Nanoscale Conducting and Insulating Domains on YbB$_6$

Recent photoemission studies on YbB$_6$ reported a metallic surface but without f-states pinned at the Fermi level, in contradiction to the theoretical prediction of YbB$_6$ as a topological Kondo insulator. Thus the topological nature of YbB$_6$ remains unclear and requires a study that can distinguish trivial surface structure and non-trivial topological effects derived from the bulk. We use scanning tunneling microscopy and spectroscopy (STM/STS) to provide a real-space microscopic picture of the surface electronic structure in YbB$_6$. We observe coexisting nanoscale metallic and insulating surface terminations. The surface conductivity of each termination reflects the degree of downward or upward band bending that is determined by the surface polarity. In addition to demonstrating that surface metallically in YbB$_6$ stems from band bending at the polar surface, our study suggests the utility of YbB$_6$ for creating spin-polarized p-n junctions at the atomic scale.