  Spin-orbit interaction is usefully classified as extrinsic or intrinsic
depending on its origin: the potential due to random impurities (extrinsic), or
the crystalline potential associated with the band or device structure
(intrinsic). In this paper we will show how by using a SU(2) formulation the
two sources of spin-orbit interaction may be described in an elegant and
unified way. As a result we obtain a simple description of the interplay of the
two types of spin-orbit interaction, and a physically transparent explanation
of the vanishing of the d.c. spin Hall conductivity in a Rashba two-dimensional
electron gas when spin relaxation is neglected, and its reinstatement when spin
relaxation is allowed. Furthermore, we obtain an explicit formula for the
transverse spin polarization created by an electric current, which generalizes
the standard formula obtained by Edelstein and Aronov and Lyanda-Geller by
including extrinsic spin-orbit interaction and spin relaxation.
