Superconductivity and electrical resistivity in alkali metal doped fullerides: Phonon mechanism

摘要

We consider a two-peak model for the phonon density of states to investigate the nature of electron pairing mechanism for superconducting state in fullerides. We first study the intercage interactions between the adjacent C_(60) cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate phonon frequencies from the dynamical matrix for the intermoleeular alkali-C_(60) phonons. Electronic parameter as repulsive parameter and the attractive coupling strength are obtained within the random phase approximation. Transition temperature, T_c, is obtained In a situation when the free electrons In lowest molecular orbital are coupled with alkali-C_(60) phonons as 5 K, which is much lower as compared to reported T_c (approx= 20 K). The superconducting pairing Is mainly driven by the high frequency intramolecular phonons and their effects enhance it to 22 K. To illustrate the usefulness of the above approach, the carbon isotope exponent and the pressure effect are also estimated. Temperature dependence of electrical resistivity is then analysed within the same model phonon spectrum. It Is inferred from the two-peak model for phonon density of states that high frequency intramolecular phonon modes play a major role in pairing mechanism with possibly some contribution from alkati-C_(60) phonon to describe most of the superconducting and normal state properties of doped fullerides.