Excess Currents in Superconducting Tunnel Junctions

We have studied the excess currents that flow for $V<{\ensuremath{\Delta}}_{1}+{\ensuremath{\Delta}}_{2}$ in two superconductor (${S}_{1}\ensuremath{-}I\ensuremath{-}{S}_{2}$) tunnel junctions. The features of interest are "multiparticle" tunneling, which is observed as a current increase at ${\ensuremath{\Delta}}_{1}$ and ${\ensuremath{\Delta}}_{2}$, and the subharmonic effects which occur at $\frac{2\ensuremath{\Delta}}{n}$ when ${\ensuremath{\Delta}}_{1}={\ensuremath{\Delta}}_{2}$. Strong evidence is presented that subharmonic junctions contain metallic shorts, and we show that the $\frac{2\ensuremath{\Delta}}{n}$ series can be observed even in the absence of single-particle tunneling. By extending the study to asymmetrical junctions (${\ensuremath{\Delta}}_{1}\ensuremath{\ne}{\ensuremath{\Delta}}_{2}$) we conclude that at present there is no satisfactory explanation of the subharmonic effects.

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