Second-order phase transition in the Heisenberg model on a triangular   lattice with competing interactions
  We discover an example where the dissociation of the Z2 vortices occurs at the second-order phase transition point. We investigate the nature of phase transition in a classical Heisenberg model on a distorted triangular lattice with competing interactions. The order parameter space of the model is SO(3)xZ2. The dissociation of the Z2 vortices which comes from SO(3) and a second-order phase transition with Z2 symmetry breaking occur at the same temperature. We also find that the second-order phase transition belongs to the universality class of the two-dimensional Ising model. 
Ryo Tamura, Shu Tanaka, Naoki Kawashima
cond-mat.stat-mech cond-mat.mtrl-sci