Riedel singularity in tin-tin oxide-tin tunnel junctions

The Riedel peak in the Josephson supercurrent in Sn-Sn oxide-Sn tunnel junctions has been investigated by studying the amplitudes of microwave-induced steps. At low temperatures, the peak rises to a value approximately 2.8 times the zero-voltage supercurrent amplitude. This can be accounted for quantitatively in terms of the known bulk gap anisotropy of tin, reduced by a factor of 2–3 due to dirty-superconductor averaging and a further factor of about 17 due to preferred crystallographic orientation of the films comprising the junctions. At high temperatures, the truncation of the peak is in quantitative agreement with theoretical predictions of the effect of quasiparticle damping processes. Complementary measurements of the slope of the quasiparticle tunnel current at the gap voltage cannot be satisfactorily explained in a similar way, despite the common origin of the quasiparticle current jump at the gap voltage and the Riedel peak in the quasiparticle density-of-states peak at the gap edge. The quasiparticle current jump appears to be broadened by some mechanism which does not significantly affect the Riedel peak.Research supported by the Army Research Office (Durham) and the Advanced Research Projects Agency.