New class of 3D topological insulator in double perovskite

TL;DR

This paper predicts a new class of 3D topological insulators in double perovskites that can achieve large band gaps through effective spin-orbit coupling, expanding the potential for lighter-element TIs.

Contribution

It introduces a novel class of topological insulators in double perovskites with large band gaps, based on a generic lattice model and first-principles calculations.

Findings

Band gap up to 0.55 eV predicted in double perovskites.

Dirac cones are robust against surface dangling bonds.

Model applies to A2BiXO6 compounds with stable topological properties.

Abstract

We predict a new class of three-dimensional topological insulators (TIs) in which the spin-orbit coupling (SOC) can more effectively generate a large band gap at $\Gamma$ point. The band gap of conventional TI such as Bi$_2$Se$_3$ is mainly limited by two factors, the strength of SOC and, from electronic structure perspective, the band gap when SOC is absent. While the former is an atomic property, we find that the latter can be minimized in a generic rock-salt lattice model in which a stable crossing of bands {\it at} the Fermi level along with band character inversion occurs for a range of parameters in the absence of SOC. Thus, large-gap TI's or TI's comprised of lighter elements can be expected. In fact, we find by performing first-principle calculations that the model applies to a class of double perovskites A$_2$BiXO$_6$ (A = Ca, Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, more detailed calculations considering realistic surface structure indicate that the Dirac cones are robust against the presence of dangling bond at the boundary with a specific termination.