Central peak signatures from vortices in 2D easy-plane antiferromagnets

We investigate the dynamics of a classical, anisotropic Heisenberg model. Assuming a dilute gas of ballistically moving vortices above the Kosterlitz-Thouless transition temperature, we calculate the dynamic form factors \(S(\vec q,\omega )\) and test them by combined Monte Carlo-molecular dynamics simulations. For both in-plane and out-of-plane correlations we predict and observe central peaks (CP) which are, however, produced by quite different mechanisms, depending on whether the correlations are globally or locally sensitive to the presence of vortices. The positions of the peaks in q-space depend on the type of interaction and on the velocity dependence of the vortex structure. For a ferromagnet both CP's are centered at q = 0; for an antigerromagnet the static vortex structure is responsible for a CP at the Bragg points, while deviations from it due to the vortex motion produce a CP at q = 0. By fitting the CP's to the simulation data we obtain the correlation length and the mean vortex velocity.