Magnetic Flux Flow and Fluctuation Effects in Thin-Film Superconductors

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Abstract
Resistance measurements have been used to investigate magnetic flux pinning, flux motion, and fluctuation effects in thin films of superconducting indium, thallium, and aluminum which were condensed onto cold substrates and maintained below 20 °K throughout the measurements. The depinning threshold of vortex motion was studied as a function of transport-current density, applied perpendicular magnetic field, and temperature. The absence of the peak effect in films is explained in terms of Pearl's theory of long-range electromagnetic interactions between vortices in films. Our critical-state data do not satisfy the empirical equation of Kim et al. which relates the current density to the critical depinning field in bulk samples. Vortex guiding has been observed, and Hall-effect measurements are also reported. Measurements on superimposed films of indium and thallium suggest that the surfaces of these films play an important role in flux pinning. Measurements of the resistive transition in most of our samples showed a peak in resistance near the transition temperature, similar to resistance peaks noticed by some other investigators. This peak disappeared as a perpendicular magnetic field was applied. An explanation of the peak is suggested; it involves small-angle electron scattering from regions in which superconducting fluctuations are occuring.