Phase transitions in the fully frustratedXYmodel studied with use of the microcanonical Monte Carlo technique

We investigate the phase transitions of the frustrated XY model in the square lattice with frustration parameter f=1/2. The system has doubly degenerate ground states in addition to those associated with U(1) symmetry. We study the system via a high-precision Monte Carlo technique. It is found that the system has two separate transitions. At the lower temperature 0.440(2)J/${\mathit{k}}_{\mathit{B}}$, we find a Kosterlitz-Thouless transition with a larger-than-universal jump in the helicity modulus. The critical index η(${\mathit{T}}_{\mathrm{KT}}$) of the spin-spin correlation function is 0.220(2), less than the universal value of 1/4. At higher temperature 0.454(2)J/${\mathit{k}}_{\mathit{B}}$, we find a second-order vortex-lattice melting transition with critical indices α=0.363(10), β=0.089(8), γ=1.448(24), and ν=0.813(5).