Spin conductance, dynamic spin stiffness and spin diffusion in itinerant magnets

We discuss analogies between the chargeand spin response functions of itinerant magnets. We show that the spin-analog of the charge stiffness is not given by the usual spin stiffness ρs, but by the dynamic spin stiffness Ds, which is obtained from the dynamic spin conductance Gs(ω) in the limit of vanishing frequency ω. The low-frequency behavior of Gs(ω) 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 limω→0ReGs(ω). We exploit this fact to develop a new extrapolation scheme for the spin diffusion coefficient in the paramagnetic phase of the Hubbard model. Typeset using REVTEX 1