Emergent photovoltage on SmB6 surface upon bulk-gap evolution revealed by pump-and-probe photoemission spectroscopy

TL;DR

This study reveals a long-lived photovoltage effect on SmB6's surface linked to bulk-gap evolution, suggesting potential for optical control of its topological surface states.

Contribution

First observation of a persistent surface photovoltage effect on SmB6 related to its bulk-gap formation, indicating optically-active band bending and potential for optical gating.

Findings

Surface photovoltage appears below ~90 K.

Photovoltage persists for over 200 microseconds.

Optical gating of metallic surface states is possible.

Abstract

Recent studies suggest that an exemplary Kondo insulator SmB6 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 Ohm cm at 4 K, making SmB6 a candidate for a so-called bulk-insulating TI. We here investigate optical-pulse responses of SmB6 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 microsec, which is long enough to be detected by electronics devices, and could be utilized for optical gating of the novel metallic surface.