Electron-electron interactions in superconducting fullerides

Summary form only given. We discuss the role of electron-electron interactions in the superconducting fullerenes, examining in addition to the familiar diagonal Hubbard (U) and extended Hubbard (V) interactions, the effects off-diagonal electron-electron interactions, representing nearest neighbor bond-bond repulsion (W) and mixed bond-site repulsion (X). Our motivation comes from studies of related novel electronic systems in reduced dimensions (including conducting polymers) in which these terms have been argued to induce superconductivity (the X term) and to affect dramatically bond alternation (the W term). Using exact diagonalization techniques on smaller homologs and perturbation theory on C/sub 60/ itself, we investigate the extent to which these additional interactions (1) affect previous conclusions on the possible existence of an effective negative-U molecular superconducting state in alkali atom-doped fullerides and (2) can explain the observation of ferromagnetism in fullerenes treated with other dopants. We close by comparing and contrasting the electronic interaction models for fulleride superconductivity with other proposed mechanisms.