First-order transition in an xy model with nearest-neighbor interactions

An x-y model with nearest-neighbor interaction V()=2J{1-[cos2(2)]p2} is studied by Monte Carlo simulation. The model has a transition from a paramagnetic to an algebraic (massless) phase. An increase of the parameter p, which changes the shape of the potential, brings about a change in the nature of the transition from continuous to first order. © 1984 The American Physical Society.