Optical evidence of the weak coupling pairing mechanism in the superconductors K/sub 3/C/sub 60/ and Rb/sub 3/C/sub 60/

Summary form only given, as follows. We will report on our optical investigations on the alkali-doped C₆₀ superconductors over a broad energy spectral range (i.e., from 10 cm^-1 up to 10⁵ cm^-1) and at temperatures below and above T c?? The samples were in form of pressed pellet and single crystal. The normal state response (T >> Tc) is characterized by various interband transitions at high frequencies (v > 5x10³ cm^-1) and by a broad excitation in the mid-IR, superimposed to a Drude contribution. Below Tc a clear enhancement of the reflectivity is observed in the FIR at frequencies below 100 cm^-1, with an onset as a function of temperature coincident with Tc. A superconducting gap of about 73 and 48 cm^-1 is inferred for the Rb- and K-doped C₆₀ single crystals, respectively. This is in full agreement with a weak coupling limit of a BCS singlet superconductor and also suggests that the relevant excitation, responsible for the superconducting pairing, is significantly larger than the single particle gap energy. Moreover, we present our calculation of the electrodynamic response above and below Tc with the standard Eliashberg electron - phonon theory of superconductivity, which strongly supports a pairing mechanism mediated by high frequency intramolecular phonon modles.