TY  - JOUR
A1  - Clark, Oliver J.
A1  - Freyse, Friedrich
A1  - Yashina, L. V.
A1  - Rader, Oliver
A1  - Sanchez-Barriga, Jaime
T1  - Robust behavior and spin-texture stability of the topological surface state in Bi2Se3 upon deposition of gold
JF  - npj quantum materials
N2  - The Dirac point of a topological surface state (TSS) is protected against gapping by time-reversal symmetry. Conventional wisdom stipulates, therefore, that only through magnetisation may a TSS become gapped. However, non-magnetic gaps have now been demonstrated in Bi2Se3 systems doped with Mn or In, explained by hybridisation of the Dirac cone with induced impurity resonances. Recent photoemission experiments suggest that an analogous mechanism applies even when Bi2Se3 is surface dosed with Au. Here, we perform a systematic spin- and angle-resolved photoemission study of Au-dosed Bi2Se3. Although there are experimental conditions wherein the TSS appears gapped due to unfavourable photoemission matrix elements, our photon-energy-dependent spectra unambiguously demonstrate the robustness of the Dirac cone against high Au coverage. We further show how the spin textures of the TSS and its accompanying surface resonances remain qualitatively unchanged following Au deposition, and discuss the mechanism underlying the suppression of the spectral weight.
KW  - Electronic properties and materials
KW  - Topological matter
Y1  - 2022
U6  - https://doi.org/10.1038/s41535-022-00443-9
SN  - 2397-4648
VL  - 7
IS  - 1
PB  - Nature Publishing Group
CY  - London
ER  - 
TY  - JOUR
A1  - Clark, Oliver J.
A1  - Wadgaonkar, Indrajit
A1  - Freyse, Friedrich
A1  - Springholz, Gunther
A1  - Battiato, Marco
A1  - Sanchez-Barriga, Jaime
T1  - Ultrafast thermalization pathways of excited bulk and surface states in the ferroelectric rashba semiconductor GeTe
JF  - Advanced materials
N2  - A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time- and angle-resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe. A complex thermalization pathway is observed, wherein three different timescales can be clearly distinguished: intraband thermalization, interband equilibration, and electronic cooling. These dynamics exhibit an unconventional temperature dependence: while the cooling phase speeds up with increasing sample temperature, the opposite happens for interband thermalization. It is demonstrated how, due to the Rashba effect, an interdependence of these timescales on the relative strength of both electron-electron and electron-phonon interactions is responsible for the counterintuitive temperature dependence, with spin-selection constrained interband electron-electron scatterings found both to dominate dynamics away from the Fermi level, and to weaken with increasing temperature. These findings are supported by theoretical calculations within the Boltzmann approach explicitly showing the opposite behavior of all relevant electron-electron and electron-phonon scattering channels with temperature, thus confirming the microscopic mechanism of the experimental findings. The present results are important for future applications of ferroelectric Rashba semiconductors and their excitations in ultrafast spintronics.
KW  - ferroelectric semiconductors
KW  - Rashba effect
KW  - spin- and angle-resolved photoemission
KW  - spin-orbit coupling
KW  - time-resolved photoemission
KW  - ultrafast dynamics
Y1  - 2022
U6  - https://doi.org/10.1002/adma.202200323
SN  - 0935-9648
SN  - 1521-4095
VL  - 34
IS  - 24
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - GEN
A1  - Varykhalov, Andrei
A1  - Freyse, Friedrich
A1  - Aguilera, Irene
A1  - Battiato, Marco
A1  - Krivenkov, Maxim
A1  - Marchenko, Dmitry
A1  - Bihlmayer, Gustav
A1  - Blugel, Stefan
A1  - Rader, Oliver
A1  - Sanchez-Barriga, Jaime
T1  - Effective mass enhancement and ultrafast electron dynamics of Au(111) surface state coupled to a quantum well
T2  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - We show that, although the equilibrium band dispersion of the Shockley-type surface state of two-dimensional Au(111) quantum films grown on W(110) does not deviate from the expected free-electron-like behavior, its nonequilibrium energy-momentum dispersion probed by time- and angle-resolved photoemission exhibits a remarkable kink above the Fermi level due to a significant enhancement of the effective mass. The kink is pronounced for certain thicknesses of the Au quantum well and vanishes in the very thin limit. We identify the kink as induced by the coupling between the Au(111) surface state and emergent quantum-well states which probe directly the buried gold-tungsten interface. The signatures of the coupling are further revealed by our time-resolved measurements which show that surface state and quantum-well states thermalize together behaving as dynamically locked electron populations. In particular, relaxation of hot carriers following laser excitation is similar for both surface state and quantum-well states and much slower than expected for a bulk metallic system. The influence of quantum confinement on the interplay between elementary scattering processes of the electrons at the surface and ultrafast carrier transport in the direction perpendicular to the surface is shown to be the reason for the slow electron dynamics.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1354 
