Formal aspects of spin currents in material media
  The relativistic generalization of the broken continuity equation that describes spin currents is exploited in general material media. The field theoretical techniques for the usual U(1) gauge theory with fermions in material media is employed. The spin current equations are obtained from the study of the symmetries of electrodynamics in material media. Moreover, we obtain the conditions that constrain the spin current to be conserved. One of the novel results is that the analogue spin current for the photon can be also defined. In the latter case, the conditions are useful constraints on the fields and the properties of the media. 
T. R. S. Santos, R. F. Sobreiro, V. J. Vasquez Otoya
hep-th cond-mat.mes-hall