Holstein and Frohlich bipolarons

We explore the properties of the bipolaron in a simplified one-dimensional (1D) Frohlich-Hubbard model in comparison with results in the Holstein-Hubb ard model, where the former has a longer range electron-phonon interaction. We solve the model with an exact diagonalization method on an infinite 1D lattice. In the strong coupling regime, the effective mass of the Frohlich bipolaron is much smaller than the Holstein bipolaron mass. In contrast to the Holstein model where only a singlet bipolaron is bound, in the Frohlich model, a triplet bipolaron can also form a bound state. (C) 2001 Elsevier Science B.V. All rights reserved.