Supercurrents through thick, clean S—N—S sandwiches

Abstract

Measurements of the maximum supercurrents which may be carried through clean lead/ copper /lead sandwiches, prepared from rolled copper foils up to 40 /μm thick, cleaned under vacuum, have been made with a Slug voltmeter, at temperatures down to 0.34 K. The magnitude of the critical supercurrent density is subject to considerable random variation, but is described to order of magnitude by the semi-empirical expression j e = A exp (-DT/α), where A = 10 10±1 A m -2 , and α = (1.0 ± 0.2) x 10 -6 K m providing strong evidence that the process is controlled by a temperature-dependent coherence length ζ, T = α/T. No supercurrents have been observed in the lead-gold system, and those for tin-copper are much smaller than those for both soldered and evaporated lead and lead/bismuth alloy on copper, which are all of comparable magnitude. These effects may be due to the growth of intermetallic compounds at the interfaces.