Rock-salt SnS and SnSe: Native Topological Crystalline Insulators

TL;DR

This study predicts that lighter rock-salt SnS and SnSe can act as topological crystalline insulators with surface Dirac cones protected by crystal symmetry, expanding the class of TCIs beyond heavy-element compounds.

Contribution

The paper demonstrates, through first-principles calculations, that SnS and SnSe are intrinsic TCIs with multiple Dirac cones, showing a topological transition influenced by volume expansion.

Findings

SnS and SnSe are topological crystalline insulators with multiple Dirac cones.

SOC is necessary to open the bulk band gap but not to induce topological order.

Volume expansion can drive a transition from TCI to trivial insulator.

Abstract

Unlike time-reversal topological insulators, surface metallic states with Dirac cone dispersion in the recently discovered topological crystalline insulators (TCIs) are protected by crystal symmetry. To date, TCI behaviors have been observed in SnTe and the related alloys Pb$_{1-x}$Sn$_{x}$Se/Te, which incorporate heavy elements with large spin-orbit coupling (SOC). Here, by combining first-principles and {\it ab initio} tight-binding calculations, we report the formation of a TCI in the relatively lighter rock-salt SnS and SnSe. This TCI is characterized by an even number of Dirac cones at the high-symmetry (001), (110) and (111) surfaces, which are protected by the reflection symmetry with respect to the ($\bar{1}$10) mirror plane. We find that both SnS and SnSe have an intrinsically inverted band structure and the SOC is necessary only to open the bulk band gap. The bulk band gap evolution upon volume expansion reveals a topological transition from an ambient pressure TCI to a topologically trivial insulator. Our results indicate that the SOC alone is not sufficient to drive the topological transition.