Experimental Demonstration of the Riedel Peak

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 2α=2eVrfhf>25, significant deviations from Bessel function behavior [JN(2α)] 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 fJ=4Δh, i.e., the Riedel peak.