Superconductivity in alkali metal doped fullerenes (K3C60): a phonon mechanism

The nature of pairing mechanism and the superconducting state parameters of alkali metal doped fullereness was investigated using the strong coupling theory. Doping of alkali metal (K) in the parent compound introduces free e lectrons in the unfilled band of C-60. It was shown that these electrons di stort the K3C60 lattice and yield low energy intermolecular vibrations. The developed approach properly incorporates the experimental observations on bulk modulus as well as the lattice parameters. The Coulomb repulsive param eter, mu*, and the effective coupling strength, lambda(eff), are obtained f or the intermolecular phonon frequency omega(er), We then evaluated the tra nsition temperature, T-c, for the phonon frequency omega(er) (=51.42 cm(-1) ) as 6.0 K, which is lower when compared with the published data. By introd ucing the intermolecular phonons, omega(ra), in an ad hoc way, the T-c enha nces from 6 to 21.1 K. The analytical results on T-c allow one to visualize the relative interplay between the strength of inter- and intramolecular p honons. The approach was further applied to estimate the carbon isotope exp onent and the energy gap ratio and were found to be consistent with the pub lished data. We found that both the low frequency intermolecular and high f requency intramolecular phonons participate in the phonon mediated electron pairing, T-c mainly arises from high frequency intramolecular phonons. In this article, the implications of the aforementioned analysis are discussed . (C) 1999 Elsevier Science Ltd. All rights reserved.