Radiation-Induced Structure in the dc Josephson Current

We analyze theoretically a method for measuring the temperature dependence of the imaginary part of the gap at the gap edge, ${\Delta }_2\mathrm{}\left(T\right)={\Delta }_2\left({\Delta }_0\mathrm{}\right(T), T)$, based upon the behavior of the critical dc Josephson current carried by a superconducting tunnel junction in the presence of a high-frequency electromagnetic field. It is shown that the form of this temperature dependence provides a low-frequency probe of the effective electron-phonon coupling.