Topological insulators with inversion symmetry

TLDR

Topological insulators are materials with a bulk excitation gap induced by spin‑orbit coupling, distinguished from ordinary insulators by Z₂ topological invariants that in two dimensions reduce to a single invariant separating trivial insulators from the quantum spin‑Hall phase, and in three dimensions to four invariants distinguishing ordinary, weak, and strong topological insulators, all of which host robust gapless surface or edge states immune to weak disorder and interactions. The authors aim to demonstrate that inversion symmetry greatly simplifies the evaluation of these Z₂ invariants. They show that the invariants can be obtained from the parity of occupied Bloch wave functions at the time‑reversal invariant points in the Brillouin zone. Using this method, they predict several strong topological insulators, including the Bi₁₋ₓSbₓ alloy, α‑Sn, and HgTe under uniaxial strain.

Abstract

Topological insulators are materials with a bulk excitation gap generated by the spin-orbit interaction that are different from conventional insulators. This distinction is characterized by ${Z}_{2}$ topological invariants, which characterize the ground state. In two dimensions, there is a single ${Z}_{2}$ invariant that distinguishes the ordinary insulator from the quantum spin-Hall phase. In three dimensions, there are four ${Z}_{2}$ invariants that distinguish the ordinary insulator from ``weak'' and ``strong'' topological insulators. These phases are characterized by the presence of gapless surface (or edge) states. In the two-dimensional quantum spin-Hall phase and the three-dimensional strong topological insulator, these states are robust and are insensitive to weak disorder and interactions. In this paper, we show that the presence of inversion symmetry greatly simplifies the problem of evaluating the ${Z}_{2}$ invariants. We show that the invariants can be determined from the knowledge of the parity of the occupied Bloch wave functions at the time-reversal invariant points in the Brillouin zone. Using this approach, we predict a number of specific materials that are strong topological insulators, including the semiconducting alloy ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ as well as $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ and HgTe under uniaxial strain. This paper also includes an expanded discussion of our formulation of the topological insulators in both two and three dimensions, as well as implications for experiments.