Quantum spin Hall edge states and interlayer coupling in twisted-bilayer WTe$_2$

TL;DR

This study investigates the topological properties of twisted bilayer WTe$_2$, revealing tunable quantum spin Hall edge states influenced by twist angle and interlayer interactions, using STM/STS and first principles calculations.

Contribution

It demonstrates the existence of QSH edge states in twisted bilayer WTe$_2$ and shows how their topological nature can be tuned by twist angle and interlayer coupling.

Findings

QSH edge states are observed in twisted bilayer WTe$_2$

Moiré patterns modify local band structure

Topology can be tuned via twist angle and interlayer distance

Abstract

The quantum spin Hall (QSH) effect, characterized by topologically protected spin-polarized edge states, was recently demonstrated in monolayers of the transition metal dichalcogenide (TMD) WTe$_2$. However, the robustness of this topological protection remains largely unexplored in van der Waals heterostructures containing one or more layers of a QSH insulator. In this work, we use scanning tunneling microscopy and spectroscopy (STM/STS) to explore the topological nature of twisted bilayer (tBL) WTe$_2$ which is produce from folded monolayers, as well as, tear-and-stack fabrication. At the tBL bilayer edge, we observe the characteristic spectroscopic signature of the QSH edge state that is absent in topologically trivial as-grown bilayer. For small twist angles, a rectangular moir\'e pattern develops, which results in local modifications of the band structure. Using first principles calculations, we quantify the interactions in tBL WTe$_2$ and its topological edge states as function of interlayer distance and conclude that it is possible to tune the topology of WTe$_2$ bilayers via the twist angle as well as interlayer interactions.