New Family of Quantum Spin Hall Insulators in Two-dimensional Transition-Metal Halide with Large Nontrivial Band Gaps

TL;DR

This paper predicts a new family of 2D transition-metal halide quantum spin Hall insulators with large nontrivial band gaps, promising for room-temperature spintronics and easier experimental realization.

Contribution

It introduces a novel family of 2D QSH insulators in transition-metal halides with large gaps and a unique d-d band inversion mechanism, expanding the material options for topological devices.

Findings

Large nontrivial gaps of 0.12-0.4 eV in MX monolayers

Some bulk compounds are already synthesized experimentally

The mechanism involves a novel d-d band inversion

Abstract

Topological insulators (TIs) are promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, currently realized 2D TIs, quantum spin Hall (QSH) insulators, suffer from ultra-high vacuum and extremely low temperature. Thus, seeking for desirable QSH insulators with high feasibility of experimental preparation and large nontrivial gap is of great importance for wide applications in spintronics. Based on the first-principles calculations, we predict a novel family of two-dimensional (2D) QSH insulators in transition-metal halide MX (M = Zr, Hf; X = Cl, Br, and I) monolayers with large nontrivial gaps of 0.12$-$0.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV) and larger than ZrTe$_5$ (0.1 eV) monolayers and graphene-based sandwiched heterstructures (30$-$70 meV). Their corresponding 3D bulk materials are weak topological insulators from stacking QSH layers, and some of bulk compounds have already been synthesized in experiment. The mechanism for 2D QSH effect in this system originates from a novel d$-$d band inversion, which is different from conventional band inversion between s$-$s orbitals, or p$-$p orbitals. The realization of pure layered MX monolayers may be prepared by exfoliation from their 3D bulk phases, thus holding great promise for nanoscale device applications and stimulating further efforts on transition metal-based QSH materials.