nesting in an electronically excited charge density wave
    Rettig, L. and Cortés, R. and Chu, J.-H. and Fisher, I.R. and Schmitt, F. and Moore, R.G. and Shen, Z.-X. and Kirchmann, P.S. and Wolf, M. and Bovensiepen, U.
    Nature Communications 7 (2016)
    Non-equilibrium conditions may lead to novel properties of materials with broken symmetry ground states not accessible in equilibrium as vividly demonstrated by non-linearly driven mid-infrared active phonon excitation. Potential energy surfaces of electronically excited states also allow to direct nuclear motion, but relaxation of the excess energy typically excites fluctuations leading to a reduced or even vanishing order parameter as characterized by an electronic energy gap. Here, using femtosecond time- and angle-resolved photoemission spectroscopy, we demonstrate a tendency towards transient stabilization of a charge density wave after near-infrared excitation, counteracting the suppression of order in the non-equilibrium state. Analysis of the dynamic electronic structure reveals a remaining energy gap in a highly excited transient state. Our observation can be explained by a competition between fluctuations in the electronically excited state, which tend to reduce order, and transiently enhanced Fermi surface nesting stabilizing the order. © 2016, Nature Publishing Group. All rights reserved.
    view abstract10.1038/ncomms10459
  • Timing photoemission-Final state matters
    Bovensiepen, U. and Ligges, M.
    Science 353 (2016)
    view abstract10.1126/science.aag1090
  • Non-Fermi-liquid scattering rates and anomalous band dispersion in ferropnictides
    Fink, J. and Charnukha, A. and Rienks, E.D.L. and Liu, Z.H. and Thirupathaiah, S. and Avigo, I. and Roth, F. and Jeevan, H.S. and Gegenwart, P. and Roslova, M. and Morozov, I. and Wurmehl, S. and Bovensiepen, U. and Borisenko, S. and Vojta, M. and Büchner, B.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Angle-resolved photoemission spectroscopy is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictide systems. We find the scattering rate for any given band to depend linearly on energy but to be independent of the control parameter. We demonstrate that the linear energy dependence gives rise to a weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. The resulting small effective Fermi energy favors a BCS [J. Bardeen, Phys. Rev. 108, 1175 (1957)PHRVAO0031-899X10.1103/PhysRev.108.1175] -Bose-Einstein [S. N. Bose, Z. Phys. 26, 178 (1924)EPJAFV1434-600110.1007/BF01327326] crossover state in the superconducting phase. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.201106
  • Separation of ultrafast spin currents and spin-flip scattering in Co/Cu(001) driven by femtosecond laser excitation employing the complex magneto-optical Kerr effect
    Wieczorek, J. and Eschenlohr, A. and Weidtmann, B. and Rösner, M. and Bergeard, N. and Tarasevitch, A. and Wehling, T.O. and Bovensiepen, U.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Ultrafast magnetization dynamics in metallic heterostructures consists of a combination of local demagnetization in the ferromagnetic constituent and spin-dependent transport contributions within and in between the constituents. Separation of these local and nonlocal contributions is essential to obtain microscopic understanding and for potential applications of the underlying microscopic processes. By comparing the ultrafast changes of the polarization rotation and ellipticity in the magneto-optical Kerr effect we observe a time-dependent magnetization profile M(z,t) in Co/Cu(001) films by exploiting the effective depth sensitivity of the method. By analyzing the spatiotemporal correlation of these profiles we find that on time scales before hot electron thermalization (<100 fs) the transient magnetization of Co films is governed by spin-dependent transport effects, while after hot electron thermalization (>200 fs) local spin-flip processes dominate. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.174410
  • Ultrafast electron dynamics at water covered alkali adatoms adsorbed on Cu(111)
    Meyer, M. and Agarwal, I. and Wolf, M. and Bovensiepen, U.
    Physical Chemistry Chemical Physics 17 (2015)
    Here we report on the ultrafast electron dynamics of the alkalis Na, K, and Cs coadsorbed with D2O on Cu(111) surfaces, which we investigated with femtosecond time-resolved two-photon photoemission. The well known transient electronic binding energy stabilization in bare adsorbed alkalis is enhanced by the presence of water which acts as a solvent and increases the transient energy gain. We observe for all adsorbed alkalis a transient binding energy stabilization of 100-300 meV. The stabilization rates range from 1 to 2 eV ps-1. Here the heavier alkali exhibits a slower stabilization which we explain by their weaker static alkali-water interaction observed in thermal desorption spectroscopy. The population dynamics at low water coverage is described by a single exponential. With increasing water coverage the behavior becomes non-exponential suggesting an additional excited state due to electron solvation. This journal is © the Owner Societies 2015.
    view abstract10.1039/c4cp05356g
  • Ultrafast laser-excited spin transport in Au/Fe/MgO(001): Relevance of the Fe layer thickness
    Alekhin, A. and Bürstel, D. and Melnikov, A. and Diesing, D. and Bovensiepen, U.
