STM imaging of nanoscale conducting and insulating domains with enhanced Rashba Islands in YbB6*

Hexaborides possess a diverse set of exotic properties, ranging from superconductivity to magnetism to non-trivial topology. However, the lack of a natural cleavage plane, as well as the surface-dependent polarity, have obscured their study. YbB6 is a predicted topological insulator, with some experimental evidence for conducting surface states of yet-unproven origin. Using scanning tunneling microscopy (STM) on the cleaved surface of YbB6, we image several coexisting terminations with distinct atomic structures, separated by sharp boundaries. Our spectroscopic measurements show significant band-bending between the terminations, resulting in both conducting and fully-gapped regions, which rule out a topologically-protected surface state. Yet we observe van Hove singularities on two terminations, suggesting the presence of Rashba spin-split quantum well states. We demonstrate the creation of enhanced Rashba islands within nanoscale charging rings using the local gating effect of the STM tip. Our study rules out the possibility that YbB6 is a strong topological insulator, but suggests the utility of YbB6 for creating spin-polarized p-n junctions at the atomic scale.