Tunneling Induced by Longitudinal Microwave Phonons in Al-Pb, Al-Sn, and Pb-Pb Superconducting Diodes

Abstract

Measurements are presented of the change $\Delta I_s\mathrm{}\left(V\right)\mathrm{}$ in the tunneling current $I\left(V\right)\mathrm{}$ in superconducting diodes due to longitudinal microwave phonons as a function of voltage bias $V$, acoustic power, and temperature. The measurements were performed on Al-Pb, Al-Sn, and Pb-Pb diodes in the temperature range 1.3-2.1°K. The voltage dependence, which was heretofore thought to be described by $\Delta I_s\mathrm{}\left(V\right)\mathrm{}\propto \frac{d^{2}I}{d{V}^2}$, was found to exhibit departures from this law. This effect is most striking in Pb-Pb diodes and somewhat less apparent when one of the superconductors making up the diode is aluminum. Furthermore, the magnitude of the measured $\Delta I_s$ is much larger than one would predict from present theory. The tunnel-diode current-pulse echoes exhibit a nonexponential decay which is strongly temperature-dependent. None of this behavior is predicted by the theory.