Tomasz Story (26.02, 10:00)
Affiliation: Institute of Physics, Polish Academy of Sciences, Poland Title: Topological states in IV-VI semiconductors Abstract:New class of 3D topological materials - topological crystalline insulators (TCI) – was experimentally discovered by angle- and spin-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) techniques. In contrast to topological materials based on Bi and Sb chalcogenides, in TCIs the topological protection of surface electronic states is warranted not by time reversal but by crystalline (mirror-plane) symmetry [1]. The TCI states were found experimentally in IV-VI narrow-gap semiconductors Pb1-xSnxTe (x=0.3-1) and Pb1-xSnxSe (x=0.18-0.4) for (001) and (111) crystal surfaces of both bulk crystals and epitaxial layers [2-4]. These materials possess direct bulk energy gap at four L-points of the Brillouin zone and undergo a band inversion at specific Sn content and temperature. In the inverted bands regime one observes topological in-gap states with 4 surface Dirac cones. These topological states were also observed for (001) surface of bulk SnTe crystals in high magnetic field studies of Shubnikov - de Haas and de Haas - van Alphen quantum oscillations [5]. In very recent magnetoconductance studies of CdTe/SnTe/CdTe//GaAs (001) layered heterostructures the experimentally observed contribution of weak antilocalisation effect was assigned to topological/trivial SnTe/CdTe interface states [6]. Variety of new proposals involving IV-VI materials relies on layered quantum heterostructures and the effects of strain, hybridization or size quantization. It requires detailed knowledge of the topological states at realistic surfaces (with atomic steps or other defects) as well buried topological interface states. The unique electronic properties of atomic steps were demonstrated [7] with STM/STS technique, identifying 1D channel of high density electronic states along the odd-monolayer-high atomic steps at (001) surface of Pb1-xSnxSe. By in-situ capping the TCI surface of Pb 0.7 Sn 0.3 Se bulk crystal with an ultrathin topologically trivial layer of PbSe ARPES studies were carried out to demonstrate the robustness of Dirac cones at Pb1-xSnxSe/PbSe interface and the important role of atomic steps that may induce a collapse of the pair of split Dirac cones into a single one [8]. Several theoretical proposals will be discussed that still await experimental confirmation: quantum spin Hall effect in IV-VI quantum well structures [9], Dirac or Weyl semimetallic phase [10], ferromagnetic TCI, and higher order topological insulator states [11]. [1] T.H. Hsieh, H. Lin, J. Liu, W. Duan, A. Bansil, L. Fu, Nat. Commun. 3, 982 (2012).
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[7] P. Sessi, D. Di Sante, A. Szczerbakow et al., Science 354, 1269 (2016).
[8] C.M. Polley, R. Buczko, A. Forsman et al., ACS Nano 12, 617 (2018).
[9] S. Safaei, M. Galicka, P. Kacman, R. Buczko, New J. Phys. 17, 063041 (2015).
[10] A. Łusakowski, P. Bogusławski, T. Story, Phys. Rev. B 98 125203 (2018).
[11] F. Schindler, A.M. Cook, M.G. Vergniory et al., Sci. Adv. 4, eaat0346 (2018).