Robust s(+/-) superconductivity in a two-band Hubbard-Frohlich model of alkali-doped organics

The damaging effect of strong electron-electron repulsion on regular, electron-phonon superconductivity is a standard tenet. In spite of that, an increasing number of compounds such as fullerides and more recently alkali-doped aromatics exhibit superconductivity despite very narrow bands and very strong electron repulsion. Here, we explore superconducting solutions of a model Hamiltonian inspired by the electronic structure of alkali-doped aromatics. The model is a two-site, two-narrow-band metal with a single intersite phonon, leading to attraction-mediated, two-order parameter superconductivity. On top of that, the model includes a repulsive onsite Hubbard U, whose effect on the superconductivity we study. Starting within mean field, we find that s(+/-) superconductivity is the best solution surviving the presence of U, whose effect is canceled out by the opposite signs of the two order parameters. The correlated Gutzwiller study that follows is necessary because without electron correlations, the superconducting state would in this model be superseded by an antiferromagnetic insulating state with lower energy. The Gutzwiller correlations lower the energy of the metallic state, with the consequence that the s(+/-) superconducting state is stabilized and even strengthened for small Hubbard U.