A New Universality at a first order phase transition: The spinflop transition in an anisotropic Heisenberg antiferromagnet
David P. Landau (Center for Simulational Physics, University of Georgia, Athens, USA)

A great triumph of statistical physics in the latter part of the 20th century was the understanding of critical behavior and universality at 2nd order phase transitions. In contrast, 1st order transitions were believed to have no common features. However, we argue that the classic, 1st order "spin-flop" transition (between the antiferromagnetic and the rotationally degenerate, canted state) in an anisotropic antiferromagnet in a magnetic field exhibits a new kind of universality. We present a finite-size scaling theory for a 1st-order phase transition where a continuous symmetry is broken using an approximation of Gaussian probability distributions with a phenomenological degeneracy factor q included. Predictions are compared with high resolution Monte Carlo simulations of the three-dimensional, XXZ Heisenberg antiferromagnet in a field to study the finite-size behavior for L X L X L simple cubic lattices for systems as large as 10^6 spins. Our Monte Carlo data agree with theoretical predictions for asymptotic large L behavior. The field dependence of all moments of the order parameters as well as the fourth-order cumulants exhibit universal intersections at the spin-flop transition with values that can be expressed in terms of the factor q that characterizes the relative degeneracy of the ordered phases.
Our theory yields q = pi, and we present numerical evidence that is compatible with this prediction. The agreement between the theory and simulation implies a heretofore unknown universality.