Pairing mechanism and superconductivity of Rb3C60 fullerides

The nature of electron pairing mechanism and the superconducting transition temperature (T-c) of alkali metal (Rb) doped fullerenes is studied within the framework of strong coupling theory. Chemical substitution of alkali me tal in the:parent compound introduces free electrons in the lowest unoccupi ed molecular orbital and for Rb3C60, the band is filled up to the Fermi lev el. The intercage interactions between the adjacent C-60 cages and expansio n of lattice due to the intercalation of Rb atoms are investigated using ne arest-neighbor interactions. The free electrons in lowest molecular orbital are coupled with intermolecular phonons. The renormalized Coulomb repulsiv e parameter mu* and the attractive coupling strength lambda are obtained fo r the intermolecular phonon frequency omega(er). T-c is then estimated as 8 .6 K, which is lower as compared with the published data of 30 K. The elect rons also couple with the intramolecular phonons and introducing them in ad hoc way, T-c enhances to 34 K. Analytical results on T-c allow one to visu alise the relative interplay between the strength of inter- and intramolecu lar phonons. To illustrate the usefulness of the above approach the carbon isotope effect exponent and the energy gap ratio are estimated which are co nsistent with the experimental data. The present analysis reveals that both low frequency intermolecular and high frequency intramolecular phonons par ticipate in the pairing mechanism, T-c mainly arises from high frequency in tramolecular phonons.