Electric field effects on the band gap and edge states of monolayer 1T'-WTe2

TL;DR

This study investigates how electric fields from gating influence the band gap and edge states in monolayer 1T'-WTe2, revealing significant band structure modifications and spin splitting effects relevant for topological and spintronic applications.

Contribution

The paper provides the first combined STM and first principles analysis of gating effects on monolayer 1T'-WTe2, showing electric field-induced band polarization and spin splitting.

Findings

Gate voltage significantly alters the bulk band gap.

Electric fields induce doping and inversion symmetry breaking.

Calculated spin splitting reaches tens of meV, useful for spintronics.

Abstract

Monolayer 1T'-WTe2 is a quantum spin Hall insulator with a gapped bulk and gapless helical edge states persisting to temperatures around 100 K. Recent studies have revealed a topological-to-trivial phase transition as well the emergence of an unconventional, potentially topological superconducting state upon tuning the carrier concentration with gating. However, despite extensive studies, the effects of gating on the band structure and the helical edge states have not yet been established. In this work we present a combined low-temperature STM and first principles study of back-gated monolayer 1T'-WTe2 films grown on graphene. Consistent with a quantum spin Hall system, the films show well-defined bulk gaps and clear edge states that span the gap. By directly measuring the density of states with STM spectroscopy, we show that the bulk band gap magnitude shows substantial changes with applied gate voltage, which is contrary to the na\"ive expectation that a gate would rigidly shift the bands relative to the Fermi level. To explain our data, we carry out density functional theory and model Hamiltonian calculations which show that a gate electric field causes doping and inversion symmetry breaking which polarizes and spin-splits the bulk bands. Interestingly, the calculated spin splitting from the effective Rashba-like spin-orbit coupling can be in the tens of meV for the electric fields in the experiment, which may be useful for spintronics applications. Our work reveals the strong effect of electric fields on the bulk band structure of monolayer 1T'-WTe2, which will play a critical role in our understanding of gate-induced phenomena in this system.