Quantum Critical Point of the XY Model and Condensation of Field-Induced   Quasiparticles in Dimer Compounds
  The quantum critical point of the three-dimensional XY model in a symmetry-preserving field is investigated. The results of Monte Carlo simulations with the directed-loop algorithm show that the quantum critical behavior is characterized by the mean-field values of critical exponents. The system-size dependence of various quantities is compared to a simple field-theoretical argument that supports the mean-field scaling. 
Naoki Kawashima