First-order phase transition in easy-plane quantum antiferromagnets
  Quantum phase transitions in Mott insulators do not fit easily into the Landau-Ginzburg-Wilson paradigm. A recently proposed alternative to it is the so called deconfined quantum criticality scenario, providing a new paradigm for quantum phase transitions. In this context it has recently been proposed that a second-order phase transition would occur in a two-dimensional spin 1/2 quantum antiferromagnet in the deep easy-plane limit. A check of this conjecture is important for understanding the phase structure of Mott insulators. To this end we have performed large-scale Monte Carlo simulations on an effective gauge theory for this system, including a Berry phase term that projects out the $S=1/2$ sector. The result is a first-order phase transition, thus contradicting the conjecture. 
S. Kragset, E. Smorgrav, J. Hove, F. S. Nogueira, and A. Sudbo