Universal jump in the helicity modulus of the two-dimensional quantum XY model

The helicity modulus of the S=1/2 XY model is precisely estimated through a world-line quantum Monte Carlo method enhanced by a cluster update algorithm. The obtained estimates for various system sizes and temperatures are well fitted by a scaling form with L replaced by log (L/${\mathrm{L}}_0$), which is inferred from the solution of the Kosterlitz renormalization group equation. The validity of the Kosterlitz-Thouless theory for this model is confirmed.