Low Energy Excitations and Phase Transitions in the Frustrated Two-Dimensional XY Model

We study the critical properties of the two-dimensional (2D) XY model in a transverse magnetic field with filling factors f=1/3 and 2/5. To obtain a comparison with recent experiments, we investigate the effect of weak quenched bond disorder for f=2/5. A finite-size scaling analysis of extensive Monte Carlo simulations strongly suggests that the critical exponents of the phase transition for f=1/3 and for f=2/5 with disorder are those of the pure 2D Ising model. The relevant low energy excitations are domain walls, and we show that their properties determine the nature of the phase transition.