Thermal depinning of a single superconducting vortex

Abstract

Thermal depinning has been studied for a single vortex trapped in a superconducting thin film in order to determine the value of the superconducting order parameter and the superfluid density when the vortex depins and starts to move around the film. For the Pb film in Pb/Al/Al2O3/PbBi junction having a gold line, the vortex depins from the artificial pinning site (Au line) and reproducibly moves through the same sequence of other pinning sites before it leaves the junction area of the Pb film. Values of the normalized order parameter Δ/Δ° vary from Δ/Δ°=0.20 at the first motion of the vortex to Δ/Δ°=0.16 where the vortex finally leaves the junction. Equivalently, the value of the normalized superfluid density changes from 4% to 2.5% for this sample in this same temperature interval. For the Nb film in Nb/Al/Al2O3/Nb junction, thermal depinning occurs when the value of Δ/Δ° is approximately 0.22 and the value of ρs/ρso is approximately 5%. These values are about 20% larger than those of a Pb sample having a gold line, but the values are really very close. For the Nb sample, grain boundaries are important pinning sites whereas, for the Pb sample with a gold line, pinning may have been dominated by an array Pb3AU precipitates. Because roughly the same answer was obtained for these rather different kinds of pinning site, there is a reasonable chance that this is a general value within factors of 2 for a wide range of materials.