第16回 物理工学科教室談話会(講師:Prof. Dr. Friedhelm Bechstedt)

日時: 平成30年1月9日(火)15:00~16:30
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].

[1] S. Küfner, F.B., et al., Phys. Rev. B 90, 125312 (2014); 89, 195312 (2014); 91, 035311 (2015); 93, 045304 (2016).
[2] L. Matthes, F.B., et al., Phys. Rev. B 93, 121106(R) (2016); 90, 165431 (2014); 94, 085410 (2016).
[3] F. Matusalem, F.B., et al., Phys. Rev. B 94, 241403(R) (2016); Scientific Reports 7, 15700 (2017).
紹介教員:押山 淳 教授、今田 正俊 教授