Středa-like formula in the spin Hall effect

A generalized Středa formula is derived for the spin transport in spin-orbit coupled systems. As compared with the original Středa formula for charge transport, there is an extra contribution of the spin Hall conductance whenever the spin is not conserved. For recently studied systems with quantum spin Hall effect in which the z -component spin is conserved, this extra contribution vanishes and the quantized value of spin Hall conductivity can be reproduced in the present approach. However, as spin is not conserved in general, this extra contribution cannot be neglected, and the quantization is not exact.