PHASE-TRANSITION AND THERMODYNAMICS OF QUANTUM XY-MODEL IN 2 DIMENSIONS

The two-dimensional spin-1/2 XY model is investigated via an extensive quantum Monte Carlo simulation on square lattices as large as 128 x 128. The transverse susceptibility and correlation length show precise thermodynamic scaling behaviors of a phase transition of the Kosterlitz-Thouless type at kT(c)/J = 0.353(3). The correlation functions near the transition point exhibit a universal scaling behavior. Various critical exponents are determined and our results demonstrate that the universality class of the two-dimensional quantum system with continuous symmetry is the same as its classical analog. The specific heat exhibits a finite peak around kT/J = 0.45, decreases very rapidly near T(c), and falls off as T2 near T = 0, validating the spin-wave treatment. The ground-state energy is -0.5543(1)J per spin.