Emergent photovoltage on SmB 6 surface upon bulk-gap evolution revealed by pump-and-probe photoemission spectroscopy

Recent studies suggest that an exemplary Kondo insulator SmB 6 belongs to a new class of topological insulators (TIs), in which non-trivial spin-polarized metallic states emerge on surface upon the formation of Kondo hybridization gap in the bulk. Remarkably, the bulk resistivity reaches more than 20 Ω cm at 4 K, making SmB 6 a candidate for a so-called bulk-insulating TI. We here investigate optical-pulse responses of SmB 6 by pump-and-probe photoemission spectroscopy. Surface photovoltage effect is observed below ~90 K. This indicates that an optically-active band bending region develops beneath the novel metallic surface upon the bulk-gap evolution. The photovoltaic effect persists for >200 µs, which is long enough to be detected by electronics devices and could be utilized for optical gating of the novel metallic surface.