Bipolarons in the extended Holstein Hubbard model

There is growing evidence that electron-phonon coupling plays an important role in determining exotic properties of novel materials such as colossal magnetoresistance 1 and high-Tc compounds. 2 Since electrons in these materials are strongly correlated, the interplay between an attractive electron-phonon interaction and Coulomb repulsion may be important in determining physics at finite doping. In particular, when the electron-phonon interaction is local, as is the case in the Holstein model, finite Coulomb repulsion leads to the formation of an intrasite bipolaron, 3‐5 with an effective mass of the order of the polaron effective mass. 5 It has been recently discovered that a longer-range electron-phonon interaction leads to a decrease in the effective mass of a polaron in the strong-coupling regime. 6,7 The lower mass can have important consequences, because lighter polarons and bipolarons are more likely to remain mobile and less likely to trap on impurities or from mutual repulsion. Motivated by this discovery, we investigate a sim