Vortex corrections to universal scaling of magnetic fluctuations in 2D   XY model
  The vortex contribution to the probability density function of longitudinal magnetization fluctuations is examined in finite 2D XY systems close to the Kosterlitz-Thouless-Berezinskii transition temperature. Within the temperature range studied their relevance is limited to rare fluctuations, where they increase the probability of events exceeding four standard deviations below the mean magnetization. 
Peter Holdsworth, Mauro Sellitto