Off-diagonal interactions and pair-binding in superconducting fullerides

We have examined the effect of including off-diagonal (non-density-density) electron-electron (c-e) interactions on putative electronically-driven superconducting states in the fullerides by studying the spectra of truncated tetrahedral and icosahedral “Hubbard molecules”. Our perturbation theoretic and exact diagonalization calculations indicate that for moderate screening lengths and intermediate interaction strengths these terms can contribute significantly to pair-binding energies and to energy level splittings, and consequently the occurence of superconductivity in such models is very sensitive to the precise values of the parameters. We illustrate this in detail in the case of C60−2. We also find that, although the individual levels shift substantially, the critical interaction strengths for the Hundʹs rule violating level crossings are quite insensitive to the inclusion of these additional interactions.