OPTICAL-PROPERTIES OF THE ALKALI-METAL-DOPED SUPERCONDUCTING FULLERENES - K3C60 AND RB3C60

We have measured the optical reflectivity over a broad spectral range of single-phase K3C60 and Rb3C60 compounds, and have evaluated the opt ical conductivity both below and above the superconducting transition temperature. We identify various features in the excitation spectrum o f the normal state: a Drude contribution at low frequency, a midinfrar ed absorption, and several interband transitions. In the superconducti ng state our results are in full agreement with a BCS singlet ground s tate, with the measured pp values consistent with the weak-coupling li mit. We also evaluate several intrinsic parameters characterizing both the normal and superconducting state, such as the plasma frequency, t he relaxation scattering rate, and the mean free path, besides the sup erconducting gaps and the penetration depth. We compare these quantiti es with similar ones from other experiments and find satisfactory agre ement. Moreover, we present our calculation of the electrodynamic resp onse above and below T(c) with the standard Eliashberg electron-phonon theory of superconductivity, which strongly supports a pairing mechan ism mediated by high-frequency intramolecular phonon modes.