Quantum spin Hall effect in twisted bilayer WTe₂ measured with STM

Tungsten telluride is a two-dimensional (2D) material that exhibits a range of properties, depending on the number of layers. Bilayers are reported to be a topologically trivial semimetal that exhibit a ferroelectric effect¹. In the monolayer (ML) limit, WTe₂ has been shown to be a quantum spin Hall (QSH) insulator², resulting in topologically protected edge modes. In this work, we report scanning tunneling microscopy and spectroscopy (STM/STS) studies of twisted bilayers of WTe₂ for a variety of configurations. We find that tunneling spectra of the twisted bilayers resemble that of decoupled monolayers for twist angles ≥5°, and observe edge states to exist at the boundaries of the twisted bilayer regions, in contrast to the case of the trivial bilayer. To study exfoliated WTe₂ in STM, which is air sensitive, we utilize a novel transfer technique³ that enables stacking in an inert gas environment but maintains an atomically pristine surface available for surface probe measurements.

¹Fei, Z. et al. Nature 560, 336–339 (2018)
²Tang, S. et al. Nature Phys 13, 683–687 (2017)
³Lüpke, F. et al. arXiv:1903.00493 (2019)