Experimental Demonstration of the Riedel Peak

Abstract

Careful measurements have been made of the power and frequency dependence of the ac Josephson steps in a superconducting tunnel junction exposed to microwave radiation. At values of $2\ensuremath{\alpha}=\frac{2e{V}_{\mathrm{rf}}}{\mathrm{hf}}>25$, significant deviations from Bessel function behavior [${J}_{N}(2\ensuremath{\alpha})$] begin to occur. These deviations are correctly predicted in the detailed microscopic theory of Werthamer and are a consequence of the logarithmic singularity in the ac Josephson current at ${f}_{\mathrm{J}}=\frac{4\ensuremath{\Delta}}{h}$, i.e., the Riedel peak.