TY - JOUR A1 - Sajedi, Maryam A1 - Krivenkov, Maxim A1 - Marchenko, Dmitry A1 - Varykhalov, Andrei A1 - Sanchez-Barriga, Jaime A1 - Rienks, Emile D. L. A1 - Rader, Oliver T1 - Absence of a giant Rashba effect in the valence band of lead halide perovskites JF - Physical review : B, Condensed matter and materials physics N2 - 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. Y1 - 2020 U6 - https://doi.org/10.1103/PhysRevB.102.081116 SN - 2469-9950 SN - 2469-9969 VL - 102 IS - 8 PB - American Institute of Physics; American Physical Society (APS) CY - Woodbury, NY 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 - 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 - 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, F. A1 - Varykhalov, Andrei A1 - Rader, Oliver A1 - Springholz, G. 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 - Varykhalov, Andrei A1 - Gudat, Wolfgang A1 - Adamchuk, V. K. A1 - Rader, Oliver T1 - Magic numbers in two-dimensional self-organization of C-60 molecules N2 - 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 Y1 - 2006 UR - http://prb.aps.org/pdf/PRB/v73/i24/e241404 U6 - https://doi.org/10.1103/Physrevb.73.241404 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 - JOUR A1 - Shikin, A. M. A1 - Varykhalov, Andrei A1 - Prudnikova, G. V. A1 - Adamchuk, V. K. A1 - Gudat, Wolfgang A1 - Rader, Oliver T1 - Photoemission from stepped W(110) : Initial or final state effect? N2 - 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 Y1 - 2004 SN - 0031-9007 ER -