CONDUCTIVITY COHERENCE FACTORS IN THE CONVENTIONAL SUPERCONDUCTORS NB AND PB

We have performed microwave measurements at 60 GHz of the surface impedance (Z(s) = R(s) - iX(s)) on superconducting Nb and Pb. The temperature dependence of the complex conductivity (sigma = sigma1 + isigma2) is inferred from the data down to T/T(c) approximately 0.25 for both materials. The imaginary part, sigma2(T), gives the temperature dependence 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 describe the electrodynamics of weak-coupling BCS superconductors. Furthermore, we calculate the complex conductivity within the framework of the standard Eliashberg electron-phonon theory of superconductivity. The experimental results obtained on Nb are in good agreement with the BCS weak-coupling theory while the data on Pb are analyzed in the anomalous (Pippard) limit and show deviations from BCS laws, accounted for only by the Eliashberg strong-coupling description of the electrodynamics of the superconducting state.