We investigate the phase transition in a triangular superconducting array with a quarter of the flux quantum per plaquette. The system has infinitely-degenerate ground states in addition to those associated with the U(1) symmetry. We study the system both in the mean-field limit, where it is equivalent to a tight-binding electron, and via the Monte Carlo method. It is found that the system exhibits a single phase transition the nature of which is vortex-antivortex unbinding. The existence of peculiar domain-wall-type excitations, which induce unbinding and thus lower the transition temperature, is also pointed out.

• Received 22 January 1992

DOI:https://doi.org/10.1103/PhysRevB.46.1240