場所：工学部6号館3階 372 （セミナー室C）
講師：Prof. Dr. Friedhelm Bechstedt
所属：University of Jena, Germany
題目：Topological surface and interface states from first principles
Topological insulators (TIs) have opened a new fascinating field for solid-state physicists. They are based on small-gap semiconductors with large spin-orbit interaction (SOI). At their surfaces and interfaces metallic edge states with linear bands (Dirac cones) and spin polarization are formed. Two classes of TIs are investigated, (i) the three-dimensional (3D) zero-gap semiconductors -Sn and HgTe with inverted bands, and (ii) two-dimensional (2D) graphene-like honeycomb crystals such as germanene, their chemically functionalized derivatives, and their one-dimensional (1D) nanoribbons. Topological invariants are computed ab initio. Thereby, quasiparticle effects are important for the correct band ordering. The boundary states of -Sn surfaces [1a] and -Sn or HgTe quantum well interfaces formed with CdTe (see Fig. 1) [1b, c, d] are investigated with respect to the appearance of topological states, their localization and spin polarization. We demonstrate that the graphene-like, buckled group-IV-derived crystals with small gap and strong SOI realize the quantum spin Hall (QSH) phase [2a, b, c]. Their ribbons indeed show topological edge states [2b], which however influence the quantization of the spin Hall conductivity [2c]. The conservation of the topological character of the 2D systems is discussed after deposition on passivated or graphene-covered SiC substates [3a, b].
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紹介教員：押山 淳 教授、今田 正俊 教授