    Springer Proceedings in Physics 159 (2015)
    Propagation dynamics of spin-dependent optical excitations is investigated by back-pump front-probe experiments in Au/Fe/Mg0(001). We observe a decrease for all pump-probe signals detected at the Au surface, if the Fe thickness in increased. Relaxation processes within Fe limit the emission region of ballistic spins at the Fe/Au interface to ∼1 nm. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-07743-7_75
  • Ultrafast non-local spin dynamics in metallic bilayers by linear and non-linear magneto-optics
    Melnikov, A. and Alekhin, A. and Bürstel, D. and Diesing, D. and Wehling, T.O. and Rungger, I. and Stamenova, M. and Sanvito, S. and Bovensiepen, U.
    Springer Proceedings in Physics 159 (2015)
    We make a step towards the understanding of spin dynamics induced by spin-polarized hot carriers in metals. Exciting the Fe layer of Au/Fe/MgO(001) structures with femtosecond laser pulses, we demonstrate the ultrafast spin transport from Fe into Au using time-resolved MOKE and mSHG for depthsensitive detection of the transient magnetization. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-07743-7_12
  • Unoccupied electronic structure and momentum-dependent scattering dynamics in Pb/Si(557) nanowire arrays
    Syed, A.S. and Trontl, V.M. and Ligges, M. and Sakong, S. and Kratzer, P. and Lükermann, D. and Zhou, P. and Avigo, I. and Pfnür, H. and Tegenkamp, C. and Bovensiepen, U.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    The unoccupied electronic structure of quasi-one-dimensional reconstructions of Pb atoms on a Si(557) surface is investigated by means of femtosecond time- and angle-resolved two-photon photoemission. Two distinct unoccupied electronic states are observed at E-EF=3.55 and 3.30 eV, respectively. Density functional theory calculations reveal that these states are spatially located predominantly on the lead wires and that they are energetically degenerated with an energy window of reduced electronic density of states in Si. We further find momentum-averaged lifetimes of 24 and 35 fs of these two states, respectively. The photoemission yield and the population dynamics depend on the electron momentum component perpendicular to the steps of the Si substrate, and the momentum-dependent dynamics cannot be described by means of rate equations. We conclude that momentum- and direction-dependent dephasing of the electronic excitations, likely caused by elastic scattering at the step edges on the vicinal surface, modifies the excited-state population dynamics in this system. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.134301
  • Coherent dynamics of the charge density wave gap in tritellurides
    Rettig, L. and Chu, J.-H. and Fisher, I.R. and Bovensiepen, U. and Wolf, M.
    Faraday Discussions 171 (2014)
    The dynamics of the transient electronic structure in the charge density wave (CDW) system RTe3 (R = rare-earth element) is studied using time- and angle-resolved photoemission spectroscopy (trARPES). Employing a three-pulse pump-probe scheme we investigate the effect of the amplitude mode oscillations on the electronic band structure and, in particular, on the CDW energy gap. We observe coherent oscillations in both lower and upper CDW band with opposite phases, whereby two dominating frequencies are modulating the CDW order parameter. This demonstrates the existence of more than one collective amplitude mode, in contrast to a simple Peierls model. Coherent control experiments of the two amplitude modes, which are strongly coupled in equilibrium, demonstrate independent control of the modes suggesting a decoupling of both modes in the transient photoexcited state. © The Royal Society of Chemistry 2014.
    view abstract10.1039/c4fd00045e
  • Comparing ultrafast surface and bulk heating using time-resolved electron diffraction
    Streubühr, C. and Kalus, A. and Zhou, P. and Ligges, M. and Hanisch-Blicharski, A. and Kammler, M. and Bovensiepen, U. and Horn-Von Hoegen, M. and Von Der Linde, D.
    Applied Physics Letters 104 (2014)
    From measurements of the transient Debye-Waller effect in Bismuth, we determine the buildup time of the random atomic motion resulting from the electronic relaxation after short pulse laser excitation. The surface sensitive reflection high energy electron diffraction and transmission electron diffraction yield a time constant of about 12 ps and 3 ps, respectively. The different energy transfer rates indicate relatively weak coupling between bulk and surface vibrational modes. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4872055
  • Electron-phonon coupling in quantum-well states of the Pb/Si(1 1 1) system
    Ligges, M. and Sandhofer, M. and Sklyadneva, I. and Heid, R. and Bohnen, K.-P. and Freutel, S. and Rettig, L. and Zhou, P. and Echenique, P.M. and Chulkov, E.V. and Bovensiepen, U.
