Ultrafast transient photocarrier dynamics of the bulk-insulating topological insulator Bi(1.5)Sb(0.5)Te(1.7)Se1.3

Abstract

Using optical-pump terahertz-probe spectroscopy, we investigated an ultrafast photocarrier relaxation behavior in a Bi1.5Sb0.5Te1.7Se1.3 (BSTS) single crystal, which is one of the most bulk-insulating topological insulators. Compared to n-type bulk-metallic Bi2Se3, we found that BSTS endows distinct behaviors in its photocarrier dynamics; the relaxation time turns out to be an order of magnitude longer, and the transient conductance spectrum exhibits a nonlinear increase as a function of the pumping power. Also, we observed an abrupt reduction of the photocarrier scattering rate in several picoseconds after the initial photoexcitation. We discuss these intriguing experimental observations based on a bulk-to-surface carrier injection assisted by the built-in electric field near the surface and electron-phonon scattering.