@article{ClarkFreyseYashinaetal.2022,
  author    = {Clark, Oliver J. and Freyse, Friedrich and Yashina, L. V. and Rader, Oliver and Sanchez-Barriga, Jaime},
  title     = {Robust behavior and spin-texture stability of the topological surface state in Bi2Se3 upon deposition of gold},
  series = {npj quantum materials},
  volume    = {7},
  journal   = {npj quantum materials},
  number    = {1},
  publisher = {Nature Publishing Group},
  address   = {London},
  issn      = {2397-4648},
  doi       = {10.1038/s41535-022-00443-9},
  pages     = {7},
  year      = {2022},
  abstract  = {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.},
  language  = {en}
}
@article{HlawenkaSiemensmeyerWeschkeetal.2018,
  author    = {Hlawenka, Peter and Siemensmeyer, Konrad and Weschke, Eugen and Varykhalov, Andrei and Sanchez-Barriga, Jaime and Shitsevalova, Natalya Y. and Dukhnenko, A. V. and Filipov, V. B. and Gabani, Slavomir and Flachbart, Karol and Rader, Oliver and Rienks, Emile D. L.},
  title     = {Samarium hexaboride is a trivial surface conductor},
  series = {Nature Communications},
  volume    = {9},
  journal   = {Nature Communications},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2041-1723},
  doi       = {10.1038/s41467-018-02908-7},
  pages     = {7},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@misc{HlawenkaSiemensmeyerWeschkeetal.2018,
  author    = {Hlawenka, Peter and Siemensmeyer, Konrad and Weschke, Eugen and Varykhalov, Andrei and S{\´a}nchez-Barriga, Jaime and Shitsevalova, Natalya Y. and Dukhnenko, A.V. and Filipov, V. B. and Gab{\´a}ni, Slavomir and Flachbart, Karol and Rader, Oliver and Rienks, Emile D. L.},
  title     = {Samarium hexaboride is a trivial surface conductor},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {612},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-42421},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-424213},
  pages     = {7},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{KrivenkovGoliasMarchenkoetal.2017,
  author    = {Krivenkov, Maxim and Golias, Evangelos and Marchenko, Dmitry and Sanchez-Barriga, Jaime and Bihlmayer, Gustav and Rader, Oliver and Varykhalov, Andrei},
  title     = {Nanostructural origin of giant Rashba effect in intercalated graphene},
  series = {2D Materials},
  volume    = {4},
  journal   = {2D Materials},
  number    = {3},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {2053-1583},
  doi       = {10.1088/2053-1583/aa7ad8},
  pages     = {11},
  year      = {2017},
  abstract  = {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.},
  language  = {en}
}
@article{KrivenkovMarchenkoSanchezBarrigaetal.2021,
  author    = {Krivenkov, Maxim and Marchenko, Dimitry and S{\´a}nchez-Barriga, Jaime and Golias, Evangelos and Rader, Oliver and Varykhalov, Andrei},
  title     = {Origin of the band gap in Bi-intercalated graphene on Ir(111)},
  series = {2D Materials},
  volume    = {8},
  journal   = {2D Materials},
  number    = {3},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {2053-1583},
  doi       = {10.1088/2053-1583/abd1e4},
  pages     = {15},
  year      = {2021},
  abstract  = {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.},
  language  = {en}
}
@phdthesis{Rader2005,
  author    = {Rader, Oliver},
  title     = {Electron quantization and localization in metal films and nanostructures},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001912},
  school      = {Universit{\"a}t Potsdam},
  year      = {2005},
  abstract  = {Es ist seit einigen Jahren bekannt, dass Elektronen unter bestimmten Bedingungen in d{\"u}nne Filme eingeschlossen werden k{\"o}nnen, 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 f{\"u}hren k{\"o}nnen, wie der oszillatorischen magnetischen Kopplung in modernen Festplatten-Lesek{\"o}pfen. In dieser Arbeit wird untersucht, inwieweit die der Quantisierung in zweidimensionalen Filmen zu Grunde liegenden Konzepte auf niedrigere Dimensionalit{\"a}t {\"u}bertragbar sind. Das bedeutet, dass schrittweise von zweidimensionalen Filmen auf eindimensionale Nanostrukturen {\"u}bergegangen wird. Diese Nanostrukturen sind zum einen die Terrassen auf atomar gestuften