THE KOSTERLITZ-THOULESS UNIVERSALITY CLASS

We examine the Kosterlitz-Thouless universality class and show that es sential scaling at this type of phase transition is not self-consisten t unless multiplicative logarithmic corrections are included. In the c ase of specific heat these logarithmic corrections are identified anal ytically. To identify those corresponding to the susceptibility we set up a numerical method involving the finite-size scaling of Lee-Yang z eroes. We also study the density of zeroes and introduce a new concept called index scaling. We apply the method to the XY model and the clo sely related step model in two dimensions. The critical parameters (in cluding logarithmic corrections) of the step model are compatible with those of the XY model indicating that both models belong to the same universality class. This result then raises questions over how a vorte x binding scenario can be the driving mechanism for the phase transiti on. Furthermore, the logarithmic corrections identified numerically by our methods of fitting are not in agreement with the renormalization group predictions of Kosterlitz and Thouless.