Robust $s\pm$ Superconductivity in a Two-Band Hubbard-Fr{\"o}hlich Model of Alkali Doped Organics

TL;DR

This paper investigates how $s \u00b1$ superconductivity can persist in a two-band model of alkali-doped organics despite strong electron-electron repulsion, highlighting the role of electron correlations in stabilizing the superconducting state.

Contribution

It demonstrates that $s \u00b1$ superconductivity remains stable in a two-band Hubbard-Fr46hlich model due to electron correlations, even with strong on-site repulsion.

Findings

$s b1$ superconductivity survives strong Hubbard $U$ effects.

Electron correlations stabilize and strengthen the superconducting state.

Without correlations, an antiferromagnetic insulator would dominate.

Abstract

The damaging effect of strong electron-electron repulsion on regular, electron-phonon %$s$-wave superconductivity is a standard tenet. In spite of that, an increasing number of compounds such as fullerides and more recently alkali-doped aromatics exhibit %$s$-wave or presumably $s$ wave 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 on-site Hubbard $U$, whose effect on the superconductivity we study. Starting within mean field, we find that $s \pm$ 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study that follows is necessary because without electron correlations the superconducting state would in this model be superseded by an antiferromagnetic insulating state with lower energy.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 $s \pm$ superconducting state is stabilized and even strengthened for small Hubbard $U$.