Oberfl{\"a}chen, aber auch Atomketten, die auf diese Terrassen aufgebracht werden, bis hin zu einer vollst{\"a}ndigen Bedeckung mit atomar d{\"u}nnen Nanostreifen. Daneben werden Selbstorganisationseffekte ausgenutzt, um zu perfekt eindimensionalen Atomanordnungen auf Oberfl{\"a}chen zu gelangen. Die winkelaufgel{\"o}ste Photoemission ist als Untersuchungsmethode deshalb so geeignet, weil sie das Verhalten der Elektronen in diesen Nanostrukturen in Abh{\"a}ngigkeit von der Raumrichtung zeigt, und unterscheidet sich darin beispielsweise von der Rastertunnelmikroskopie. Damit ist es m{\"o}glich, deutliche und manchmal {\"u}berraschend große Effekte der eindimensionalen Quantisierung bei verschiedenen exemplarischen Systemen zum Teil erstmals nachzuweisen. Die f{\"u}r zweidimensionale Filme wesentliche Rolle von Bandl{\"u}cken im Substrat wird f{\"u}r Nanostrukturen best{\"a}tigt. Hinzu kommt jedoch eine bei zweidimensionalen Filmen nicht vorhandene Ambivalenz zwischen r{\"a}umlicher Einschr{\"a}nkung der Elektronen in den Nanostrukturen und dem Effekt eines {\"U}bergitters aus Nanostrukturen sowie zwischen Effekten des Elektronenverhaltens in der Probe und solchen des Messprozesses. Letztere sind sehr groß und k{\"o}nnen die Photoemissionsspektren dominieren. Abschließend wird der Effekt der verminderten Dimensionalit{\"a}t speziell f{\"u}r die d-Elektronen von Mangan untersucht, die zus{\"a}tzlich starken Wechselwirkungseffekten unterliegen. Auch hierbei treten {\"u}berraschende Ergebnisse zu Tage.},
  language  = {en}
}
@article{RaderFauthGouldetal.2005,
  author    = {Rader, Oliver and Fauth, K. and Gould, C. and Ruster, C. and Schott, G. M. and Schmidt, G. and Brunner, K. and Molenkamp, Laurens W. and Schutz, G. and Kronast, F. and Durr, H. A. and Eberhardt, W. and Gudat, Wolfgang},
  title     = {Identification of extrinsic Mn contributions in Ga1-xMnxAs by field-dependent magnetic circular X-ray dichroism},
  issn      = {0368-2048},
  year      = {2005},
  abstract  = {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},
  language  = {en}
}
@article{RaderPampuchShikinetal.2004,
  author    = {Rader, Oliver and Pampuch, Carsten and Shikin, A. M. and Gudat, Wolfgang and Okabayashi, J. and Mizokawa, T. and Fujimori, A. and Hayashi, T. and Tanaka, M. and Tanaka, A. and Kimura, A.},
  title     = {Resonant photoemission of Ga1-xMnxAs at the Mn L edge},
  year      = {2004},
  abstract  = {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},
  language  = {en}
}
@article{RienksWimmerSanchezBarrigaetal.2019,
  author    = {Rienks, Emile D. L. and Wimmer, S. and Sanchez-Barriga, Jaime and Caha, O. and Mandal, Partha Sarathi and Ruzicka, J. and Ney, A. and Steiner, H. and Volobuev, V. V. and Groiss, H. and Albu, M. and Kothleitner, G. and Michalicka, J. and Khan, S. A. and Minar, J. and Ebert, H. and Bauer, G. and Freyse, Friedrich and Varykhalov, Andrei and Rader, Oliver and Springholz, Gunther},
  title     = {Large magnetic gap at the Dirac point in Bi2Te3/MnBi2Te4 heterostructures},
  series = {Nature : the international weekly journal of science},
  volume    = {576},
  journal   = {Nature : the international weekly journal of science},
  number    = {7787},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {0028-0836},
  doi       = {10.1038/s41586-019-1826-7},
  pages     = {423 -- 428},
  year      = {2019},
  abstract  = {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.},
  language  = {en}
}
@article{SajediKrivenkovMarchenkoetal.2022,
  author    = {Sajedi, Maryam and Krivenkov, Maxim and Marchenko, Dmitry and Sanchez-Barriga, Jaime and Chandran, Anoop K. and Varykhalov, Andrei and Rienks, Emile D. L. and Aguilera, Irene and Bl{\"u}gel, Stefan and Rader, Oliver},
  title     = {Is there a polaron signature in Angle-Resolved Photoemission of CsPbBr3?},
  series = {Physical review letters},
  volume    = {128},
  journal   = {Physical review letters},
  number    = {17},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {0031-9007},
  doi       = {10.1103/PhysRevLett.128.176405},
  pages     = {7},
  year      = {2022},
  abstract  = {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.},
  language  = {en}
}