Boson-fermion mixed site states

It is shown that an essential aspect in the study of polarons, which belong to multiparticle systems, is a correct selection of the Fock space from a manifold of unitarily inequivalent representations. Using the quantum field theory, we have studied a single-site polaron model with an allowance for the electron correlations at a lattice site. This model takes into account the formation of a charged site boson state in addition to the single-particle fermion states. It is demonstrated that, provided the initial site sates are half-occupied, the formation of boson-polaron mixed site states takes place. The thermodynamics of these states is considered.