Unitary transformation scheme for polaron and bipolaron correlation models

On the basis of the modified Lang-Firsov-transformed Holstein-Hubbard model, effective polaron and bipolaron models are derived within the unified framework of Schrieffer-Wolff-type transformations in the strong correlation limit. The resulting polaronic t-J and bipolaronic local pairing models obtained for large positive and negative on-site interactions, respectively, exhibit different renormalizations of the hopping and interaction terms in the moderately heavy and light polaron and bipolaron cases.