Low-temperature nuclear spin-lattice relaxation in glasses - homogeneous and inhomogeneous averaging

A phenomenological model is presented which is able to explain both temperature and magnetic field dependences of nuclear spin-lattice relaxation rates T₁^?1 observed recently in glasses at low temperatures. The model assumes randomly distributed defect centres, typical of glasses, producing internal fluctuating fields. The only restrictive assumption is a power law for the distance dependence of the responsible interaction. In accordance with experiment, homogeneous and inhomogeneous relaxation, corresponding to exponential and non-exponential relaxation behaviour, is predicted in the cases of rapid and slow spin diffusion, respectively. Describing the fluctuating interactions by a BPP-type spectral density and assuming an Arrhenius law for the correlation time, explicit expressions for T_-1^? are obtained. It is shown that by comparison of homogeneous and inhomogeneous relaxation data information on the type of interaction and the density of defect centres can be obtained.