First- and second-order phase transitions in the d=2 XY model

The d=2 XY model is generalized to include an operator that couples directly to the vortex core energy, while maintaining the full symmetry of the original model. In addition to the usual Kosterlitz-Thouless transition, the model exhibits a true first-order transition and a pseudo first-order transition. The excitations of the model include fractionally charged vortices. Implications for the d=2 melting problem are discussed. © 1982 The American Physical Society.