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Instruments / Technical Area	Publication Details	Published
I09-Surface and Interface Structural Analysis	Design and realization of topological Dirac fermions on a triangular lattice Maximilian Bauernfeind , Jonas Erhardt , Philipp Eck , Pardeep K. Thakur , Judith Gabel , Tien-Lin Lee , Jörg Schäfer , Simon Moser , Domenico Di Sante , Ralph Claessen , Giorgio Sangiovanni Nature Communications 12 DOI: 10.1038/s41467-021-25627-y Diamond Proposal Number(s): [26419, 25151] Open Access Show Abstract ▼ Abstract: Large-gap quantum spin Hall insulators are promising materials for room-temperature applications based on Dirac fermions. Key to engineer the topologically non-trivial band ordering and sizable band gaps is strong spin-orbit interaction. Following Kane and Mele’s original suggestion, one approach is to synthesize monolayers of heavy atoms with honeycomb coordination accommodated on templates with hexagonal symmetry. Yet, in the majority of cases, this recipe leads to triangular lattices, typically hosting metals or trivial insulators. Here, we conceive and realize “indenene”, a triangular monolayer of indium on SiC exhibiting non-trivial valley physics driven by local spin-orbit coupling, which prevails over inversion-symmetry breaking terms. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice and unveil how a hidden honeycomb connectivity emerges from interference patterns in Bloch px ± ipy-derived wave functions.	Sep 2021
I05-ARPES	Tailoring the topological surface state in ultrathin α -Sn(111) films V. A. Rogalev , F. Reis , F. Adler , M. Bauernfeind , J. Erhardt , A. Kowalewski , M. R. Scholz , L. Dudy , L. B. Duffy , T. Hesjedal , M. Hoesch , G. Bihlmayer , J. Schäfer , R. Claessen Physical Review B 100 DOI: 10.1103/PhysRevB.100.245144 Diamond Proposal Number(s): [10244, 10289, 12892, 15285] Show Abstract ▼ Abstract: We report on the electronic structure of α -Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energy angle-resolved photoemission spectroscopy allows for the direct observation of the linearly dispersing two-dimensional (2D) topological surface state (TSS) that exists between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200 meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin-zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of α -Sn films where the hybridization gap in the TSS coexists with the topologically nontrivial electronic structure and one can expect the presence of a one-dimensional helical edge state.	Dec 2019
I09-Surface and Interface Structural Analysis	Nontrivial topological valence bands of common diamond and zinc-blende semiconductors Tomáš Rauch , Victor A. Rogalev , Maximilian Bauernfeind , Julian Maklar , Felix Reis , Florian Adler , Simon Moser , Johannes Weis , Tien-Lin Lee , Pardeep K. Thakur , Jörg Schäfer , Ralph Claessen , Jürgen Henk , Ingrid Mertig Physical Review Materials 3, 064203 DOI: 10.1103/PhysRevMaterials.3.064203 Diamond Proposal Number(s): [19512] Show Abstract ▼ Abstract: The diamond and zinc-blende semiconductors are well-known and have been widely studied for decades. Yet, their electronic structure still surprises with unexpected topological properties of the valence bands. In this joint theoretical and experimental investigation, we demonstrate for the benchmark compounds InSb and GaAs that the electronic structure features topological surface states below the Fermi energy. Our parity analysis shows that the spin-orbit split-off band near the valence band maximum exhibits a strong topologically nontrivial behavior characterized by the Z 2 invariants ( 1 ; 000 ) . The nontrivial character is a consequence of the nonzero spin-orbit coupling and is imposed by the chosen constituents, in contrast to the conventional topological phase transition mechanism which relies on tuning parameters in the system Hamiltonian. Ab initio-based tight-binding calculations resolve topological surface states in the occupied electronic structure of InSb and GaAs, further confirmed experimentally by soft x-ray angle-resolved photoemission from both materials. Our findings are valid for all other materials whose valence bands are adiabatically linked to those of InSb, i.e., many diamond and zinc-blende semiconductors, as well as other related materials, such as half-Heusler compounds.	Jun 2019

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