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- ItemTwo-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.
- ItemMomentum-resolved superconducting gap in the bulk of Ba1-xK xFe2As2 from combined ARPES and μSR measurements(Milton Park : Taylor & Francis, 2009) Evtushinsky, D.V.; Inosov, D.S.; Zabolotnyy, V.B.; Viazovska, M.S.; Khasanov, R.; Amato, A.; Klauss, H.-H.; Luetkens, H.; Niedermayer, Ch.; Sun, G.L.; Hinkov, V.; Lin, C.T.; Varykhalov, A.; Koitzsch, A.; Knupfer, M.; Büchner, B.; Kordyuk, A.A.; Borisenko, S.V.Here we present a calculation of the temperature-dependent London penetration depth, λ(T), in Ba1-xKxFe 2As2 (BKFA) on the basis of the electronic band structure (Zabolotnyy et al 2009 Nature 457 569, Zabolotnyy et al 2009 Physica C 469 448) and momentum-dependent superconducting gap (Evtushinsky et al 2009 Phys. Rev. B 79 054517) extracted from angleresolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of λ(T) by muon spin rotation (μSR) (Khasanov et al 2009 Phys. Rev. Lett. 102 187005). The value of λ(T = 0), calculated with no adjustable parameters, equals 270 nm, while the directly measured one is 320 nm; the temperature dependence λ(T) is also easily reproduced. Such agreement between the two completely different approaches allows us to conclude that ARPES studies of BKFA are bulk-representative. Our review of the available experimental studies of the superconducting gap in the new ironbased superconductors in general allows us to state that most of them bear two nearly isotropic gaps with coupling constants 2ΔkBTc = 2.5±1.5 and 7±2.