NMR RELAXATION IN DISORDERED-SYSTEMS

We investigate for the first time spin-lattice relaxation caused by particles diffusing in disordered environments. For noninteracting particles on a fractal substrate with fractal dimension d(f) and diffusion exponent d(w), we find that the NMR correlation functions G(q)(t) are asymptotically described by G(q)(t) approximately t(-gamma) with gamma = 2/d(w) in d = 2 and gamma = (6 - d(f))/d(w) in d = 3. This behavior leads to an anomalous frequency dependence of 1/T1 at high temperatures, 1/T1 approximately omega-(1-gamma). For charged particles, we have investigated G(q)(t) by Monte Carlo simulations. We find that due to the interplay between structural disorder and Coulomb interaction the decay of G(q)(t) is dramatically slowed down when p(c) is approached.