Pairing Mechanism and Superconductivity of Rb3C60 Fullerides

Abstract

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 metal in the parent compound introduces free electrons in the lowest unoccupied molecular orbital and for Rb3C60, the band is filled up to the Fermi level. The intercage interactions between the adjacent C60 cages and expansion of lattice due to the intercalation of Rb atoms are investigated using nearest-neighbor interactions. The free electrons in lowest molecular orbital are coupled with intermolecular phonons. The renormalized Coulomb repulsive parameter μ* and the attractive coupling strength λ are obtained for the intermolecular phonon frequency ωer. T c is then estimated as 8.6 K, which is lower as compared with the published data of 30 K. The electrons 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 visualise the relative interplay between the strength of inter- and intramolecular phonons. To illustrate the usefulness of the above approach the carbon isotope effect exponent and the energy gap ratio are estimated which are consistent with the experimental data. The present analysis reveals that both low frequency intermolecular and high frequency intramolecular phonons participate in the pairing mechanism, T c mainly arises from high frequency intramolecular phonons.