The Classical Equivalent‐Neighbor XXZ Model: Exact Results for Dynamic Correlation Functions

The dynamics of the classical X X Z 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