Photoelectromagnetic effect induced by terahertz radiation in (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ topological insulators

The mobility of surface charge carriers is estimated based on an analysis of the photoelectromagnetic effect in three-dimensional (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ (0 $\le x\le$ 0.55) topological insulators. A high degree of degeneracy of the carrier gas in combination with a low energy of the exciting terahertz quantum provide a nonequilibrium process associated exclusively with thermal heating of the carrier. Under these conditions, the photovoltage is determined by the mobility gradient of the surface and bulk carriers. The photovoltage and, consequently, the mobility gradient disappear completely with an increase in the bulk mobility up to 10$^5$ cm$^2$ V$^{-1}$ s$^{-1}$. Photovoltage is clearly observed in the samples with comparatively low bulk mobility under the same experimental conditions.