Observation of Topologically Protected States at Crystalline Phase Boundaries in Single-layer WSe2

TL;DR

This study demonstrates the existence of topologically protected helical states at phase boundaries in single-layer WSe2, verified through ARPES and STM, offering a platform for exploring boundary state behaviors in 2D materials.

Contribution

It provides experimental evidence of topologically protected states at crystalline phase boundaries in single-layer WSe2, a new quantum spin Hall insulator with well-defined edge states.

Findings

Helical states observed at phase boundaries in WSe2

Band inversion confirmed by ARPES around 120 meV gap

Direct imaging of helical edge states via STM

Abstract

Transition metal dichalcogenide (TMD) materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional (2D) single-layer limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1T'-WSe2. We observe helical states at the crystallographically-aligned interface between quantum a spin Hall insulating domain of 1T'-WSe2 and a semiconducting domain of 1H-WSe2 in contiguous single layers grown using molecular beam epitaxy (MBE). The QSHI nature of single-layer 1T'-WSe2 was verified using ARPES to determine band inversion around a 120 meV energy gap, as well as STM spectroscopy to directly image helical edge-state formation. Using this new edge-state geometry we are able to directly confirm the predicted penetration depth of a helical interface state into the 2D bulk of a QSHI for a well-specified crystallographic direction. The clean, well-ordered topological/trivial interfaces observed here create new opportunities for testing predictions of the microscopic behavior of topologically protected boundary states without the complication of structural disorder.