Non-ergodic quasi-equilibria in short linear spin 1/2 chains

The long-term behavior of longitudinal spin order is studied by numerical simulations in closed quantum systems of dipolar coupled homonuclear spins 1/2, which are arranged as linear chains in the presence of a strong static magnetic field. For chains of spins that are locally polarized on spin i, sigma(0)=I-iz, the long-term behavior involves low amplitude oscillations about quasi-equilibrium states, which correspond to the projection of sigma(0) on the constants of the motion of the total Hamiltonian. These states are dependent on the initial conditions and are often far from a thermal equilibrium distribution proportional to F-z = Sigma(j)I(jz), reflecting inherent non-ergodic behavior. Apparently ergodic behavior can be induced externally when the dipolar interactions are randomly varied in time.