    Journal of Physics Condensed Matter 26 (2014)
    The electron-phonon coupling parameters in the vicinity of the γ̄ point, λ(γ̄), for electronic quantum well states in epitaxial lead films on a Si(1 1 1) substrate are measured using 5, 7 and 12ML films and femtosecond laser photoemission spectroscopy. The λ (γ̄) values in the range of 0.6-0.9 were obtained by temperature-dependent line width analysis of occupied quantum well states and found to be considerably smaller than the momentum averaged electron-phonon coupling at the Fermi level of bulk lead, (λ = 1.1-1.7). The results are compared to density functional theory calculations of the lead films with and without interfacial stress. It is shown that the discrepancy can not be explained by means of confinement effects or simple structural modifications of the Pb films and, thus, is attributed to the influence of the substrate on the Pb electronic and vibrational structures. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/26/35/352001
  • Femtospex: A versatile optical pump-soft x-ray probe facility with 100fs x-ray pulses of variable polarization
    Holldack, K. and Bahrdt, J. and Balzer, A. and Bovensiepen, U. and Brzhezinskaya, M. and Erko, A. and Eschenlohr, A. and Follath, R. and Firsov, A. and Frentrup, W. and Le Guyader, L. and Kachel, T. and Kuske, P. and Mitzner, R. and Müller, R. and Pontius, N. and Quast, T. and Radu, I. and Schmidt, J.-S. and Schüßler-Langeheine, C. and Sperling, M. and Stamm, C. and Trabant, C. and Föhlisch, A.
    Journal of Synchrotron Radiation 21 (2014)
    Here the major upgrades of the femtoslicing facility at BESSYII (Khan et al., 2006) are reviewed, giving a tutorial on how elliptical-polarized ultrashort soft X-ray pulses from electron storage rings are generated at high repetition rates. Employing a 6kHz femtosecond-laser system consisting of two amplifiers that are seeded by one Ti:Sa oscillator, the total average flux of photons of 100fs duration (FWHM) has been increased by a factor of 120 to up to 10 6 photons s -1 (0.1% bandwidth) -1 on the sample in the range from 250 to 1400eV. Thanks to a new beamline design, a factor of 20 enhanced flux and improvements of the stability together with the top-up mode of the accelerator have been achieved. The previously unavoidable problem of increased picosecond-background at higher repetition rates, caused by 'halo' photons, has also been solved by hopping between different 'camshaft' bunches in a dedicated fill pattern ('3+1 camshaft fill') of the storage ring. In addition to an increased X-ray performance at variable (linear and elliptical) polarization, the sample excitation in pump-probe experiments has been considerably extended using an optical parametric amplifier that supports the range from the near-UV to the far-IR regime. Dedicated endstations covering ultrafast magnetism experiments based on time-resolved X-ray circular dichroism have been either upgraded or, in the case of time-resolved resonant soft X-ray diffraction and reflection, newly constructed and adapted to femtoslicing requirements. Experiments at low temperatures down to 6K and magnetic fields up to 0.5T are supported. The FemtoSpeX facility is now operated as a 24h user facility enabling a new class of experiments in ultrafast magnetism and in the field of transient phenomena and phase transitions in solids. © 2014 International Union of Crystallography.
    view abstract10.1107/S1600577514012247
  • Photo-enhanced antinodal conductivity in the pseudogap state of high-T c cuprates
    Cilento, F. and Dal Conte, S. and Coslovich, G. and Peli, S. and Nembrini, N. and Mor, S. and Banfi, F. and Ferrini, G. and Eisaki, H. and Chan, M.K. and Dorow, C.J. and Veit, M.J. and Greven, M. and Van Der Marel, D. and Comin, R. and Damascelli, A. and Rettig, L. and Bovensiepen, U. and Capone, M. and Giannetti, C. and Parmigiani, F.
