Multiple phase transitions in the XY model with nematic-like couplings

Critical behavior of the two-dimensional generalized XY model involving solely nematic-like terms of the second, third and fourth orders is studied by Monte Carlo method. We find that such a system can undergo three successive phase transitions. At higher temperatures there is a phase transition of the Berezinskii-Kosterlitz-Thouless type to the q=4 nematic-like phase, followed by two more transitions of the Ising type to the q=2 nematic-like and ferromagnetic phases, respectively. The q nematic-like phases are characterized by spin alignments with angles 2kπ/q, where k≤q is an integer. The ferromagnetic phase appears at low temperatures even without the presence of magnetic interactions owing to a synergic effect of the nematic-like terms.