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-s pin system in the type-II super conductors subjected to the external magnet ic held of the intermediate range. We showed that at the beginning of the r elaxation process the direct relaxation regime is implemented with nonexpon ential time dependence. Then the relaxation regime changes to the diffusion one. The analytical expressions are obtained for the dependencies of relax ation rate on the magnetic held value. Experiments performed on the fluorin e doped Y-Ba-Cu-O system yield results in agreement with theory, both in re spect of the NMR and the vortex models.