Bipolarons in the extended Holstein hubbard model - art. no. 094507

We numerically and analytically calculate the properties of the bipolaron i n an extended Hubbard-Holstein model, which has a longer-ran-ge electron-ph onon coupling like the Frohlich model. In the strong-coupling regime, the e ffective mass of the bipolaron in the extended model is much smaller than t he Holstein bipolaron mass. In contrast to the Holstein bipolaron, the bipo laron in the extended model has a lower binding energy and remains bound wi th substantial binding energy even in the large-U limit. In comparison with the Holstein model where only a singlet bipolaron is bound, in the extende d Holstein model a triplet bipolaron can also form a bound state. We discus s the possibility of phase separation in the case of finite electron doping .