Tunneling at Point Contacts between Superconductors

Abstract

We have investigated the tunneling behavior which can occur at anodized point contacts between various superconducting metals and alloys (Nb, Ta, Nb-Ta, and Ti-Mo). A large excess background current almost always exists, but the usual tunneling characteristics are observed. The values of the superconducting energy gaps $\Delta$ determined from this work agree well with those determined from observations of tunneling between thin films. In the case of contacts between like metals, a subharmonic sequence of features located at $V=\frac{2\Delta }{n}$ ($n$ an integer) is generally observed. For unlike metals we observe structure at ${\Delta }_1+{\Delta }_2$, ${\Delta }_1$, and $\frac{{\Delta }_2}{n} ({\Delta }_2<{\Delta }_1)$. When ${\Delta }_2<\frac{1}{3}({\Delta }_1+{\Delta }_2)$, no structure is discernible at $\frac{({\Delta }_1+{\Delta }_2)}{\mathrm{}(2n+1)\mathrm{}}$. These results agree with recent thin-film tunneling work of Rowell and Feldman. The various subharmonic features appear to be peaks in the dc current-voltage characteristics of the junctions. The behavior of the junctions in a magnetic field was always characteristic of superconducting weak links probably formed by metallic shorts. Available evidence points to absorption of weak-link radiation within the junctions as the probable cause of the subharmonic features, but the nature of the physical mechanisms involved remains unclear.