KW  - AG
KW  - films
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-549892
SN  - 1866-8372
IS  - 1
ER  - 
TY  - JOUR
A1  - Varykhalov, Andrei
A1  - Freyse, Friedrich
A1  - Aguilera, Irene
A1  - Battiato, Marco
A1  - Krivenkov, Maxim
A1  - Marchenko, Dmitry
A1  - Bihlmayer, Gustav
A1  - Blugel, Stefan
A1  - Rader, Oliver
A1  - Sanchez-Barriga, Jaime
T1  - Effective mass enhancement and ultrafast electron dynamics of Au(111) surface state coupled to a quantum well
JF  - Physical Review Research
N2  - We show that, although the equilibrium band dispersion of the Shockley-type surface state of two-dimensional Au(111) quantum films grown on W(110) does not deviate from the expected free-electron-like behavior, its nonequilibrium energy-momentum dispersion probed by time- and angle-resolved photoemission exhibits a remarkable kink above the Fermi level due to a significant enhancement of the effective mass. The kink is pronounced for certain thicknesses of the Au quantum well and vanishes in the very thin limit. We identify the kink as induced by the coupling between the Au(111) surface state and emergent quantum-well states which probe directly the buried gold-tungsten interface. The signatures of the coupling are further revealed by our time-resolved measurements which show that surface state and quantum-well states thermalize together behaving as dynamically locked electron populations. In particular, relaxation of hot carriers following laser excitation is similar for both surface state and quantum-well states and much slower than expected for a bulk metallic system. The influence of quantum confinement on the interplay between elementary scattering processes of the electrons at the surface and ultrafast carrier transport in the direction perpendicular to the surface is shown to be the reason for the slow electron dynamics.
KW  - AG
KW  - Flims
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevResearch.2.013343
SN  - 0031-9007
VL  - 2
IS  - 1
SP  - 1
EP  - 9
PB  - American Physical Society
CY  - Ridge, NY
ER  - 
TY  - JOUR
A1  - Rienks, Emile D. L.
A1  - Wimmer, S.
A1  - Sanchez-Barriga, Jaime
A1  - Caha, O.
A1  - Mandal, Partha Sarathi
A1  - Ruzicka, J.
A1  - Ney, A.
A1  - Steiner, H.
A1  - Volobuev, V. V.
A1  - Groiss, H.
A1  - Albu, M.
A1  - Kothleitner, G.
A1  - Michalicka, J.
A1  - Khan, S. A.
A1  - Minar, J.
A1  - Ebert, H.
A1  - Bauer, G.
A1  - Freyse, Friedrich
A1  - Varykhalov, Andrei
A1  - Rader, Oliver
A1  - Springholz, Gunther
T1  - Large magnetic gap at the Dirac point in Bi2Te3/MnBi2Te4 heterostructures
JF  - Nature : the international weekly journal of science
N2  - Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications(1-8). The edge states are hosted by a magnetic energy gap at the Dirac point(2), but hitherto all attempts to observe this gap directly have been unsuccessful. Observing the gap is considered to be essential to overcoming the limitations of the QAHE, which so far occurs only at temperatures that are one to two orders of magnitude below the ferromagnetic Curie temperature, T-C (ref. (8)). Here we use low-temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below T-C. Surprisingly, our analysis reveals large gap sizes at 1 kelvin of up to 90 millielectronvolts, which is five times larger than theoretically predicted(9). Using multiscale analysis we show that this enhancement is due to a remarkable structure modification induced by Mn doping: instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap(10). Mn-doped Bi2Se3 (ref. (11)) and Mn-doped Sb2Te3 form similar heterostructures, but for Bi2Se3 only a nonmagnetic gap is formed and the magnetization is in the surface plane. This is explained by the smaller spin-orbit interaction by comparison with Mn-doped Bi2Te3. Our findings provide insights that will be crucial in pushing lossless transport in topological insulators towards room-temperature applications.
Y1  - 2019
U6  - https://doi.org/10.1038/s41586-019-1826-7
SN  - 0028-0836
SN  - 1476-4687
VL  - 576
IS  - 7787
SP  - 423
EP  - 428
PB  - Nature Publ. Group
CY  - London
ER  -