$C_n$-symmetric higher-order topological crystalline insulators in atomically thin transition-metal dichalcogenides

TL;DR

This paper predicts that atomically thin transition-metal dichalcogenides are higher-order topological insulators with protected corner states, offering a new platform for experimental detection of topological phenomena in 2D materials.

Contribution

It introduces a theoretical prediction of large-gap higher-order topological crystalline insulators in 2H group-VIB TMDs based on first-principles calculations and symmetry analysis.

Findings

Existence of nontrivial topological indices in TMDs.

Presence of quantized fractional charge at corner states.

Corner states are observable in triangular flakes with armchair edges.

Abstract

Based on first-principles calculations and symmetry analysis, we predict atomically thin ($1-N$ layers) 2H group-VIB TMDs $MX_2$ ($M$ = Mo, W; $X$ = S, Se, Te) are large-gap higher-order topological crystalline insulators protected by $C_3$ rotation symmetry. We explicitly demonstrate the nontrivial topological indices and existence of the hallmark corner states with quantized fractional charge for these familiar TMDs with large bulk optical band gaps ($1.64-1.95$ eV for the monolayers), which would facilitate the experimental detection by STM. We find that the well-defined corner states exist in the triangular finite-size flakes with armchair edges of the atomically thin ($1-N$ layers) 2H group-VIB TMDs, and the corresponding quantized fractional charge is the number of layers $N$ divided by 3 modulo integers, which will simply double including spin degree of freedom.