Thermal depinning of a single superconducting vortex in Nb

Thermal depinning has been studied for a single vortex trapped in a superconducting Nb 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. The value depends on the pinning site, but typically the location of the vortex begins to change when the order parameter is about 24% of the T=0 value and the superfluid density is about 5% of the T=0 value. For all the pinning sites probed, the vortex would exit the film when the superfluid density was under 1%. This implies that there is a window between 9.1 and 9.2 K where vortices will spontaneously leave the film in a microtesla environment even though the film is still superconducting.