Spin transverse force on spin current in an electric field

As a relativistic quantum mechanical effect, it is shown that the electric field exerting a transverse force on an electron spin 1/2 only if the electron is moving and the spin is polarized along the electric field. The spin force, analogue to the Lorentz for on an electron charge in a magnetic field, is perpendicular to the electric field and the spin current polarized anong the electric field. This spin-dependent force can be used to understand the zitterbewegung of electron wave packet with spin-orbit coupling and is relevant to the generation of the charge Hall effect driven by the spin current in semiconductors.