Nuclear spin-lattice relaxation and spin diffusion in the mixed state of type-II layered superconductors

The theory of nuclear-spin diffusion is extended to the case of a nuclear-spin system in the type-II superconductors subjected to the external magnetic field of the intermediate range. We showed that at the beginning of the relaxation process the direct relaxation regime is implemented with nonexponential time dependence. Then the relaxation regime changes to the diffusion one. The analytical expressions are obtained for the dependencies of relaxation rate on the magnetic field value. Experiments performed on the fluorine doped Y-Ba-Cu-O system yield results in agreement with theory, both in respect of the NMR and the vortex models.