  We report on the observation of a terahertz radiation induced photon drag
effect in epitaxially grown $n$- and $p$-type
(Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ three dimensional topological insulators
with different antimony concentrations $x$ varying from 0 to 1. We demonstrate
that the excitation with polarized terahertz radiation results in a $dc$
electric photocurrent. While at normal incidence a current arises due to the
photogalvanic effect in the surface states, at oblique incidence it is
outweighed by the trigonal photon drag effect. The developed microscopic model
and theory show that the photon drag photocurrent is due to the dynamical
momentum alignment by time and space dependent radiation electric field and
implies the radiation induced asymmetric scattering in the electron momentum
space.
