KOSTERLITZ-THOULESS TRANSITION OF QUANTUM XY MODEL IN 2 DIMENSIONS

The two-dimensional S = 1/2 XY model is investigated with an extensive quantum Monte Carlo simulation. The helicity modulus is precisely est imated through a continuous-time loop algorithm for systems up to 128 x 128 near and below the critical temperature. The critical temperatur e is estimated as T-KT = 0.3427(2)J. The obtained estimates for the he licity modulus are well fitted hy a scaling form derived from the Kost erlitz renormalization group equation. The validity of the Kosterlitz- Thouless theory for this model is confirmed.