Transfer of Spin Order in Solids. Deterministic and Diffusive Behavior
It is wellknown that the transfer of spin order in solids, normally called “spin diffusion”, is a fully deterministic quantum mechanical process. It can be described as the evolution of a nonequilibrium state under the dipolar interaction. In consequence, time-reversal and spin-echo experiments are feasible, and entropy should remain constant. Nevertheless, due to the complexity of the network of dipolar interactions, an irreversible behavior results in practical situations. This suggests the description by diffusion equations.