Chirality and Z2 vortices in a Heisenberg spin model on the kagome lattice

The phase diagram of the classical J(1)-J(2) model on the kagome lattice is investigated by using extensive Monte Carlo simulations. In a realistic range of parameters, this model has a low-temperature chiral-ordered phase without long-range spin order. We show that the critical transition marking the destruction of the chiral order is preempted by the first-order proliferation Of Z(2) point defects. The core energy of these vortices appears to vanish when approaching the T=0 phase boundary, where both Z(2) defects and gapless magnons contribute to disordering the system at very low temperatures. This situation might be typical of a large class of frustrated magnets. Possible relevance for real materials is also discussed.