FREE VORTEX AND VORTEX-PAIR LIFETIMES IN CLASSICAL 2-DIMENSIONAL EASY-PLANE MAGNETS

We report numerical simulation results for free-vortex lifetimes in th e critical region of classical two-dimensional easy-plane ferro- and a ntiferromagnets having three-component order parameters. The fluctuati ons in the vortex number density in a spin dynamics simulation were us ed to estimate the lifetimes. The observed lifetimes are of the same o rder of magnitude but smaller than the characteristic timescale above which a phenomenological ideal vortex-gas theory that has been used to account for the central peak in the dynamic structure factor S-alpha alpha(q,omega) is expected to be valid. For strong anisotropy, where t he vortices are in-plane, the free vortex lifetimes for ferromagnets a nd antiferromagnets are the same, while for weak anisotropy, where the vortices have non-zero out-of-easy-plane components, the lifetimes in antiferromagnets are smaller than in ferromagnets. The dependence of the free-vortex and total vortex densities on the size dependent corre lation length in the critical region is examined. We also determined t he lifetimes of vortex-antivortex pairs for T = T-KT and well below T- KT. The observed timescales are very short, and the observed pair dens ities are extremely small. These results suggest that pair creation an d annihilation are unlikely to play any role in the central peak in S- xx(q,omega) observed in computer simulations for the ferromagnetic mod el for T less than or equal to T-KT.