Topological-Insulator Spin Transistor

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 TI spin transistor. This switching behavior is explained using a minimal model of competing contributions from the topological surface state and trivial Rashba-split states.