Flux motion through the superconducting surface sheath

The response ḃ of Pb-2% In alloy specimens in the superconducting surface-sheath state to a small ac field ḣ with a triangular waveform has been studied. Features noted with this approach include a precursor, a plateau, and an exponential approach of ḃ to ḣ. These observations and others point to the necessity of a more complete model for flux passage through the sheath. An analytical expression, based on a model of creep of flux spots in the sheath, is found to give an excellent quantitative description for Pb-2% In alloy specimens. The activation energy for the process is 10⁻³ eV<U₀<10⁻² eV and the activation distance is greater than 10 \AA{}. This latter value is regarded as low but may result from geometrical effects within the sheath.