Observing Topological State Residing at Step Edge of bulk WTe₂

Topological states emerge at the boundary of solids as a consequence of the nontrivial topology of the bulk. In two dimensional topological insulators, the topological edge states support quantum spin Hall effect, where the electrons at the edge of the system possess different spins when propagating along opposite directions. Spectroscopic imaging scanning tunneling microscopy (SISTM) can access the electronic states of the system with high energy and spatial resolution, and thus offers an idea probe to the topological edge states. Recently, theory predicts a topological edge state on single layer transition metal dichalcogenides with 1T’ structure. However, its existence still lacks experimental proof. Here, we report the direct observations of the topological states at the step edge of WTe₂ by SISTM. A one-dimensional electronic state residing at the step edge of WTe₂ is observed, which exhibits remarkable robustness against edge imperfections. First principles calculations rigorously verify the edge state has a topological origin, and its topological nature is unaffected by the presence of the substrate. Our study supports the existence of topological edge states in 1T’-WTe₂, which may envision in-depth study of its topological physics and device applications.