Subharmonic Structure in Superconducting Tunnel Junctions

Abstract

We discuss recent experiments which show that the intensity of the structure at $eV=\frac{2\Delta }{n}$ ($2\Delta =\mathrm{energy} \mathrm{gap}$, $n=2,3,4,\dots$) in a superconducting tunnel junction is proportional to the tunnel matrix element in the strong-coupling region. It is shown that for the odd series at $eV=\frac{2\Delta }{\mathrm{}(2n+1)\mathrm{}}$ this behavior can be understood in terms of the accepted theory. However, the familiar explanation for the even series at $eV=\frac{2\Delta }{2n}$ based on the sharp increase of the surface resistance of the superconducting films at $\hslash \omega =2\mathrm{}\Delta$ due to quasiparticle excitation is not consistent with the above experimental findings. A mechanism for the even series is proposed which involves the breaking of a tunneling Josephson pair caused by the oscillating electromagnetic field in the junction.