Graphene-like Dirac states and Quantum Spin Hall Insulators in the square-octagonal MX2 (M=Mo, W; X=S, Se, Te) Isomers

TL;DR

This study investigates square-octagonal MX2 structures, revealing graphene-like Dirac bands and quantum spin Hall phases, thus demonstrating new topological properties in transition metal dichalcogenides.

Contribution

It introduces the square-octagonal MX2 isomers as a new platform exhibiting Dirac states and QSH phases, expanding the understanding of topological phenomena in these materials.

Findings

Graphene-like Dirac bands observed in square-octagonal MX2

Spin-orbit coupling induces nontrivial topological gaps

Realization of quantum spin Hall insulator phases

Abstract

We studied the square-octagonal lattice of the transition metal dichalcogenide MX$_2$ (with $M$=Mo, W; $X$=S, Se and Te), as an isomer of the normal hexagonal compound of MX$_2$. By band structure calculations, we observe the graphene-like Dirac band structure in a rectangular lattice of MX$_2$ with nonsymmorphic space group symmetry. Two bands with van Hove singularity points cross each at the Fermi energy, leading to two Dirac cones that locates at opposite momenta. Spin-orbit coupling can open a nontrivial gap at these Dirac points and induce the quantum spin Hall (QSH) phase, the 2D topological insulator. Here, square-octagonal MX$_2$ structures realize the interesting graphene physics, such as Dirac bands and QSH effect, in the transition metal dichalcogenides.