  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.

Statistical Mechanics;Materials Science