Dr. Manuel Ligges

Ultrafast Phenomena in Solids and at Interfaces
University of Duisburg-Essen


  • 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
  • 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
  • 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
  • Precision Plasmonics with Monomers and Dimers of Spherical Gold Nanoparticles: Nonequilibrium Dynamics at the Time and Space Limits
    Schumacher, L. and Jose, J. and Janoschka, D. and Dreher, P. and Davis, T.J. and Ligges, M. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X. and Meyer Zu Heringdorf, F. and Sokolowski-Tinten, K. and Schlücker, S.
    Journal of Physical Chemistry C 123 (2019)
    Monomers and dimers of spherical gold nanoparticles (NPs) exhibit highly uniform plasmonic properties at the single-particle level due to their high structural homogeneity (precision plasmonics). Recent investigations in precision plasmonics have largely focused on static properties using conventional techniques such as transmission electron microscopy and optical dark-field microscopy. In this Feature Article, we first highlight the application of femtosecond time-resolved electron diffraction for monitoring the nonequilibrium dynamics of spherical gold NPs after ultrafast optical excitation. The analysis of the transient diffraction patterns allows us to directly obtain quantitative information on the incoherent excitation of the lattice, that is, heating upon electron-lattice equilibration, as well as on the development of strain due to lattice expansion on picosecond time scales. The controlled assembly of two spherical gold NPs into a dimer with a few nanometers gap leads to unique optical properties. Specifically, extremely high electric fields (hot spot) in the gap are generated upon resonant optical excitation. Conventional optical microscopy cannot spatially resolve this unique hot spot due to the optical diffraction limit. We therefore employed nonlinear photoemission electron microscopy to visualize hot spots in single dimers of spherical gold NPs. A quantitative comparison of different single dimers confirms the homogeneity of the hot spots on the single-particle level. Overall, these initial results are highly encouraging because they pave the way to investigate nonequilibrium dynamics in highly uniform plasmonic nanostructures at the time and space limits. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b01007
  • 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
  • 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
  • Electron-lattice energy relaxation in laser-excited thin-film Au-insulator heterostructures studied by ultrafast MeV electron diffraction
    Sokolowski-Tinten, K. and Shen, X. and Zheng, Q. and Chase, T. and Coffee, R. and Jerman, M. and Li, R.K. and Ligges, M. and Makasyuk, I. and Mo, M. and Reid, A.H. and Rethfeld, B. and Vecchione, T. and Weathersby, S.P. and Dürr, H.A. and Wang, X.J.
    Structural Dynamics 4 (2017)
    We apply time-resolved MeV electron diffraction to study the electron-lattice energy relaxation in thin film Au-insulator heterostructures. Through precise measurements of the transient Debye-Waller-factor, the mean-square atomic displacement is directly determined, which allows to quantitatively follow the temporal evolution of the lattice temperature after short pulse laser excitation. Data obtained over an extended range of laser fluences reveal an increased relaxation rate when the film thickness is reduced or the Au-film is capped with an additional insulator top-layer. This behavior is attributed to a cross-interfacial coupling of excited electrons in the Au film to phonons in the adjacent insulator layer(s). Analysis of the data using the two-temperature-model taking explicitly into account the additional energy loss at the interface(s) allows to deduce the relative strength of the two relaxation channels. © Author(s) 2017.
    view abstract10.1063/1.4995258
  • 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
  • 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
  • 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
  • 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
  • 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
  • Timing photoemission-Final state matters
    Bovensiepen, U. and Ligges, M.
    Science 353 (2016)
    view abstract10.1126/science.aag1090
  • Thickness-dependent electron-lattice equilibration in laser-excited thin bismuth films
    Sokolowski-Tinten, K. and Li, R.K. and Reid, A.H. and Weathersby, S.P. and Quirin, F. and Chase, T. and Coffee, R. and Corbett, J. and Fry, A. and Hartmann, N. and Hast, C. and Hettel, R. and Horn-Von Hoegen, M. and Janoschka, D. and Lewandowski, J.R. and Ligges, M. and Meyer zu Heringdorf, F.-J. and Shen, X. and Vecchione, T. and Witt, C. and Wu, J. and Dürr, H.A. and Wang, X.J.
    New Journal of Physics 17 (2015)
