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  - Hlawenka, Peter
A1  - Siemensmeyer, Konrad
A1  - Weschke, Eugen
A1  - Varykhalov, Andrei
A1  - Sanchez-Barriga, Jaime
A1  - Shitsevalova, Natalya Y.
A1  - Dukhnenko, A. V.
A1  - Filipov, V. B.
A1  - Gabani, Slavomir
A1  - Flachbart, Karol
A1  - Rader, Oliver
A1  - Rienks, Emile D. L.
T1  - Samarium hexaboride is a trivial surface conductor
JF  - Nature Communications
N2  - SmB6 is predicted to be the first member of the intersection of topological insulators and Kondo insulators, strongly correlated materials in which the Fermi level lies in the gap of a many-body resonance that forms by hybridization between localized and itinerant states. While robust, surface-only conductivity at low temperature and the observation of surface states at the expected high symmetry points appear to confirm this prediction, we find both surface states at the (100) surface to be topologically trivial. We find the (Gamma) over bar state to appear Rashba split and explain the prominent (X) over bar state by a surface shift of the many-body resonance. We propose that the latter mechanism, which applies to several crystal terminations, can explain the unusual surface conductivity. While additional, as yet unobserved topological surface states cannot be excluded, our results show that a firm connection between the two material classes is still outstanding.
Y1  - 2018
U6  - https://doi.org/10.1038/s41467-018-02908-7
SN  - 2041-1723
VL  - 9
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - GEN
A1  - Hlawenka, Peter
A1  - Siemensmeyer, Konrad
A1  - Weschke, Eugen
A1  - Varykhalov, Andrei
A1  - Sánchez-Barriga, Jaime
A1  - Shitsevalova, Natalya Y.
A1  - Dukhnenko, A.V.
A1  - Filipov, V. B.
A1  - Gabáni, Slavomir
A1  - Flachbart, Karol
A1  - Rader, Oliver
A1  - Rienks, Emile D. L.
T1  - Samarium hexaboride is a trivial surface conductor
T2  - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - SmB6 is predicted to be the first member of the intersection of topological insulators and Kondo insulators, strongly correlated materials in which the Fermi level lies in the gap of a many-body resonance that forms by hybridization between localized and itinerant states. While robust, surface-only conductivity at low temperature and the observation of surface states at the expected high symmetry points appear to confirm this prediction, we find both surface states at the (100) surface to be topologically trivial. We find the (Gamma) over bar state to appear Rashba split and explain the prominent (X) over bar state by a surface shift of the many-body resonance. We propose that the latter mechanism, which applies to several crystal terminations, can explain the unusual surface conductivity. While additional, as yet unobserved topological surface states cannot be excluded, our results show that a firm connection between the two material classes is still outstanding.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 612 
KW  - topological Kondo-insulator
KW  - SmB 6
KW  - photoemission
KW  - states
KW  - gap
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-424213
SN  - 1866-8372
IS  - 612
ER  - 
TY  - JOUR
A1  - Krivenkov, Maxim
A1  - Golias, Evangelos
A1  - Marchenko, Dmitry
A1  - Sanchez-Barriga, Jaime
A1  - Bihlmayer, Gustav
A1  - Rader, Oliver
A1  - Varykhalov, Andrei
T1  - Nanostructural origin of giant Rashba effect in intercalated graphene
JF  - 2D Materials
N2  - To enhance the spin-orbit interaction in graphene by a proximity effect without compromising the quasi-free-standing dispersion of the Dirac cones means balancing the opposing demands for strong and weak graphene-substrate interaction. So far, only the intercalation of Au under graphene/Ni(111) has proven successful, which was unexpected since graphene prefers a large separation (similar to 3.3 angstrom) from a Au monolayer in equilibrium. Here, we investigate this system and find the solution in a nanoscale effect. We reveal that the Au largely intercalates as nanoclusters. Our density functional theory calculations show that the graphene is periodically stapled to the Ni substrate, and this attraction presses graphene and Au nanoclusters together. This, in turn, causes a Rashba effect of the giant magnitude observed in experiment. Our findings show that nanopatterning of the substrate can be efficiently used for engineering of spin-orbit effects in graphene.
