Infinite-temperature dynamics of the equivalent-neighborXYZmodel

The dynamics of the classical XYZ model with uniform interaction is investigated by the recursion method and, in part, by exact analysis. The time evolution is anharmonic for arbitrary N (number of spins); only the cases N=2 and ∞ are completely integrable. For the special (uniaxially symmetric) equivalent-neighbor XXZ model, the nonlinearities in the equations of motion disappear in the limit N→∞, and the spin autocorrelation functions are determined exactly for infinite temperature: The function 〈${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$(t)${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$〉 exhibits a Gaussian decay to a nonzero constant, and the function 〈${\mathit{S}}_{\mathit{i}}^{\mathit{x}}$(t)${\mathit{S}}_{\mathit{i}}^{\mathit{x}}$〉 decays to zero, algebraically or like a Gaussian, depending on the amount of uniaxial anisotropy. For the general XYZ case, the T=∞ dynamical behavior includes four different universality classes, categorized according to the decay law of the spectral densities at high frequencies. That decay law governs the growth rate of the sequence of recurrents that determine the relaxation function in the continued-fraction representation. The four universality classes may serve as prototypes for a classification of the dynamics of classical and quantum many-body systems in general.