Nanoscale Conducting and Insulating Domains on YbB₆

Hexaborides possess aexhibit diverse set of exotic properties, including f-state magnetism and, in some cases, non-trivial topology. Hhowever, the lack of a natural neutral cleavage plane, along with surface dependent polarity, has obscured their study. YbB₆ has beenwas predicted to host topological surface states, and indeed conducting surface states have beenwere observed experimentally, but their origin has yet to be determined. We use scanning tunneling microscopy and spectroscopy to provide the first real-space atomically resolved picture of the surface electronic structure in YbB₆. We observe several coexisting nanoscale surface terminations that can be identified by the relative downward or upward band bending determined by the surface polarity. Sharp metal-to-insulating transitions are observed on the surface of YbB₆ along with the onset of a van Hove singularity identifying spin-split Rashba surface states. Our study suggests the utility possibility of engineering YbB₆ for creating spin-polarized p-n junctions at the atomic scale on YbB₆.