Unusual temperature dependence of the band structure associated with local atomic distortion in monolayer 1T'-WTe2

TL;DR

This study reveals an unusual temperature-dependent band structure in monolayer 1T'-WTe2, linked to local atomic distortions, supporting its 2D topological insulator phase and impacting its transport properties.

Contribution

It demonstrates the temperature-dependent band shifts in monolayer 1T'-WTe2 and attributes these to local atomic distortions, providing new insights into its electronic structure.

Findings

Support for 2D topological insulator phase with inverted band structure

Large downward shift of valence bands upon cooling

Correlation of band shifts with local atomic distortions

Abstract

The ground state of monolayer 1T'-WTe2 has been a target of intensive debate on whether or not it is a two-dimensional topological insulator (2D TI) associated with exciton formation. We investigated the band structure of an epitaxial monolayer 1T'-WTe2 film grown on graphene/SiC(0001) in a wide temperature range of T = 40 - 400 K by angle-resolved photoemission spectroscopy (ARPES). We observed an electron band above the Fermi level (EF) slightly away from the {\Gamma} point, together with four hole bands below EF just at the {\Gamma} point. This signifies an indirect band gap exceeding 0.1 eV in support of the 2D-TI phase with the inverted band structure. We uncovered an unexpectedly large downward shift of valence bands upon cooling, accompanied with an upward shift of the conduction band. Comparison of the ARPES-derived band structure with first-principles band calculations suggests that the observed band shift is ascribed to the systematic local atomic distortion of tungsten atoms, which should be incorporated into the interpretation of unusual transport properties of 1T'-WTe2.