The spin current is described by a second-rank tensor q ij, where the first index refers to the direction of flow, and the second one to the spin component that is flowing. Thus q xy denotes the flow density of the y -component of spin in the x -direction. Introduce also the vector q i of charge flow density (which is related to the normal current density j = e q), where e is the elementary charge. The coupling between spin and charge currents is due to spin-orbit interaction. It may be described in a very simple way by introducing a single dimensionless coupling parameter ʏ.