Ergodic theorem for an impurity spin subsystem in a paramagnet

A model for verifying and developing the fundamental ideas underlying the ergodic hypothesis is proposed. The model describes the dynamics of the spin subsystem formed by impurity charges with spin I and a small g factor in a crystal immersed in a strong constant external magnetic field under conditions where the spin system of the nuclei in the crystal is isolated from the other degrees of freedom. The additive integral of motion is the projection of the total spin of the subsystem onto the external field. Attention is focused mainly on the case of I = 1/2. It is shown that the ergodic hypothesis holds if the correlation radius is finite in the initial state and that the ergodic hypothesis is violated if the initial state is sharply localized or has global correlation. The nonergodicity of the Li-8-Li-6 spin subsystem, which is a convenient object for experimental investigations of spin dynamics, is revealed. An estimate is obtained for the time for transition from a sharply localized disturbance of the canonical distribution to a quasiequilibrium state. (C) 1999 American Institute of Physics. [S1063-7761(99)02010-7].