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Author: Büchner, B. × End date: 2023 × Start date: 2020 × Has files: Yes × Subject: Electronic structure ×

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Now showing 1 - 4 of 4
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    Two-dimensional ferromagnetic extension of a topological insulator
    (College Park, MD : APS, 2023) Kagerer, P.; Fornari, C. I.; Buchberger, S.; Tschirner, T.; Veyrat, L.; Kamp, M.; Tcakaev, A. V.; Zabolotnyy, V.; Morelhão, S. L.; Geldiyev, B.; Müller, S.; Fedorov, A.; Rienks, E.; Gargiani, P.; Valvidares, M.; Folkers, L. C.; Isaeva, A.; Büchner, B.; Hinkov, V.; Claessen, R.; Bentmann, H.; Reinert, F.
    Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a three-dimensional (3D) topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface, as through proximity of a ferromagnetic (FM) layer. However, experimental evidence of such a proximity-induced Dirac mass gap is missing, likely due to an insufficient overlap of TSS and the FM subsystem. Here, we take a different approach, namely ferromagnetic extension (FME), using a thin film of the 3D TI Bi2Te3, interfaced with a monolayer of the lattice-matched van der Waals ferromagnet MnBi2Te4. Robust 2D ferromagnetism with out-of-plane anisotropy and a critical temperature of Tc≈15 K is demonstrated by x-ray magnetic dichroism and electrical transport measurements. Using angle-resolved photoelectron spectroscopy, we observe the opening of a sizable magnetic gap in the 2D FM phase, while the surface remains gapless in the paramagnetic phase above Tc. Ferromagnetic extension paves the way to explore the interplay of strictly 2D magnetism and topological surface states, providing perspectives for realizing robust quantum anomalous Hall and chiral Majorana states.
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    Nonlocal dielectric function and nested dark excitons in MoS2
    (London : Nature Publishing Group, 2019) Koitzsch, A.; Pawlik, A.-S.; Habenicht, C.; Klaproth, T.; Schuster, R.; Büchner, B.; Knupfer, M.
    Their exceptional optical properties are a driving force for the persistent interest in atomically thin transition metal dichalcogenides such as MoS2. The optical response is dominated by excitons. Apart from the bright excitons, which directly couple to light, it has been realized that dark excitons, where photon absorption or emission is inhibited by the spin state or momentum mismatch, are decisive for many optical properties. However, in particular the momentum dependence is difficult to assess experimentally and often remains elusive or is investigated by indirect means. Here we study the momentum dependent electronic structure experimentally and theoretically. We use angle-resolved photoemission as a one-particle probe of the occupied valence band structure and electron energy loss spectroscopy as a two-particle probe of electronic transitions across the gap to benchmark a single-particle model of the dielectric function ϵ(q, ω) against momentum dependent experimental measurements. This ansatz captures key aspects of the data surprisingly well. In particular, the energy region where substantial nesting occurs, which is at the origin of the strong light–matter interaction of thin transition metal dichalcogenides and crucial for the prominent C-exciton, is described well and spans a more complex exciton landscape than previously anticipated. Its local maxima in (q≠0,ω) space can be considered as dark excitons and might be relevant for higher order optical processes. Our study may lead to a more complete understanding of the optical properties of atomically thin transition metal dichalcogenides.
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    Topological Electronic Structure and Intrinsic Magnetization in MnBi4Te7: A Bi2Te3 Derivative with a Periodic Mn Sublattice
    (College Park, MD : American Physical Society, 2019) Vidal, R.C.; Zeugner, A.; Facio, J.I.; Ray, R.; Haghighi, M.H.; Wolter, A.U.B.; Corredor, Bohorquez, L.T.; Caglieris, F.; Moser, S.; Figgemeier, T.; Peixoto, T.R.F.; Vasili, H.B.; Valvidares, M.; Jung, S.; Cacho, C.; Alfonsov, A.; Mehlawat, K.; Kataev, V.; Hess, C.; Richter, M.; Büchner, B.; Van Den Brink, J.; Ruck, M.; Reinert, F.; Bentmann, H.; Isaeva, A.
    Combinations of nontrivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances, material candidates are emerging. Yet, so far a compound that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized. MnBi2Te4 has been established as the first antiferromagnetic topological insulator and constitutes the progenitor of a modular (Bi2Te3)n(MnBi2Te4) series. Here, for n=1, we confirm a nonstoichiometric composition proximate to MnBi4Te7. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topologically nontrivial surface state on the MnBi4Te7(0001) surface, analogous to the nonmagnetic parent compound Bi2Te3. Our results establish MnBi4Te7 as the first band-inverted compound with intrinsic net magnetization providing a versatile platform for the realization of magnetic topological states of matter.
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    Charge-transfer energy in iridates: A hard x-ray photoelectron spectroscopy study
    (College Park, ML : American Physical Society, 2020) Takegami, D.; Kasinathan, D.; Wolff, K.K.; Altendorf, S.G.; Chang, C.F.; Hoefer, K.; Melendez-Sans, A.; Utsumi, Y.; Meneghin, F.; Ha, T.D.; Yen, C.H.; Chen, K.; Kuo, C.Y.; Liao, Y.F.; Tsuei, K.D.; Morrow, R.; Wurmehl, S.; Büchner, B.; Prasad, B.E.; Jansen, M.; Komarek, A.C.; Hansmann, P.; Tjeng, L.H.
    We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir4+ or Ir5+ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight-binding calculations including only the Ir 5d, O 2p, and O 2s orbitals with parameters based on the downfolding of the density-functional band structure results. We found that, regardless of the A and B cations, the A2BIrO6 iridates have essentially zero O 2p to Ir 5d charge-transfer energies. Hence double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.