The classical equivalent‐neighbor XXZ model: Exact results for dynamic correlation functions

The dynamics of the classical XXZ model with uniform interaction is nonlinear for N≥2 spins and nonintegrable for N≥3. However, the nonlinearities disappear in the thermodynamic limit N→∞, and the spin autocorrelation functions can be determined exactly for infinite temperature. The function 〈S^z_i(t)S^z_i〉 exhibits a Gaussian decay to a nonzero constant, and the function 〈S^x_i(t)S^x_i〉 decays algebraically to zero or like a Gaussian, depending on the type (easy axis or easy plane) and amount of uniaxial anisotropy.