    Nature Communications 5 (2014)
    A major challenge in understanding the cuprate superconductors is to clarify the nature of the fundamental electronic correlations that lead to the pseudogap phenomenon. Here we use ultrashort light pulses to prepare a non-thermal distribution of excitations and capture novel properties that are hidden at equilibrium. Using a broadband (0.5-2 eV) probe, we are able to track the dynamics of the dielectric function and unveil an anomalous decrease in the scattering rate of the charge carriers in a pseudogap-like region of the temperature (T) and hole-doping (p) phase diagram. In this region, delimited by a well-defined T* neq (p) line, the photoexcitation process triggers the evolution of antinodal excitations from gapped (localized) to delocalized quasiparticles characterized by a longer lifetime. The novel concept of photo-enhanced antinodal conductivity is naturally explained within the single-band Hubbard model, in which the short-range Coulomb repulsion leads to a k-space differentiation between nodal quasiparticles and antinodal excitations. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms5353
  • Photoinduced changes in the cuprate electronic structure revealed by femtosecond time- and angle-resolved photoemission
    Rameau, J.D. and Freutel, S. and Rettig, L. and Avigo, I. and Ligges, M. and Yoshida, Y. and Eisaki, H. and Schneeloch, J. and Zhong, R.D. and Xu, Z.J. and Gu, G.D. and Johnson, P.D. and Bovensiepen, U.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The dressing of quasiparticles in solids is investigated by changes in the electronic structure E(k) driven by femtosecond laser pulses. Employing time- and angle-resolved photoemission on an optimally doped cuprate above Tc, we observe two effects with different characteristic temporal evolutions and, therefore, different microscopic origins. First, a marked change in the effective mass due to the 70-meV kink in E(k) is found to occur during the experiment's 100-fs temporal resolution and is assigned to laser-driven perturbation of an electronic interaction dressing the bare band. Second, a change in kF is explained by effective photodoping due to particle-hole asymmetry and offers opportunities for ultrafast optoelectronic switches based on an optically driven insulator-superconductor transition. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.115115
  • Role of spin-lattice coupling in the ultrafast demagnetization of Gd1-xTbx alloys
    Eschenlohr, A. and Sultan, M. and Melnikov, A. and Bergeard, N. and Wieczorek, J. and Kachel, T. and Stamm, C. and Bovensiepen, U.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    After excitation by femtosecond laser pulses, Gd and Tb exhibit ultrafast demagnetization in two steps, with the time constant of the second step linked to the coupling strength of the 4f magnetic moments to the lattice. In time-resolved magneto-optical Kerr effect measurements of Gd1-xTbx alloys, we observe a decrease in this time constant from 33 to 9 ps with Tb content x increasing from 0 to 0.7. We explain this behavior by the stronger spin-lattice coupling of Tb compared to Gd, which increases the effective spin-lattice coupling in Gd1-xTbx with x. In contrast, the faster time constant of the first demagnetization step exhibits no dependence on x. Additional time- and element-resolved x-ray magnetic circular dichroism measurements show a two-step demagnetization of Gd and Tb in Gd0.6Tb0.4 with an equivalent time scale of the second step but a different magnitude of demagnetization which persists for 15 ps. We explain this by an increased coupling of the Gd 4f magnetic moments to the lattice compared to pure Gd, via interatomic exchange coupling to the neighboring Tb 4f moments mediated by 5d electrons, which has limited efficiency and allows an estimation of a characteristic time scale of the interatomic exchange coupling. We assign the first demagnetization step to the dynamics of the laser-excited 5d electrons, while the second demagnetization step is dominated by the strength of spin-lattice coupling of the 4f electrons. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.214423
  • 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 Kapteyn, H.C. and Kipp, L. and Fink, J. and Bauer, M. and Bovensiepen, U. and Rossnagel, K.
    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 abstract10.1103/PhysRevLett.112.207001
  • 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 (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 abstract10.1016/j.elspec.2014.04.006
  • 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 abstract10.1088/0953-8984/25/9/094003
  • Coupling of spin and vibrational degrees of freedom of adsorbates at metal surfaces probed by vibrational sum-frequency generation
    Öström, H. and Krenz, M. and Radu, I. and Bovensiepen, U. and Wolf, M. and Frischkorn, C.
    Applied Physics Letters 103 (2013)
    Vibrational spectroscopy using sum-frequency generation has been used to investigate the coupling between a ferromagnetic thin film and adsorbed molecules, here CO on Ni/Cu(100). The CO stretching vibration exhibits a strong magnetic contrast with a pronounced temperature dependence, underlining the high sensitivity of this adsorbate-specific spectroscopy method. Our results indicate that the strong temperature dependence is caused by dynamical changes in the surface chemical bond when the CO stretch vibration is coupled to thermally excited external vibrational modes. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4822120
  • Electron-phonon coupling in 122 Fe pnictides analyzed by femtosecond time-resolved photoemission
    Rettig, L. and Cortés, R. and Jeevan, H.S. and Gegenwart, P. and Wolf, T. and Fink, J. and Bovensiepen, U.
    New Journal of Physics 15 (2013)
    Based on the results from femtosecond time-resolved photoemission, we compare three different methods for the determination of the electron-phonon coupling constant λ in Eu- and Ba-based 122 FeAs compounds. We find good agreement between all three methods, which reveal a small λ < 0.2. This makes simple electron-phonon-mediated superconductivity unlikely in these compounds. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/15/8/083023
  • Electronic structure and quantum criticality in Ba(Fe1-x-y CoxMny)2As2, an ARPES study
    Rienks, E.D.L. and Wolf, T. and Koepernik, K. and Avigo, I. and Hlawenka, P. and Lupulescu, C. and Arion, T. and Roth, F. and Eberhardt, W. and Bovensiepen, U. and Fink, J.
