Evidence of a topological edge state in a Rashba compound

The presence of a gapless edge state within a bulk energy gap, namely the bulk-boundary correspondence, is a hallmark of topology. Using scanning tunneling microscopy, we uncover such an edge state in a Rashba compound containing a Rashba-split surface state. In an atomically resolved surface, we observe a small gap near the Fermi level. On the other hand, a monolayer atomic step edge features an in-gap, gapless edge state. We argue the topological nature of the edge state based on two experimental findings:

(i) an energy gap opens when applying a magnetic field, pointing to a time-reversal symmetry protected nature of the edge state.

(ii) the edge spectra show intriguing differences depending on the height of the step edge, consistent with the hybridization of the edge state.