Critical-Current Behavior in Narrow Thin-Film Superconductors

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Abstract
Critical currents in planar thin films of tin and lead have been measured as a function of temperature using very narrow strip samples. There is a critical sample width of the order of one micron above which the critical-current behavior is dominated by flux-flow processes. For narrower samples the temperature dependence of the average critical current density is in excellent agreement with the usual relation deduced from free-energy consideration, and the low-temperature values are in good agreement with the simple depairing criterion. At higher temperatures the observed critical current densities are significantly larger than the Bardeen-Rogers predictions based on calculations of the BCS energy-gap variation with current. It is proposed that in these narrow films flux vortices due to the self-field of the current at the film edges are too large to enter the film and by their subsequent motion degrade the critical current. The approximate size of such vortices is calculated and good agreement with the experimental critical widths is obtained. The observation, in wide films only, of "training" phenomena similar to those seen in bulk type-II superconductors also tends to support this hypothesis.

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