Vortex signatures in dynamic structure factors for two-dimensional easy-plane ferromagnets

The XY and the anisotropic Heisenberg models are considered above the Kosterlitz-Thouless transition temperature. Under the assumption of a gas of freely moving vortices, it is shown that the dynamic structure factor exhibits a central peak for both in-plane and out-of-plane correlations, in good agreement with the results of a combined Monte Carlo–molecular dynamics simulation. These results are also consistent with recent neutron-scattering data on ${\mathrm{Rb}}_2$ ${\mathrm{CrCl}}_4$ and ${\mathrm{BaCo}}_2$(${\mathrm{AsO}}_4$ ${)}_2$, which show qualitatively the same wave-vector and temperature dependences.