Thermodynamic properties of the extended Hubbard model with strong intra-atomic attraction and an arbitrary electron density

We have used a canonical transformation to derive an effective extended Hubbard Hamiltonian with intra-atomic repulsion and Ising-type interatomic exchange interaction in an effective external magnetic field from the extended Hubbard model with intra-atomic attraction. The absence of magnetic ordering in the original Hamiltonian is transformed into the condition of one quasiparticle per atom. In the strong---intra-atomic---attraction limit, we can further derive from the transformed Hamiltonian an antiferromagnetic anisotropic Heisenberg Hamiltonian in an external field and with a given value of magnetization. A phase diagram of the charge-ordered, the singlet-superconducting, the mixed, and the nonordered phase is then obtained with the mean-field approximation. We continue to investigate the collective excitations in the MFA states. For various special cases our results reduce to the known results or the exact solutions.

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