Two-particle tunneling in superconducting PbIn-oxide-Pb junctions and their microwave dependence

Superconducting PbIn-oxide-Pb tunnel junctions show a sharp structure in their current-voltage characteristics at a voltage corresponding to $\Delta$, half the energy gap. The current rise in the voltage region between $\Delta$ and $2\Delta$ is in excellent agreement with the theoretical model for two-particle tunneling. On the application of microwave radiation, the step magnitude varies with the radiation power as the squared Bessel function of order zero as predicted by theory.