A BPP(Bloembergen-Purcell-Pound) model for nuclear spin relaxation dueto diffusion in disordered systems: combined barrier- andsite-energy disorder

Abstract

Nuclear spin relaxation rates due to magnetic interactions betweendiffusing spins in ordered systems are often interpreted in terms ofan exponential correlation function (the BPP model). The commonextension of this to disordered systems is to average the relaxationrate for a jump rate with energy E over a distribution N(E) ofenergies in the disordered system. A more rigorous extension of theBPP model to disordered systems is described which incorporatessimultaneous barrier- and site-energy distributions. An efficientcomputational scheme is developed to evaluate the triple integralsrequired. Results are presented for the correlation functions andrelaxation rates for some typical values of parameters for both ofthe above extensions of the BPP model to disordered systems. Thereare significant differences between the results for the two models.The model presented here provides a means of analysing experimentalrelaxation data to deduce information about both barrier- andsite-energy distributions.