The role of electron–vibron interaction and local pairing in conductivity and superconductivity of alkali-doped fullerides

Abstract

We investigate the competition between the electron–vibron interaction (interaction with the Jahn–Teller phonons) and the Coulomb repulsion in a system with the local pairing of electrons on the 3-fold degenerate lowest unoccupied molecular orbital (LUMO). The el.–vib. interaction and the local pairing radically change conductivity and magnetic properties of alkali-doped fullerides AnC60, which would have to be antiferromagnetic Mott insulators: we have shown that materials with n=1,5 and A=K,Rb are conductors but not superconductors; n=3 and A=K,Rb are conductors and superconductors at low temperatures, but with A=Cs they are Mott–Jahn–Teller insulators at normal pressure; n=2,4 are nonmagnetic Mott insulators. Thus superconductivity, conductivity and insulation of these materials have common nature. Using this approach we obtain the phase diagram of A3C60 analytically, which is the result of interplay between the local pairing, the el.–vib. interaction, Coulomb correlations, and formation of small radius polarons.