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

Abstract

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( ω) is white up to a temperature dependent cross-over frequency ω ξ ( T). We identify ω ξ 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/ ω dependence.