    EPL 103 (2013)
    We used angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations to study the electronic structure of Ba(Fe 1-x-yCoxMny)2As2 for x=0.06 and 0 ≥y ≥0.07. From ARPES we derive that the substitution of Fe by Mn does not lead to hole doping, indicating a localization of the induced holes. An evaluation of the measured spectral function does not indicate a diverging effective mass or scattering rate near optimal doping. Thus, the present ARPES results indicate a continuous evolution of the quasiparticle interaction and therefore question previous quantum critical scenarios.
    view abstract10.1209/0295-5075/103/47004
  • Magnetism on Ultrafast Timescales Probed with Femtosecond X-ray Pulses
    Eschenlohr, A. and Bovensiepen, U.
    Synchrotron Radiation News 26 (2013)
    The combination of synchrotron-based soft X-ray light sources and magnetism research has been very fruitful and productive in the last few decades and has led to numerous exciting results. This success is essentially based on resonant dichroic effects, which were employed to analyze magnetic order in an element-specific manner [11, 32]. Thanks to the short wavelength, magneto-dichroic effects could also be employed in microscopy, and within the available momentum transfer even magnetic scattering was developed [11]. Therefore, soft X-ray methods were and still are essential to develop the detailed understanding of magnetism and magnetic materials in the thermodynamic ground state available today. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/08940886.2013.850378
  • Time-resolved Fermi surface mapping of the charge density wave material DyTe3
    Rettig, L. and Cortés, R. and Chu, J.-H. and Fisher, I.R. and Schmitt, F. and Kirchmann, P.S. and Moore, R.G. and Shen, Z.-X. and Wolf, M. and Bovensiepen, U.
    EPJ Web of Conferences 41 (2013)
    The femtosecond dynamics of the Fermi surface of DyTe3 and its band structure are investigated by time- and angle-resolved photoemission spectroscopy. We directly monitor the ultrafast collapse of the charge density wave gap within 200 fs. © Owned by the authors, published by EDP Sciences, 2013.
    view abstract10.1051/epjconf/20134103025
  • Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts
    Zhou, P. and Streubühr, C. and Kalus, A. and Frigge, T. and Wall, S. and Hanisch-Blicharski, A. and Kammler, M. and Ligges, M. and Bovensiepen, U. and Von Der Linde, D. and Horn-Von Hoegen, M.
    EPJ Web of Conferences 41 (2013)
    We present time-resolved RHEED from a laser excited Pb(111) surface using a pulse front tilter for the compensation of the velocity mismatch of electrons and light. The laser pulses with tilted fronts were characterized by a spatially resolving cross correlator. The response of the surface upon excitation was observed to be less than 2 ps. © Owned by the authors, published by EDP Sciences, 2013.
    view abstract10.1051/epjconf/20134110016
  • 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 (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 abstract10.1016/j.susc.2011.12.014
  • Dynamics at Solid State Surfaces and Interfaces
    Bovensiepen, U. and Petek, H. and Wolf, M.
    Dynamics at Solid State Surfaces and Interfaces 2 (2012)
    This two-volume work covers ultrafast structural and electronic dynamics of elementary processes at solid surfaces and interfaces, presenting the current status of photoinduced processes. Providing valuable introductory information for newcomers to this booming field of research, it investigates concepts and experiments, femtosecond and attosecond time-resolved methods, as well as frequency domain techniques. The whole is rounded off by a look at future developments. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved.
    view abstract10.1002/9783527646463
  • Electron- and phonon-mediated ultrafast magnetization dynamics of Gd(0001)
    Sultan, M. and Atxitia, U. and Melnikov, A. and Chubykalo-Fesenko, O. and Bovensiepen, U.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Here we report on the ultrafast magnetization dynamics of Gd(0001), which we investigated as a function of equilibrium temperature by employing the femtosecond time-resolved magneto-optical Kerr effect (MOKE) and modeling by the Landau-Lifshitz-Bloch equation in combination with the two-temperature model. Based on the observed temperature-dependent transient MOKE signals, we separate the magnetization dynamics into two regimes at delays of (i) a few picoseconds and (ii) several 100 femtoseconds. In the picosecond regime, the demagnetization time determined from the experiment increases with temperature from 0.8 ps at 50 K to 1.5 ps at 280 K. A successful description of this observation was achieved by considering the dynamics of the 4f spin system coupled to 5d conduction electrons within two coupling mechanisms: (a) through electronic scattering and (b) spin-flip scattering mediated by phonons. We conclude that at temperatures below the Debye temperature, a hot electron-mediated process describes the experimentally found demagnetization times of ≈0.8 ps well. At higher temperatures phonon-mediated processes have to be included to explain the 2 times longer demagnetization time. In the second regime at time delays of few 100 fs we find an increase in the MOKE rotation and ellipticity at 50 K at delays before demagnetization sets in. Above 50 K the transient changes in rotation and ellipticity are of opposite sign. We explain this behavior by competing magnetic and nonmagnetic contributions in the transient MOKE signals at these delays directly after optical excitation when excited phonons do not yet facilitate angular momentum transfer to the lattice. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.184407
  • Elementary relaxation processes investigated by femtosecond photoelectron spectroscopy of two-dimensional materials
    Bovensiepen, U. and Kirchmann, P.S.
