TY - JOUR A1 - Varykhalov, Andrei A1 - Shikin, A. M. A1 - Gudat, Wolfgang A1 - Moras, P. A1 - Grazioli, C. A1 - Carbone, C. A1 - Rader, Oliver T1 - Probing the ground state electronic structure of a correlated electron system by quantum well states: Ag/ Ni(111) N2 - The ground state electronic properties of the strongly correlated transition metal Ni are usually not accessible from the excitation spectra measured in photoelectron spectroscopy. We show that the bottom of the Ni d band along [111] can be probed through the energy dependence of the phase of quantum-well states in Ag/Ni(111). Our model description of the quantum-well energies measured by angle-resolved photoemission determines the bottom of the Lambda(1) d band of Ni as 2.6 eV, in full agreement with standard local density theory and at variance with the values of 1.7-1.8 eV from direct angle-resolved photoemission experiments of Ni Y1 - 2005 SN - 0031-9007 ER - TY - JOUR A1 - Varykhalov, Andrei A1 - Rader, Oliver A1 - Gudat, Wolfgang T1 - Structure and quantum-size effects in a surface carbide : W(110)/C-R(15 X 3) N2 - Results of the combined investigation of atomic and electronic structure of the W(110)/C-R(15x3) surface carbide are reported. A variety of experimental techniques has been involved such as scanning tunneling microscopy (STM), low-energy electron diffraction, x-ray photoelectron spectroscopy, and angle-resolved photoemission (ARPES). Distance-dependent STM measurements show a nontrivial geometrical behavior in the topography data, demonstrating five different patterns representing the superstructure at different values of the tip-surface separation. Atomic resolution was achieved at lower tunneling gap resistance. An unexpected spatial asymmetry in the distribution of the local density of states across the surface unit cell has been observed as well. Photoelectron spectroscopy of C1s and W4f core levels clarifies the nature of the chemical bonding in the system. The band mapping with ARPES provides information on the wave- vector dependence of the electronic states. Notable quantum size and superlattice effects were discovered in the dispersion of the valence-band states. The experimental data suggests an apparent one-dimensional character of the electronic structure. Lateral quantization and umklapp scattering are proposed as explanation. Finally, based on photoemission and STM measurements, an improved crystallographic model of the tungsten surface carbide is introduced Y1 - 2005 SN - 1098-0121 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 -