Interpreting current-induced spin polarization in topological insulator surface states

Motivated by recent experiments, we address an ambiguity between the intrinsic in-plane spin projection of 2-dimensional surface states on 3-dimensional topological insulators and their ensemble spin polarization induced by a non-equilibrium charge current. Standard expressions from a Boltzmann equation approach in the relaxation-time approximation at zero and finite temperature are used to emphasize our assertion that despite high in-plane spin projection, obtainable currentinduced ensemble spin polarization is minuscule. The only exception is in the limit when chemical potential approaches the charge neutrality Dirac point, when the carrier density itself simultaneously vanishes.