  We investigate the band dispersion and the spin texture of topologically
protected surface states in the bulk topological insulators Bi2Se3 and Bi2Te3
by first-principles methods. Strong spin-orbit entanglement in these materials
reduces the spin-polarization of the surface states to ~50% in both cases; this
reduction is absent in simple models but of important implications to
essentially any spintronic application. We propose a way of controlling the
magnitude of spin polarization associated with a charge current in thin films
of topological insulators by means of an external electric field. The proposed
dual-gate device configuration provides new possibilities for electrical
control of spin.
