CONDUCTIVITY COHERENCE FACTORS IN THE CONVENTIONAL SUPERCONDUCTORS NBAND PB

We have performed microwave measurements at 60 GHz of the surface impe dance (Z(s) = R(s) - iX(s)) on superconducting Nb and Pb. The temperat ure dependence of the complex conductivity (sigma = sigma1 + isigma2) is inferred from the data down to T/T(c) approximately 0.25 for both m aterials. The imaginary part, sigma2(T), gives the temperature depende nce of the superconducting gap and the real part, sigma1(T), exhibits the predicted conductivity coherence peak below T(c). The findings are compared with the expressions calculated by Mattis and Bardeen to des cribe the electrodynamics of weak-coupling BCS superconductors. Furthe rmore, we calculate the complex conductivity within the framework of t he standard Eliashberg electron-phonon theory of superconductivity. Th e experimental results obtained on Nb are in good agreement with the B CS weak-coupling theory while the data on Pb are analyzed in the anoma lous (Pippard) limit and show deviations from BCS laws, accounted for only by the Eliashberg strong-coupling description of the electrodynam ics of the superconducting state.