Phase slips and vortex-antivortex pairs in superconducting films with constrictions

A system of two thin coplanar superconducting films seamlessly connected by a bridge is studied. When such a system is cooled, two distinct resistive transitions are observed. The first one is identified with the Berezinskii-Kosterlitz-Thouless (BKT) transition in the film, whereas the second one is a crossover related to the quenching of thermally activated phase slips (TAPS) occurring on the bridge. Using direct and indirect methods, we have measured the resistance $R(T)$ of such samples over a range of {\em eleven orders of magnitude}. Over this broad range of resistance a good agreement is found with the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of TAPS, which was adopted for short bridges, as given by the simple expression $R(T)=R_n \text{exp} [-(c/t)(1-t)^{3/2}]$, where $t=T/T_c$ and $R_n$ is the normal resistance of the bridge.