Bipolaron localization for increasing electron-phonon coupling in a small cluster

By exact diagonalization of a small cluster, we show that an interplay of electron-phonon and onsite electron-electron interactions results in "intersite" or "onsite" two-electron (bipolaronic) solutions of the Holstein-Hubbard model, depending on the strengths of the interactions. On this basis, we argue that the decrease in the superconducting transition temperature of Bi-2212 compounds, following the enhancement of the electron-phonon interaction recently reported by Devereaux ct al. [10] might he a consequence of the above mechanism, which leads to a transition from itinerant (intersite) to bound immobile (onsite) bipolarons, thus effectively reducing the number of superconducting carriers.