Frequency dependence of the Josephson current

Careful measurements have been made of the power and frequency dependence of the ac Josephson steps in a superconducting tunnel junction exposed to 20-26-GHz microwave radiation. At values of $2\alpha =\frac{2e{V}_{rf}}{hf}>25$, significant deviations from Bessel-function behavior [$J_N\left(2\alpha \right)$] begin to occur. These deviations are caused by a sharp peak in the Josephson current at a frequency $f_J=\frac{4\Delta }{h}$, i. e., the Riedel peak. The experimental data are used to measure the shape of this peak.·Simultaneous measurements of the Josephson and quasiparticle effects are also presented. These results demonstrate that the two effects have the theoretically predicted correlation. Our experiments as a whole demonstrate the theory of superconducting tunnel junctions to be on very firm experimental ground.