FrustratedXYmodels with accidental degeneracy of the ground state

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Abstract
The uniformly frustrated XY model on a triangular lattice is studied for two values of the frustration: f=1/4 and f=1/3. These two cases are very special because the ground states of the model have accidental degeneracy not related to symmetries. This degeneracy originates from the possibility of constructing domain walls with zero energy. At any finite temperature, the accidental degeneracy is removed by the spin-wave free energy, but the low free energy of the domain walls leads to the possibility of phase transitions at temperatures that are much lower than can be expected for the other values of the frustration not leading to accidental degeneracies. These conclusions are supported by Monte Carlo simulations. The results are of relevance for the description of Josephson-junction arrays in the presence of a perpendicular magnetic field.