Quantum-classical crossover in the spin-1/2 XXZ chain

We compute, by means of exact diagonalization of systems of N = 16 and 18 spins, the correlation function [sigma(0)(z)sigma(n)(z)] at nonzero temperature for the XXZ model with anisotropy Delta. In the gapless attractive region -1 <Delta<0 for fixed separation the temperature can always be made sufficiently low so that the correlation is always negative for n not equal 0. However we find that for sufficiently large temperatures and fixed separation or for fixed temperature greater than some To(il) and sufficiently large separations the correlations are always positive. This sign changing effect has not been previously seen and we interpret it as a crossover from quantum to classical behavior. [S0163-1829(99)05101-2].