Electronic Correlations within Fermionic Lattice Models

We investigate two-site electronic correlations within generalized Hubbard model, which incorporates the conventional Hubbard model (parameters: t (hopping between nearest neighbours), U (Coulomb repulsion (attraction))) supplemented by the intersite Coulomb interactions (parameters: J⁽¹⁾ (parallel spins), J⁽²⁾ (antiparallel spins)) and the hopping of the intrasite Cooper pairs (parameter: V). As a first step we find the eigenvalues E_α and eigenvectors |E_α> of the dimer and we represent each partial Hamiltonian E_α|E _α>< E_α|(α=1,2,...,16) in the second quantization with the use of the Hubbard and spin operators. Each dimer energy level possesses its own Hamiltonian describing different two-site interactions which can be active only in the case when the level will be occupied by the electrons. A typical feature is the appearance of two generalized t-J interactions ascribed to two different energy levels which do not vanish even for U=J⁽¹⁾=J⁽²⁾=V=0 and their coupling constants are equal to ±t in this case. In the large linebreak U-limit for J⁽¹⁾=J⁽²⁾=V=0 there is only one t-J interaction with coupling constant equal to 4t^2/|U| as in the case of a real lattice. The competition between ferromagnetism, antiferromagnetism and superconductivity (intrasite and intersite pairings) is also a typical feature of the model because it persists in the case U=J⁽¹⁾=J⁽²⁾=V=0 and t≠0. The same types of the electronic, competitive interactions are scattered between different energy levels and therefore their thermodynamical activities are dependent on the occupation of these levels. It qualitatively explains the origin of the phase diagram of the model. We consider also a real lattice as a set of interacting dimers to show that the competition between magnetism and superconductivity seems to be universal for fermionic lattice models.