Phase transitions in two-dimensional uniformly frustrated<i>XY</i>spin systems

Abstract

We investigate the nature of phase transitions in a generalized uniformly frustrated square-lattice model with XY spins. The frustration is made to vary by changing the negative bond strength \ensuremath{\eta}. From ground-state (GS) analysis we find that, below the critical value \ensuremath{\eta}=(1/3), the GS is ferromagnetic, while for \ensuremath{\eta}&gt;(1/3), it is doubly degenerate with canted spin configurations. This suggests the existence of an Ising-like transition. This is confirmed by our extensive Monte Carlo simulations. In addition, there is a Kosterlitz-Thouless-like transition at higher temperature for \ensuremath{\eta}\ensuremath{\ne}1. In the fully frustrated case (\ensuremath{\eta}=1), these two transitions are merged into a single one of dominant Ising character. These conclusions follow from a finite-size-scaling analysis and a visualization of the ordering.