  When a charge current is injected into the surface state of a topological
insulator (TI), the resulting shift of the spin-momentum-locked Fermi surface
leads to the appearance of a net spin polarization. The helical spin structure
of the Dirac-cone surface state of a TI should lead to a fixed sign of this
spin polarization for a given current direction, but experimentally, both signs
that agree and disagree with the theory expectation for the surface state Dirac
cone have been observed in the past. Although the origin of the wrong sign has
not been conclusively elucidated, this observation points to the possibility
that one may switch the spin polarization at will to realize a spin transistor
operation. Here we report the observation of both signs of spin polarization in
the very same device and demonstrate the tunability between the two by
electrostatic gating, which gives a proof of principle of a topological
insulator spin transistor. This switching behaviour is explained using a
minimal model of competing contributions from the topological surface state and
trivial Rashba-split states.
