SPIN CONDUCTANCE, DYNAMIC SPIN STIFFNESS, AND SPIN-DIFFUSION IN ITINERANT MAGNETS

We discuss analogies between the charge and spin response functions of itinerant magnets. We show that the spin analog of the charge stiffne ss is not given by the usual spin stiffness rho(s), but by the dynamic spin stiffness D-s, which is obtained from the dynamic spin conductan ce G(s)(omega) in the limit of vanishing frequency omega. The low-freq uency behavior of G(s)(omega) is used to define ideal spin conductors, normal spin conductors, and spin insulators. Assuming diffusive spin dynamics, we show that the spin-diffusion coefficient is proportional to lim(omega-->0) ReG(s)(omega). We exploit this fact to develop an ex trapolation scheme for the spin-diffusion coefficient in the paramagne tic phase of the Hubbard model.