Thermally activated coherent vortex motion in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin film microbridges

Microbridges with dimensions smaller than the London penetration depth λ have been prepared in epitaxial ${\mathrm{YBaCu}}_2$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films by means of electron beam lithography. Typical peaks and kinks are observed in their differential resistance versus current characteristics which can be attributed to coherent flux motion. A vortex activation energy proportional to the inverse current 1/I is obtained. Both the current and the temperature dependence of the activation energy are consistent with the collective flux creep model if the dimensionality of the elastic vortex lattice equals 2.