Edge barrier pinning for a single superconducting vortex

Thermal depinning of a single vortex trapped in a superconducting thin film has been measured in order to study the Bean-Livingston surface barrier. There are two forces that bias the motion of the vortex in the natural pinning potential of the film. These are the image force pulling the vortex toward the edge of the film and the Lorentz force of the Meissner currents pushing the vortex toward the center of the film. With zero applied magnetic field, a vortex trapped in a clean, well-defined junction will begin to spontaneously move over large distances of 1 mu m or more at a temperature where the reduced order parameter is about Delta/Delta(0)=0.2. When Delta/Delta(0) has been further reduced to Delta/Delta(0)=0.15, the vortex exits the film, giving a vortex-free state below T-c. In zero applied field, the data show that the image force clearly causes a trapped vortex to leave the film. When a perpendicular magnetic field is applied, however, results show that new vortices were nucleated for fields higher than 20 mG. At 20 mG, the Meissner current force was only a few percent of the pinning force and this biasing force causes the vortex to exit the film at a temperature 0.050 K lower than in zero field. [S0163-1829(99)03938-7].