PHASE-TRANSITIONS IN THE FULLY FRUSTRATED XY MODEL 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/k(B), we find a Kosterlitz-Thouless transition with a larger-than-universal jump in the helicity modulus. The critical index eta(T(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/k(B), we find a second-order vortex-lattice melting transition with critical indices alpha = 0.363(10), beta = 0.089(8), gamma = 1.448(24), and nu = 0.813(5).