Vortex dynamics in two-dimensional Josephson junction arrays

The dynamic response of unfrustrated two-dimensional Josephson junction arrays close to, but above the Kosterlitz-Thouless($KT$) transition temperature is described in terms of the vortex dielectric function $\epsilon(\omega)$. The latter is calculated by considering separately the contribution of $\ll$free$\gg$ vortices interacting by a screened Coulomb potential, and the $\ll$pair motion$\gg$ of vortices that are closer to each other than the $KT$ correlation length. This procedure allows to understand various anomalous features in $\epsilon(\omega)$ and in the flux noise spectra that have been observed experimentally and in dynamic simulations.