On the Nature of Spin Currents
  Full expressions for finite frequency spin and charge conductivity in Rashba and Luttinger type systems are given. Whereas in the Rashba Hamiltonian the spin conductivity has the same frequency dependence as the dielectric polarizability, the Luttinger case is different. Moreover, for a generalized Rashba-type coupling the two quantities also exhibit different frequency dependencies. 
B.A. Bernevig