Flux Motion through the Superconducting Surface Sheath

The response $\dot{b}$ of Pb-2% In alloy specimens in the superconducting surface-sheath state to a small ac field $\dot{h}$ with a triangular waveform has been studied. Features noted with this approach include a precursor, a plateau, and an exponential approach of $\dot{b}$ to $\dot{h}$. 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 ${10}^{-3\mathrm{}} \mathrm{eV}<U_0<{10}^{-2\mathrm{}} \mathrm{eV}$ and the activation distance is greater than 10 Å. This latter value is regarded as low but may result from geometrical effects within the sheath.