Bipolaren formation in a strong coupling polaronic metal

Abstract

We study the formation of bipolarons in the Holstein-Peierls-Hubbard model on a square lattice. This model, for sufficiently small Coulomb interaction, is known to describe a narrow band metallic state, in which the carriers are heavily dressed by the strong interaction with local, dispersionless (Einstein) phonons. To deal with the absence of a small expansion parameter, we use a combination of canonical transformations and a variational treatment in terms of squeezed phonon states. The coexistence of polarons and bipolarons is considered and found compatible for reasonable values of the system parameters, namely the electronic hopping term t, the phonon frequency ω and the electron-phonon coupling constant g. The possibility of bipolaron condensation is studied within this model and found to occur in the same parameter range as the polaron-bipolaron coexistence regime. © 1999 Plenum Publishing Corporation.