  Spin-orbit interaction (SOI) in low-dimensional systems results in the
fascinating property of spin-momentum locking. In a Rashba system the inversion
symmetry normal to the plane of a two-dimensional (2D) electron gas is broken,
generating a Fermi surface spin texture reminiscent of spin vortices of
different radii. This can be exploited in a spin-based field-effect transistor
(spin- FET), where the Rashba system forms a 2D channel between ferromagnetic
(FM) source and drain electrodes. The electron spin precesses when propagating
through the Rashba channel and spin orientations (anti)parallel to the drain
give (low) high conductivity. Crucial is the possibility to tune the momentum
splitting, and consequently the precession angle, through an external
parameter. Here we show that this can be achieved in Pb quantum well states
through the doping dependence of the Schottky barrier, opening up the
possibility of a terahertz spin-FET.
