Bipolaron formation in a strong coupling polaronic metal

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 (E instein) phonons. To deal with the absence of a small expansion parameter, we use a combination of canonical transformations and a variational treatme nt in terms of squeezed phonon states. The coexistence of polarons and bipo larons is considered and found compatible for reasonable values of the syst em parameters, namely the electronic hopping term t, the phonon frequency o mega and the electron-phonon coupling constant g. The possibility of bipola ron condensation is studied within this model and found to occur in the sam e parameter range as the polaron-bipolaron coexistence regime.