KW  - quasi-free-standing graphene
KW  - Ni(111)
KW  - gold intercalation
KW  - spin-orbit interaction
KW  - nanoclusters
KW  - STM
KW  - DFT
Y1  - 2017
U6  - https://doi.org/10.1088/2053-1583/aa7ad8
SN  - 2053-1583
VL  - 4
IS  - 3
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Krivenkov, Maxim
A1  - Marchenko, Dimitry
A1  - Sánchez-Barriga, Jaime
A1  - Golias, Evangelos
A1  - Rader, Oliver
A1  - Varykhalov, Andrei
T1  - Origin of the band gap in Bi-intercalated graphene on Ir(111)
JF  - 2D Materials
N2  - Proximity to heavy sp-elements is considered promising for reaching a band gap in graphene that could host quantum spin Hall states. The recent report of an induced spin-orbit gap of 0.2 eV in Pb-intercalated graphene detectable by spin-resolved photoemission has spurred renewed interest in such systems (Klimovskikh et al 2017 ACS Nano 11, 368). In the case of Bi intercalation an even larger band gap of 0.4 eV has been observed but was assigned to the influence of a dislocation network (Warmuth et al 2016 Phys. Rev. B 93, 165 437). Here, we study Bi intercalation under graphene on Ir(111) and report a nearly ideal graphene dispersion without band replicas and no indication of hybridization with the substrate. The band gap is small (0.19 eV) and can be tuned by +/- 25 meV through the Bi coverage. The Bi atomic density is higher than in the recent report. By spin-resolved photoemission we exclude induced spin-orbit interaction as origin of the gap. Quantitative agreement of a photoemission intensity analysis with the measured band gap suggests sublattice symmetry breaking as one of the possible band gap opening mechanisms. We test several Bi structures by density functional theory. Our results indicate the possibility that Bi intercalates in the phase of bismuthene forming a graphene-bismuthene van der Waals heterostructure.
KW  - graphene
KW  - bismuth
KW  - Ir(111)
KW  - spin-orbit interaction
KW  - ARPES
KW  - STM
KW  - bismuthene
Y1  - 2021
U6  - https://doi.org/10.1088/2053-1583/abd1e4
SN  - 2053-1583
VL  - 8
IS  - 3
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - THES
A1  - Rader, Oliver
T1  - Electron quantization and localization in metal films and nanostructures
N2  - Es ist seit einigen Jahren bekannt, dass Elektronen unter bestimmten Bedingungen in dünne Filme eingeschlossen werden können, selbst wenn diese Filme aus Metall bestehen und auf Metall-Substrat aufgebracht werden. In Photoelektronenspektren zeigen diese Filme charakteristische diskrete Energieniveaus, und es hat sich herausgestellt, dass sie zu großen, technisch nutzbaren Effekten führen können, wie der oszillatorischen magnetischen Kopplung in modernen Festplatten-Leseköpfen.  In dieser Arbeit wird untersucht, inwieweit die der Quantisierung in zweidimensionalen Filmen zu Grunde liegenden Konzepte auf niedrigere Dimensionalität übertragbar sind. Das bedeutet, dass schrittweise von zweidimensionalen Filmen auf eindimensionale Nanostrukturen übergegangen wird. Diese Nanostrukturen sind zum einen die Terrassen auf atomar gestuften Oberflächen, aber auch Atomketten, die auf diese Terrassen aufgebracht werden, bis hin zu einer vollständigen Bedeckung mit atomar dünnen Nanostreifen. Daneben werden Selbstorganisationseffekte ausgenutzt, um zu perfekt eindimensionalen Atomanordnungen auf Oberflächen zu gelangen.  