Dynamical Impurity Spin Susceptibility in Metals

The longitudinal and transverse impurity spin susceptibilities are expressed exactly in terms of holomorphic self-energies. Within the s-d model these self-energies are calculated in lowest order perturbation theory thus yielding reasonable approximations for the susceptibilities valid for all temperatures and magnetic fields above the Kondo regime and in the whole frequency domain including the hydrodynamical one. The transverse susceptibility and the longitudinal nuclear spin susceptibility are found to be satisfactorily described by Bloch's equations. In the low-temperature regime the longitudinal relaxation time T-1 exhibits a nontrivial spin and temperature dependence. At low temperatures and high fields the longitudinal localized electron spin susceptibility is found to show a non-Loremzian double-peak line shape which cannot be described by Bloch's equations.