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

Measurements are presented of the change ΔIₛ(V) in the tunneling current $I(V)$ 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 ΔIₛ(V)∝d²IdV², 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 ΔIₛ 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.