Quantum to Classical Crossover in the 2D Easy-Plane XXZ Model
نویسندگان
چکیده
Ground-state and thermodynamic properties of the spin-1/2 two-dimensional easy-plane XXZmodel are investigated by both a Green's-function approach and by Lanczos diagonalization on lattices with up to 36 sites. We calculate the spatial and temperature dependences of various spin correlation functions, as well as the wave-vector dependence of the spin susceptibility for all anisotropy parameters . In the easy{plane ferromagnetic region ( 1 < < 0), the longitudinal correlators of spins at distance r change sign at a nite temperature T0( ; r). This transition, observed in the 2D case for the rst time, can be interpreted as a quantum to classical crossover.
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