Quantum to classical crossover in the 2D easy-plane XXZ model

Ground-state and thermodynamic properties of the spin-1/2 two-dimensional e asy-plane XXZ model are investigated by both a Green's-function approach an d by Lanczos diagonalization on lattices vith up to 36 sites. We calculate the spatial and temperature dependences of various spin correlation functio ns, as well as the wave-rector dependence of the spin susceptibility for al l anisotropy parameters a. In the easy-plane ferromagnetic region (-1 < <De lta> < 0), the longitudinal correlators of spins at distance r change sign at a finite temperature T-0(<Delta>, r). This transition, observed in the 2 D case for the first time, can be interpreted as a quantum to classical cro ssover.