2-PHASE-TRANSITIONS IN THE FULLY-FRUSTRATED-XY MODEL

The fully frustrated XY model on a square lattice is studied by means of Monte Carlo simulations. A Kosterlitz-Thouless transition is found at T-KT approximate to 0.446, followed by an ordinary Ising transition at a slightly higher temperature, T-c approximate to 0.452. The non-Ising exponents reported by others are explained as a failure of finite-size scaling due to the screening length associated with the nearby Kosterlitz-Thouless transition.