Phase transition and thermodynamics of quantumXYmodel in two dimensions

The two-dimensional spin-1/2 XY model is investigated via an extensive quantum Monte Carlo simulation on square lattices as large as 128×128. The transverse susceptibility and correlation length show precise thermodynamic scaling behaviors of a phase transition of the Kosterlitz-Thouless type at ${\mathit{kT}}_{\mathit{c}}$/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 ${\mathit{T}}_{\mathit{c}}$, and falls off as ${\mathit{T}}^2$ near T=0, validating the spin-wave treatment. The ground-state energy is -0.5543(1)J per spin.