First-order and Berezinskii-Kosterlitz-Thouless phase transitions in two-dimensional generalized XY models

The aim of this paper is to illustrate that generalized two-dimensional XY models (proposed by Romano and Zagrebnov) may also support a first-order phase transition. Two approaches are employed to accurately determine the critical parameter q at which such a transition takes place. Furthermore, we show that the model is characterized by three distinct regions concerning both first-order and Berezinskii-Kosterlitz-Thouless phase transitions. Finally, the underlying mechanisms governing such transitions are presented, along with an estimation of the critical temperatures.