Investigation of vortex dynamics in Josephson junction arrays with magnetic flux noise measurements

We have measured the magnetic flux noise in a two-dimensional Josephson junction array in nominally zero magnetic field, in the vicinity of the superconducting transition. This transition is of the Berezinski-Kosterlitz-Thouless (BKT) type, driven by the unbinding of thermally excited vortex-antivortex pairs. At temperatures just above the BKT transition temperature $T_{c}$, the flux-noise power spectrum $S(\omega)$ is white up to a temperature dependent cross-over frequency $\omega_{\xi}(T)$. We identify $\omega_{\xi}$ with the characteristic time related to the scale at which the cross-over from free vortex response to a response dominated by bound pairs occurs. The signature of free vortices is thus white noise while bound vortex pairs give rise to a $1/\omega$ dependence.