Die winkelaufgelöste Photoemission ist als Untersuchungsmethode deshalb so geeignet, weil sie das Verhalten der Elektronen in diesen Nanostrukturen in Abhängigkeit von der Raumrichtung zeigt, und unterscheidet sich darin beispielsweise von der Rastertunnelmikroskopie. Damit ist es möglich, deutliche und manchmal überraschend große Effekte der eindimensionalen Quantisierung bei verschiedenen exemplarischen Systemen zum Teil erstmals nachzuweisen. Die für zweidimensionale Filme wesentliche Rolle von Bandlücken im Substrat wird für Nanostrukturen bestätigt. Hinzu kommt jedoch eine bei zweidimensionalen Filmen nicht vorhandene Ambivalenz zwischen räumlicher Einschränkung der Elektronen in den Nanostrukturen und dem Effekt eines Übergitters aus Nanostrukturen sowie zwischen Effekten des Elektronenverhaltens in der Probe und solchen des Messprozesses. Letztere sind sehr groß und können die Photoemissionsspektren dominieren.  Abschließend wird der Effekt der verminderten Dimensionalität speziell für die d-Elektronen von Mangan untersucht, die zusätzlich starken Wechselwirkungseffekten unterliegen. Auch hierbei treten überraschende Ergebnisse zu Tage.
N2  - It has been known for several years that under certain conditions electrons can be confined within thin layers even if these layers consist of metal and are supported by a metal substrate. In photoelectron spectra, these layers show characteristic discrete energy levels and it has turned out that these lead to large effects like the oscillatory magnetic coupling technically exploited in modern hard disk reading heads.  The current work asks in how far the concepts underlying quantization in two-dimensional films can be transferred to lower dimensionality. This problem is approached by a stepwise transition from two-dimensional layers to one-dimensional nanostructures. On the one hand, these nanostructures are represented by terraces on atomically stepped surfaces, on the other hand by atom chains which are deposited onto these terraces up to complete coverage by atomically thin nanostripes. Furthermore, self organization effects are used in order to arrive at perfectly one-dimensional atomic arrangements at surfaces.  Angle-resolved photoemission is particularly suited as method of investigation because is reveals the behavior of the electrons in these nanostructures in dependence of the spacial direction which distinguishes it from, e. g., scanning tunneling microscopy. With this method intense and at times surprisingly large effects of one-dimensional quantization are observed for various exemplary systems, partly for the first time. The essential role of bandgaps in the substrate known from two-dimensional systems is confirmed for nanostructures. In addition, we reveal an ambiguity without precedent in two-dimensional layers between spacial confinement of electrons on the one side and superlattice effects on the other side as well as between effects caused by the sample and by the measurement process. The latter effects are huge and can dominate the photoelectron spectra.  Finally, the effects of reduced dimensionality are studied in particular for the d electrons of manganese which are additionally affected by strong correlation effects. Surprising results are also obtained here. ---------------------------- Die Links zur jeweiligen Source der im Appendix beigefügten Veröffentlichungen befinden sich auf Seite 83 des Volltextes.
T2  - Electron quantization and localization in metal films and nanostructures
KW  - elektronische Struktur
KW  - elektronische Eigenschaften
KW  - Dispersion
KW  - reduzierte Dimensionalität
KW  - Oberfläche
KW  - Nanostruktur
KW  - Quantendraht
KW  - Terrasse ...
KW  - electronic structure
KW  - electronic properties
KW  - dispersion
KW  - reduced dimensionality
KW  - 1D
KW  - 2D
KW  - surface
KW  - nanostructure
KW  - quantum wire
KW  - terrace ...
Y1  - 2005
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0001912
ER  - 
TY  - JOUR
A1  - Rader, Oliver
A1  - Fauth, K.
A1  - Gould, C.
A1  - Ruster, C.
