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)×Z₂. 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.