Attraction from repulsion: unconventional superconductivity in flat bands due to far-out pairs

Many superconducting systems of current interest feature abnormally high Tc values coexisting with high rs values and other anomalies which point to a Coulomb-repulsion-dominated superconducting pairing mechanism. It is appealing to consider a route in which the Coulomb repulsion generates attraction ``directly", rather than acting through low-energy intermediaries used in previous proposals. If achievable, such a mechanism would fully benefit from the strength of the pristine Coulomb interaction. One mechanism considered previously involves repulsion-induced pairing between several Fermi surfaces, however, the resulting pairing interaction is weakened by Thomas-Fermi screening. This talk will describe a new repulsion-dominated pairing mechanism that utilizes a nimbus of far-out pairs in the empty band positioned above the conduction band. The resulting pairing interaction is unscreened, and thus provides an enhanced Coulomb-repulsion-driven superconductivity. We further illustrate the sizable effect resulting from such mechanism by applying it to the magic-angle twisted bilayer graphene (MATBG) where the upper bands in our model capture the role of the non-flat minibands, which is usually overlooked in the literature.