First-order phase transition in a gauge theory of S = 1/2 quantum antiferromagnets in the deep easy-plane limit

We perform large-scale Monte Carlo simulations on an effective gauge theory for a deep easy-plane antiferromagnet, including a Berry phase term that projects out the S = 1/2 sector. Without a Berry phase term, the model exhibits a phase transition in the 3DXY universality class associated with proliferation of gauge-charge neutral U(1) vortices. The instantons that eliminate the phase transition in the gauge-charged sector are suppressed by the Berry phases. The result is a first-order phase transition. 1Work supported by the Norwegian Reserch Council.