Critical Currents in Superconducting Films of Indium

Abstract

Critical currents in the region near the transition temperature $T_c$ have been measured for thin indium films, of thickness 585 to 3600 Å, deposited on sapphire cylinders. The coherence length obtained from the normal resistivities is compared with previous values and the increase of $T_c$ with decreasing film thickness is determined. Measurements are reported on a 585-Å-thick film which combine the use of a compensated geometry avoiding the difficulties associated with specimen edges, and current pulses with a rise time of 7 nsec in which the transition is not obscured by specimen heating. Two regions are observed; for $(T_c-T)\le 0.11°$K the critical currents vary with the 1.5 power of the temperature difference, in accord with the Ginzburg-Landau theory, while for $0.11°\mathrm{K}\le (T_c-T)\le 0.30°\mathrm{K}$ the temperature dependence becomes nearly linear. The approach to this behavior depends on (a) shortening the measuring-pulse rise time, and, particularly, (b) reducing the fraction of normal resistance restored. Paradoxically, because of (b), even dc currents lead to similar results.