    Laser and Photonics Reviews 6 (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 abstract10.1002/lpor.201000035
  • The Electronic Structure of Solids
    Bovensiepen, U. and Biermann, S. and Perfetti, L.
    Dynamics at Solid State Surfaces and Interfaces 2 (2012)
    view abstract10.1002/9783527646463.ch1
  • Ultrafast dynamics of occupied quantum well states in Pb/Si(111)
    Rettig, L. and Kirchmann, P.S. and Bovensiepen, U.
    New Journal of Physics 14 (2012)
    We investigate the ultrafast electron dynamics of occupied quantum well states (QWSs) in Pb/Si(111) with time-resolved photoemission spectroscopy. We find an ultrafast increase in binding energy of the QWSs driven by the optical excitation, while the electronic system is in a non-equilibrium state. We explain this transient energetic stabilization in the photoexcited state by an ultrafast modification of the Fermi level pinning, triggered by charge transfer across the Pb/Si interface. In addition, we observe the excitation of a coherent surface phonon mode at a frequency of ∼2 THz, which modulates the QWS binding energy. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/14/2/023047
  • Ultrafast momentum-dependent response of electrons in antiferromagnetic EuFe 2As 2 driven by optical excitation
    Rettig, L. and Cortés, R. and Thirupathaiah, S. and Gegenwart, P. and Jeevan, H.S. and Wolf, M. and Fink, J. and Bovensiepen, U.
    Physical Review Letters 108 (2012)
    Employing the momentum sensitivity of time- and angle-resolved photoemission spectroscopy we demonstrate the analysis of ultrafast single- and many-particle dynamics in antiferromagnetic EuFe 2As 2. Their separation is based on a temperature-dependent difference of photoexcited hole and electron relaxation times probing the single-particle band and the spin density wave gap, respectively. Reformation of the magnetic order occurs at 800 fs, which is 4 times slower compared to electron-phonon equilibration due to a smaller spin-dependent relaxation phase space. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.097002
  • Ultrafast quasiparticle dynamics in the heavy-fermion compound YbRh 2Si 2
    Kummer, K. and Vyalikh, D.V. and Rettig, L. and Cortés, R. and Kucherenko, Y. and Krellner, C. and Geibel, C. and Bovensiepen, U. and Wolf, M. and Molodtsov, S.L.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    Understanding strongly correlated rare-earth intermetallic compounds requires knowledge of the nature of the fermionic quasiparticles in states near the Fermi level E F. We report on a pump-probe experiment using femtosecond time- and angle-resolved photoemission spectroscopy to determine lifetimes of hot quasiparticles in the heavy-fermion compound YbRh 2Si 2. An unoccupied band with electronlike dispersion and a band bottom 0.2 eV above E F was identified at Γ̄, in agreement with band structure calculations for the subsurface region. Hot quasiparticle lifetimes from 30 to 80 fs were found for energies between 0.4 and 0.1 eV above E F. These lifetimes generally follow the typical monotonous increase towards E F, in agreement with earlier studies on Yb and Rh elemental metals. However, at normal emission the lifetimes at around 0.2 eV exceed this trend by about +20 fs. This difference decreases with increasing photoemission angle and can be assigned to the particular band that is probed in YbRh 2Si 2. Potential microscopic scenarios are discussed. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.085139
  • Hot-electron-driven enhancement of spin-lattice coupling in Gd and Tb 4f ferromagnets observed by femtosecond x-ray magnetic circular dichroism
    Wietstruk, M. and Melnikov, A. and Stamm, C. and Kachel, T. and Pontius, N. and Sultan, M. and Gahl, C. and Weinelt, M. and Dürr, H.A. and Bovensiepen, U.
    Physical Review Letters 106 (2011)
    Femtosecond x-ray magnetic circular dichroism was used to study the time-dependent magnetic moment of 4f electrons in the ferromagnets Gd and Tb, which are known for their different spin-lattice coupling. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to 50 times compared to equilibrium. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.106.127401
  • Momentum-resolved ultrafast electron dynamics in superconducting Bi 2Sr2CaCu2O8+δ
    Cortés, R. and Rettig, L. and Yoshida, Y. and Eisaki, H. and Wolf, M. and Bovensiepen, U.
