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 $\ensuremath{\Delta}$ 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 $V=\frac{2\ensuremath{\Delta}}{n}$ ($n$ an integer) is generally observed. For unlike metals we observe structure at ${\ensuremath{\Delta}}_{1}+{\ensuremath{\Delta}}_{2}$, ${\ensuremath{\Delta}}_{1}$, and $\frac{{\ensuremath{\Delta}}_{2}}{n} ({\ensuremath{\Delta}}_{2}<{\ensuremath{\Delta}}_{1})$. When ${\ensuremath{\Delta}}_{2}<\frac{1}{3}({\ensuremath{\Delta}}_{1}+{\ensuremath{\Delta}}_{2})$, no structure is discernible at $\frac{({\ensuremath{\Delta}}_{1}+{\ensuremath{\Delta}}_{2})}{(2n+1)}$. 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.