NONEXPONENTIAL NMR RELAXATIONS IN HETEROGENEOUSLY DOPED SOLIDS

Nuclear-spin diffusion and direct relaxation by electronic spins are competing mechanisms that can lead to nonexponential nuclear-spin relaxation. Usual relaxation-time measurements are not possible in such cases. In the case of strongly heterogeneous distributions of electronic spins, we show that the magnetization is the product of a power law by an exponential at small times. This approach leads to a reliable analysis of experimental data, as we show by comparing spin-dynamics measurements in two doped conducting polymers, poly(bithiophene) and poly(3-methylthiophene).