@article{SajediKrivenkovMarchenkoetal.2020,
  author    = {Sajedi, Maryam and Krivenkov, Maxim and Marchenko, Dmitry and Varykhalov, Andrei and Sanchez-Barriga, Jaime and Rienks, Emile D. L. and Rader, Oliver},
  title     = {Absence of a giant Rashba effect in the valence band of lead halide perovskites},
  series = {Physical review : B, Condensed matter and materials physics},
  volume    = {102},
  journal   = {Physical review : B, Condensed matter and materials physics},
  number    = {8},
  publisher = {American Institute of Physics; American Physical Society (APS)},
  address   = {Woodbury, NY},
  issn      = {2469-9950},
  doi       = {10.1103/PhysRevB.102.081116},
  pages     = {6},
  year      = {2020},
  abstract  = {For hybrid organic-inorganic as well as all-inorganic lead halide perovskites a Rashba effect has been invoked to explain the high efficiency in energy conversion by prohibiting direct recombination. Both a bulk and surface Rashba effect have been predicted. In the valence band of methylammonium (MA) lead bromide a Rashba effect has been reported by angle-resolved photoemission and circular dichroism with giant values of 7-11 eV angstrom. We present band dispersion measurements of MAPbBr(3) and spin-resolved photoemission of CsPbBr3 to show that a large Rashba effect detectable by photoemission or circular dichroism does not exist and cannot be the origin of the high effciency.},
  language  = {en}
}
@misc{VarykhalovFreyseAguileraetal.2020,
  author    = {Varykhalov, Andrei and Freyse, Friedrich and Aguilera, Irene and Battiato, Marco and Krivenkov, Maxim and Marchenko, Dmitry and Bihlmayer, Gustav and Blugel, Stefan and Rader, Oliver and Sanchez-Barriga, Jaime},
  title     = {Effective mass enhancement and ultrafast electron dynamics of Au(111) surface state coupled to a quantum well},
  series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-54989},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-549892},
  pages     = {11},
  year      = {2020},
  abstract  = {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.},
  language  = {en}
}
@article{VarykhalovFreyseAguileraetal.2020,
  author    = {Varykhalov, Andrei and Freyse, Friedrich and Aguilera, Irene and Battiato, Marco and Krivenkov, Maxim and Marchenko, Dmitry and Bihlmayer, Gustav and Blugel, Stefan and Rader, Oliver and Sanchez-Barriga, Jaime},
  title     = {Effective mass enhancement and ultrafast electron dynamics of Au(111) surface state coupled to a quantum well},
  series = {Physical Review Research},
  volume    = {2},
  journal   = {Physical Review Research},
  number    = {1},
  publisher = {American Physical Society},
  address   = {Ridge, NY},
  issn      = {0031-9007},
  doi       = {10.1103/PhysRevResearch.2.013343},
  pages     = {1 -- 9},
  year      = {2020},
  abstract  = {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.},
  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{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, F. and Varykhalov, Andrei and Rader, Oliver and Springholz, G.},
  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{VarykhalovGudatAdamchuketal.2006,
  author    = {Varykhalov, Andrei and Gudat, Wolfgang and Adamchuk, V. K. and Rader, Oliver},
  title     = {Magic numbers in two-dimensional self-organization of C-60 molecules},
  doi       = {10.1103/Physrevb.73.241404},
  year      = {2006},
  abstract  = {Employing the chemically passive carbon reconstruction W(110)/C-R(15x3) as substrate for deposition of C-60 molecules, we have discovered by scanning tunneling microscopy two-dimensional self-assembly of fullerenes into uniform molecular nanoclusters with "magic" numbers. Our photoemission measurements determine van der Waals forces as the dominating interaction in this self-organizing two-dimensional molecular gas. Based on this, a theoretical determination of the cluster structures in the framework of the Girifalco model gives perfect agreement with the experiment},
  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}
}
@article{ShikinVarykhalovPrudnikovaetal.2004,
  author    = {Shikin, A. M. and Varykhalov, Andrei and Prudnikova, G. V. and Adamchuk, V. K. and Gudat, Wolfgang and Rader, Oliver},
  title     = {Photoemission from stepped W(110) : Initial or final state effect?},
  issn      = {0031-9007},
  year      = {2004},
  abstract  = {The electronic structure of the (110)-oriented terraces of stepped W(331) and W(551) is compared to the one of flat W(110) using angle-resolved photoemission. We identify a surface-localized state which develops perpendicular to the steps into a repeated band structure with the periodicity of the step superlattices. It is shown that a final-state diffraction process rather than an initial-state superlattice effect is the origin of the observed behavior and why it does not affect the entire band structure},
  language  = {en}
}