Abstract
We have performed microwave measurements at 60 GHz of the surface impedance (=-) on superconducting Nb and Pb. The temperature dependence of the complex conductivity (σ^=+i) is inferred from the data down to T/∼0.25 for both materials. The imaginary part, (T), gives the temperature dependence of the superconducting gap and the real part, (T), exhibits the predicted conductivity coherence peak below . 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.
- Received 29 March 1994
DOI:https://doi.org/10.1103/PhysRevB.50.6307
©1994 American Physical Society