Microwave-Photon-Assisted Tunneling in Sn-I-Sn Superconducting Tunnel Junctions

Abstract

We have made an experimental study of comparatively low-frequency (3.93 GHz) microwave-photon-assisted quasiparticle tunneling in superconducting Sn-I-Sn tunnel junctions. The junctions were situated in a perpendicular rf electric field of frequency $\ensuremath{\omega}$, with microwave voltages ${V}_{\mathrm{rf}}$ satisfying the condition $\frac{e{V}_{\mathrm{rf}}}{\ensuremath{\hbar}\ensuremath{\omega}}\ensuremath{\lesssim}18$. Excellent agreement with the rf power dependence predicted by the theory of Tien and Gordon has been obtained for junctions with normal resistances $\ensuremath{\gtrsim}1 \ensuremath{\Omega}$, although the calculated junction cavity fields remain an order of magnitude below field values needed to fit the data. As the junction resistance is decreased, agreement remains good at high rf power levels, but systematic discrepancies between theory and experiment occur at lower power levels. The interaction of microwave radiation with the zero-voltage Josephson current has also been studied on the same junctions, and the response compared to the theoretical predictions of Werthamer. In this case quantitative agreement with the theory is generally poor and does not appear to be correlated with sample resistance.