    Electron-phonon coupling processes determine electronic transport properties of materials and are responsible for the transfer of electronic excess energy to the lattice. With decreasing device dimensions an understanding of these processes in nanoscale materials is becoming increasingly important. Here we use time-resolved electron diffraction to directly study energy relaxation in thin bismuth films after optical excitation. Precise measurements of the transient Debye-Waller-effect for various film thicknesses and over an extended range of excitation fluences allow to separate different contributions to the incoherent lattice response. While phonon softening in the electronically excited state is responsible for an immediate increase of the r.m.s. atomic displacement within a few hundred fs, 'ordinary' electron-phonon coupling leads to subsequent heating of the material on a few ps time-scale. The data reveal distinct changes in the energy transfer dynamics which becomes faster for stronger excitation and smaller film thickness, respectively. The latter effect is attributed to a cross-interfacial coupling of excited electrons to phonons in the substrate. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/17/11/113047
  • 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
  • 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
  • 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
  • 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
  • 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
  • The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain
    Chan, W.-L. and Ligges, M. and Zhu, X.-Y.
    Nature Chemistry 4 (2012)
    One strategy to improve solar-cell efficiency is to generate two excited electrons from just one photon through singlet fission, which is the conversion of a singlet (S 1) into two triplet (T 1) excitons. For efficient singlet fission it is believed that the cumulative energy of the triplet states should be no more than that of S 1. However, molecular analogues that satisfy this energetic requirement do not show appreciable singlet fission, whereas crystalline tetracene displays endothermic singlet fission with near-unity quantum yield. Here we probe singlet fission in tetracene by directly following the intermediate multiexciton (ME) state. The ME state is isoenergetic with 2×T 1, but fission is not activated thermally. Rather, an S 1 ⇔ ME superposition formed through a quantum-coherent process allows access to the higher-energy ME. We attribute entropic gain in crystalline tetracene as the driving force for the subsequent decay of S 1 ⇔ ME into 2×T 1, which leads to a high singlet-fission yield. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/nchem.1436
  • Transient anisotropy in the electron diffraction of femtosecond laser-excited bismuth
    Zhou, P. and Streubühr, C. and Ligges, M. and Brazda, T. and Payer, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M. and Von Der Linde, D.
    New Journal of Physics 14 (2012)
    Laser excitation of thin bismuth films leads to a reduction in the diffraction intensity, which exhibits a characteristic angular anisotropy. The anisotropy depends on the polarization of the laser pulse and persists for approximately 150 ps. The effect clearly indicates coherent atomic motion in a preferential direction that we tentatively attribute to a transient shear deformation due to the photoelastic stress induced by the laser pulse. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/14/10/103031
  • Communication: Momentum-resolved quantum interference in optically excited surface states
    Chan, W.-L. and Tritsch, J. and Dolocan, A. and Ligges, M. and Miaja-Avila, L. and Zhu, X.-Y.
    Journal of Chemical Physics 135 (2011)
    Surface states play essential roles in condensed matter physics, e.g., as model two-dimensional (2D) electron gases and as the basis for topological insulators. Here, we demonstrate quantum interference in the optical excitation of 2D surface states using the model system of C60Au(111). These surface states are transiently populated and probed in a femtosecond time- and angle-resolved two-photon photoemission experiment. We observe quantum interference within the excited populations of these surface states as a function of parallel momentum vector. Such quantum interference in momentum space may allow one to control 2D transport properties by optical fields. © 2011 American Institute of Physics.
    view abstract10.1063/1.3615541
  • Observing the multiexciton state in singlet fission and ensuing ultrafast multielectron transfer
    Chan, W.-L. and Ligges, M. and Jailaubekov, A. and Kaake, L. and Miaja-Avila, L. and Zhu, X.-Y.
    Science 334 (2011)
    Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state.
    view abstract10.1126/science.1213986
  • Transient (000)-order attenuation effects in ultrafast transmission electron diffraction
    Ligges, M. and Rajkovi, I. and Streubhr, C. and Brazda, T. and Zhou, P. and Posth, O. and Hassel, C. and Dumpich, G. and Von Der Linde, D.
    Journal of Applied Physics 109 (2011)
    We discuss the observation of a transient (000)-order attenuation in time-resolved transmission electron diffraction experiments. It is shown that this effect causes a decrease of the diffraction intensity of all higher diffraction orders. This effect is not unique to specific materials as it was observed in thin Au, Ag and Cu films. © 2011 American Institute of Physics.
    view abstract10.1063/1.3554405
  • electron correlations

  • electron phonon couplings

  • electron-phonon interactions

  • low-dimensional systems

  • photoemission spectroscopy

  • quantum theory

  • solids

  • surfaces

  • ultrafast electron dynamics

  • ultrafast electron relaxation

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