    Physical Review Letters 107 (2011)
    The nonequilibrium state of the high-Tc superconductor Bi 2Sr2CaCu2O8+δ and its ultrafast dynamics have been investigated by femtosecond time- and angle-resolved photoemission spectroscopy well below the critical temperature. We probe optically excited quasiparticles at different electron momenta along the Fermi surface and detect metastable quasiparticles near the antinode, since their decay toward the nodal region through e-e scattering is blocked by phase space restrictions. The observed lack of momentum dependence in the decay rates is in agreement with relaxation dynamics dominated by Cooper pair recombination in a boson bottleneck limit. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.107.097002
  • Solvation dynamics of surface-trapped electrons at NH3 and D2O crystallites adsorbed on metals: from femtosecond to minute timescales
    Stähler, J. and Meyer, M. and Bovensiepen, U. and Wolf, M.
    Chemical Science 2 (2011)
    The creation and stabilization of localized, low-energy electrons is investigated in polar molecular environments. We create such excess electrons in excited states in ice and ammonia crystallites adsorbed on metal surfaces and observe their relaxation in real time using time-resolved photoelectron spectroscopy. The observed dynamics proceed up to minute timescales and are therefore slowed down considerably compared to ultrafast excited state relaxation in front of metal surfaces, which proceeds typically on femto- or picosecond time scales. It is the highly efficient wave function constriction of the electrons from the metal that ultimately enables the investigation of the relaxation dynamics over a large range of timescales (up to 17 orders of magnitude). Therefore, it gives novel insight into the solvated electron ground state formation at interfaces. As these long-lived electrons are observed for both, D2O and NH3 crystallites, they appear to be of general character for polar molecule-metal interfaces. Their time- and temperature-dependent relaxation is analyzed for both, crystalline ice and ammonia, and compared using an empirical model that yields insight into the fundamental solvation processes of the respective solvent. © The Royal Society of Chemistry 2011.
    view abstract10.1039/c0sc00644k
  • Ultrafast dynamics at the Na/D2O/Cu(111) interface: Electron solvation in Ice layers and Na+-mediated surface solvation
    Meyer, M. and Bertin, M. and Bovensiepen, U. and Wegkamp, D. and Krenz, M. and Wolf, M.
    Journal of Physical Chemistry C 115 (2011)
    We have studied the influence of sodium ions bound near the ice/vacuum interface on the electron solvation dynamics in amorphous D2O ice layers by means of femtosecond time-resolved two-photon photoelectron spectroscopy. Adsorption of submonolayer coverages of sodium on top of multilayers of amorphous ice leads to the formation of Na+ ions and to pronounced changes in the observed ultrafast dynamics compared to pure amorphous ice. We identify a Na+-induced species of excess electrons which exhibits a much longer lifetime compared to excess electrons in pure D2O ice and approximate the decay of the Na-induced contribution by two decay times τ2 = 880 fs and τ3 = 9.6 ps. In addition, a faster energetic stabilization of the excited electrons with a rate of σ = 0.73 eV/ps is observed. The population of these electrons depends nonlinearly on the sodium coverage. We attribute the Na-induced contribution to a transient electron/ion/water complex which is located at the ice/vacuum interface. This interpretation is corroborated by coverage-dependent measurements and by overlayer experiments. © 2010 American Chemical Society.
    view abstract10.1021/jp107253g
  • Ultrafast electron dynamics in the charge density wave material TbTe 3
    Schmitt, F. and Kirchmann, P.S. and Bovensiepen, U. and Moore, R.G. and Chu, J.-H. and Lu, D.H. and Rettig, L. and Wolf, M. and Fisher, I.R. and Shen, Z.-X.
    New Journal of Physics 13 (2011)
    Gaining insights into the mechanisms of how order and broken symmetry emerges from many-particle interactions is a major challenge in solid state physics. Most experimental techniques-such as angle-resolved photoemission spectroscopy (ARPES)-probe the single-particle excitation spectrum and extract information about the ordering mechanism and collective effects, often indirectly through theory. Time-resolved ARPES (tr-ARPES) makes collective dynamics of a system after optical excitation directly visible through their influence on the quasi-particle band structure. Using this technique, we present a systematic study of TbTe3, a metal that exhibits a charge-density wave (CDW) transition. We discuss time-resolved data taken at different positions in the Brillouin zone (BZ) and at different temperatures. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/13/6/063022
  • Ultrafast magnetization dynamics of Gd(0001): Bulk versus surface
    Sultan, M. and Melnikov, A. and Bovensiepen, U.
