Topological phase transition in 2D quantum Josephson array
  Path Integral Quantum Monte Carlo simulation is used to study thermodynamic properties and a phase diagram of 2D quantum Josephson array, described by 2+1 XY model. The helicity and vorticity moduli, correlation function of phases and other characteristics of the system as functions of quantum parameter $q$ and temperature $T$ are studied ($q=2e/\sqrt{JC_0}$, $J$ is the Josephson coupling constant, $C_0$ is the intragrain capacitance). Quantum fluctuation induced superconductor - normal phase transition is studied in detail through the use of behavior of above-mentioned quantities. No discontinuous or reentrant phase transition in $q-T$ plane is found. Analysis of the vorticity and the renormalized coupling constant leads to the conclusion that the whole line of phase transition is of Kosterlitz - Thouless type. 
A. I. Belousov, Yu. E. Lozovik