Observation of Coulomb gap in the quantum spin Hall candidate single-layer 1T'-WTe$_2$

TL;DR

This study reveals that in single-layer 1T'-WTe2, electron interactions induce a Coulomb gap at the Fermi energy, suppressing bulk conductance and facilitating the observation of topological edge states in a quantum spin Hall candidate.

Contribution

It uncovers a novel electron interaction mechanism that opens a Coulomb gap, aiding the realization of quantum spin Hall effects in materials with semi-metallic bulk bands.

Findings

Coulomb gap pinned at Fermi energy in 1T'-WTe2

Suppressed bulk conductance enhances edge state visibility

Electron interactions induce a Coulomb gap in a semi-metallic system

Abstract

The two-dimensional topological insulators (2DTI) host a full gap in the bulk band, induced by spin-orbit coupling (SOC) effect, together with the topologically protected gapless edge states. However, the SOC-induced gap is usually small, and it is challenging to suppress the bulk conductance and thus to realize the quantum spin Hall (QSH) effect. In this study, we find a novel mechanism to effectively suppress the bulk conductance. By using the quasiparticle interference (QPI) technique with scanning tunneling spectroscopy (STS), we demonstrate that the QSH candidate single-layer 1T'-WTe$_2$ has a semi-metal bulk band structure with no full SOC-induced gap. Surprisingly, in this two-dimensional system, we find the electron interactions open a Coulomb gap which is always pinned at the Fermi energy (E$_F$). The opening of the Coulomb gap can efficiently diminish the bulk state at the E$_F$ and is in favor of the observation of the quantized conduction of topological edge states.