A1  - Schott, G. M.
A1  - Schmidt, G.
A1  - Brunner, K.
A1  - Molenkamp, Laurens W.
A1  - Schutz, G.
A1  - Kronast, F.
A1  - Durr, H. A.
A1  - Eberhardt, W.
A1  - Gudat, Wolfgang
T1  - Identification of extrinsic Mn contributions in Ga1-xMnxAs by field-dependent magnetic circular X-ray dichroism
N2  - We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field- dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1-xMnxAs and observe different Mn constituents: ferromagnetic Mn with an n(d) > 5 lineshape and paramagnetic Mn with distinct n(d) = 5 lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency. (c) 2005 Elsevier B.V. All rights reserved
Y1  - 2005
SN  - 0368-2048
ER  - 
TY  - JOUR
A1  - Rader, Oliver
A1  - Pampuch, Carsten
A1  - Shikin, A. M.
A1  - Gudat, Wolfgang
A1  - Okabayashi, J.
A1  - Mizokawa, T.
A1  - Fujimori, A.
A1  - Hayashi, T.
A1  - Tanaka, M.
A1  - Tanaka, A.
A1  - Kimura, A.
T1  - Resonant photoemission of Ga1-xMnxAs at the Mn L edge
N2  - Ga1-xMnxAs, x=0.043, has been grown ex situ on GaAs(100) by low-temperature molecular-beam epitaxy. On the reprepared p(1x1) surface, resonant photoemission of the valence band shows a 20-fold enhancement of the Mn 3d contribution at the L-3 edge. The difference spectrum is similar to our previously obtained resonant photoemission at the Mn M edge, in particular a strong satellite appears and no clear Fermi edge ruling out strong Mn 3d weight at the valence-band maximum. The x-ray absorption lineshape differs from previous publications. Our calculation based on a configuration-interaction cluster model reproduces the x-ray absorption and the L-3 on-resonance photoemission spectrum for model parameters Delta, U-dd, and (pdsigma) consistent with our previous work and shows the same spectral shape on and off resonance thus rendering resonant photoemission measured at the L-3 edge representative of the Mn 3d contribution. At the same time, the results are more bulk sensitive due to a probing depth about twice as large as for photoemission at the Mn M edge. The confirmation of our previous results obtained at the M edge calls recent photoemission results into question which report the absence of the satellite and good agreement with local-density theory
Y1  - 2004
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  - 
TY  - JOUR
A1  - Sajedi, Maryam
A1  - Krivenkov, Maxim
A1  - Marchenko, Dmitry
A1  - Sanchez-Barriga, Jaime
A1  - Chandran, Anoop K.
A1  - Varykhalov, Andrei
A1  - Rienks, Emile D. L.
A1  - Aguilera, Irene
A1  - Blügel, Stefan
A1  - Rader, Oliver
T1  - Is there a polaron signature in Angle-Resolved Photoemission of CsPbBr3?
JF  - Physical review letters
N2  - The formation of large polarons has been proposed as reason for the high defect tolerance, low mobility, low charge carrier trapping, and low nonradiative recombination rates of lead halide perovskites. Recently, direct evidence for large-polaron formation has been reported from a 50% effective mass enhancement in angle-resolved photoemission of CsPbBr3 over theory for the orthorhombic structure. We present in-depth band dispersion measurements of CsPbBr3 and GW calculations, which lead to similar effective masses at the valence band maximum of 0.203 1 0.016 m0 in experiment and 0.226 m0 in orthorhombic theory. We argue that the effective mass can be explained solely on the basis of electron-electron correlation and largepolaron formation cannot be concluded from photoemission data.
Y1  - 2022
U6  - https://doi.org/10.1103/PhysRevLett.128.176405
SN  - 0031-9007
SN  - 1079-7114
VL  - 128
IS  - 17
PB  - American Physical Society
CY  - College Park
ER  -