Subharmonic Structure in Superconducting Tunneling

Abstract

The current-voltage characteristic obtained in a tunneling experiment between two equal superconductors often contains structure at $\frac{2\Delta }{n}$ where $2\Delta$ is the energy gap and $n$ is an integer. The origin of this subharmonic structure has never been satisfactorily established; however, it has often been linked with the Josephson effect. We have studied this subharmonic structure experimentally and have obtained three new results: (1) The magnitude of the structure can be made to increase proportionally to the square of the tunneling matrix element; (2) the structure is very sensitive to externally applied microwave radiation; (3) when two dissimilar superconductors $a$ and $b$ are used, the structure appears at $\frac{2{\Delta }_a}{2n}$, $\frac{2{\Delta }_b}{2n}$, and $\frac{({\Delta }_a+{\Delta }_b)}{\mathrm{}(2n+1)\mathrm{}}$. These experimental results strongly support the idea that the structure is caused by the self-detection of the Josephson radiation. To explain the magnitude of the structure, it is necessary to assume that the tunneling matrix element is so large that it is improper to linearize the Josephson equations.