THE XY-MODEL AND THE 3-STATE ANTIFERROMAGNETIC POTTS-MODEL IN 3 DIMENSIONS - CRITICAL PROPERTIES FROM FLUCTUATING BOUNDARY-CONDITIONS

We present the results of a Monte Carlo study of the three-dimensional XY model and the three-dimensional antiferromagnetic three-state Pott s model. In both cases we compute the difference of the free energies of a system with periodic and a system with antiperiodic boundary cond itions in a neighborhood of the critical coupling. From the finite-siz e scaling behaviour of this quantity we extract values for the critica l temperature and the critical exponent nu that are compatible with re cent high-statistics Monte Carlo studies of the models. The results fo r the free energy difference at the critical temperature and for the e xponent nu confirm that both models belong to the same universality cl ass.