    Physica Status Solidi (B) Basic Research 248 (2011)
    Ultrafast laser-induced demagnetization of Gd(0001) has been investigated by magneto-induced optical second harmonic generation and the magneto-optical Kerr effect which facilitate a comparison of surface and bulk dynamics. We observe pronounced differences in the transient changes of the surface and bulk sensitive magneto-optical signals which we attribute to transfer of optically excited, spin-polarized carriers between surface and bulk states of the Gd(0001) film. A fluence dependent analysis of the bulk magnetization dynamics results in a weak variation of the demagnetization time constant, which starts at about 0.7ps and increases by 10% within a fluence variation up to 1mJ/cm2. We compare these results with fluence dependent changes in the transient energy density calculated by the two temperature model. The determined characteristic times of excess energy transfer from the electron system to the lattice, which is mediated by e-ph scattering, range from 0.2 to 0.6ps. Such a more pronounced fluence dependent change in the characteristic time compared to the observed rather weakly varying demagnetization times suggests a more advanced description of the optically excited state than by the two-temperature model. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201147105
  • Ultrafast transport of laser-excited spin-polarized carriers in Au/Fe/MgO(001)
    Melnikov, A. and Razdolski, I. and Wehling, T.O. and Papaioannou, E.Th. and Roddatis, V. and Fumagalli, P. and Aktsipetrov, O. and Lichtenstein, A.I. and Bovensiepen, U.
    Physical Review Letters 107 (2011)
    Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.107.076601
  • Coherent Excitations at Ferromagnetic Gd(0001) and Tb(0001) Surfaces
    Melnikov, A. and Bovensiepen, U.
    Dynamics at Solid State Surfaces and Interfaces 1 (2010)
    view abstract10.1002/9783527633418.ch12
  • Electron Dynamics at Polar Molecule-Metal Interfaces: Competition between Localization, Solvation, and Transfer
    Sťahler, J. and Bovensiepen, U. and Wolf, M.
    Dynamics at Solid State Surfaces and Interfaces 1 (2010)
    view abstract10.1002/9783527633418.ch16
  • Femtosecond Time- and Angle-Resolved Photoemission as a Real-time Probe of Cooperative Effects in Correlated Electron Materials
    Kirchmann, P.S. and Perfetti, L. and Wolf, M. and Bovensiepen, U.
    Dynamics at Solid State Surfaces and Interfaces 1 (2010)
    view abstract10.1002/9783527633418.ch20
  • Quasiparticle lifetimes in metallic quantum-well nanostructures
    Kirchmann, P.S. and Rettig, L. and Zubizarreta, X. and Silkin, V.M. and Chulkov, E.V. and Bovensiepen, U.
    Nature Physics 6 (2010)
    Quasiparticle lifetimes in metals as described by Fermi-liquid theory are essential in surface chemistry and determine the mean free path of hot carriers. Relaxation of hot electrons is governed by inelastic electron-electron scattering, which occurs on femtosecond timescales owing to the large scattering phase space competing with screening effects. Such lifetimes are widely studied by time-resolved two-photon photoemission, which led to understanding of electronic decay at surfaces. In contrast, quasiparticle lifetimes of metal bulk and films are not well understood because electronic transport leads to experimental lifetimes shorter than expected theoretically. Here, we lift this discrepancy by investigating Pb quantum-well structures on Si(111), a two-dimensional model system. For electronic states confined to the film by the Si bandgap we find quantitative agreement with Fermi-liquid theory and ab initio calculations for bulk Pb, which we attribute to efficient screening. For states resonant with Si bands, extra decay channels open for electron transfer to Si, resulting in lifetimes shorter than expected for bulk. Thereby we demonstrate that for understanding electronic decay in nanostructures coupling to the environment is essential, and that even for electron confinement to a few ångströms Fermi-liquid theory for bulk can remain valid. © 2010 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/nphys1735
  • Two-beam high-order harmonics from solids: Coupling mechanisms
    Tarasevitch, A. and Wieczorek, J. and Kohn, R. and Bovensiepen, U. and Von Der Linde, D.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 82 (2010)
    The polarization of the two beam (driver-probe) high-order harmonic generation from solids is measured. The experiments, together with computer simulations, allow us to distinguish two different coupling mechanisms of the driver and the probe, resulting in different harmonic efficiencies and spectral slopes. We find that in the nonrelativistic regime the coupling is mostly due to the nonlinear plasma density modulation. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevE.82.056410
  • electron-phonon interactions

  • ferromagnetic materials

  • interfaces

  • lasers

  • magnetism on ultrafast time scales

  • non-equilibrium states of correlated materials

  • nonlinear optics

  • photoelectron spectroscopy

  • spectroscopy

  • spin dynamics

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