In the paper, we study the thermodynamic and electromagnetic properties of the Penson-Kolb (PK) model, i.e., the tight-binding model for fermionic particles with the pair-hopping interaction J. We focus on the case of repulsive J (i.e., J < 0), which can stabilize the eta-pairing superconductivity with Cooper-pair center-of-mass momentum q = Q, Q = (π/a,π/a,. . . ). Numerical calculations are performed for several d-dimensional hypercubic lattices: d = 2 (the square lattice, SQ), d = 3 (the simple cubic lattice) and d = ∞ hypercubic lattice (for arbitrary particle concentration 0 < n < 2 and temperature T ). The ground state J versus n phase diagrams and the crossover to the Bose-Einstein condensation regime are analyzed and the evolution of the superfluid characteristics are examined within the (broken symmetry) Hartree-Fock approximation (HFA). The critical fields, the coherence length, the London penetration depth, and the Ginzburg ratio are determined at T = 0 and T > 0 as a function of n and pairing strength. The analysis of the effects of the Fock term on the ground state phase boundaries and on selected PK model characteristics is performed as well as the influence of the phase fluctuations on the eta-pairing superconductivity is investigated. Within the Kosterlitz-Thouless scenario, the critical temperatures TKT are estimated for d = 2 SQ lattice and compared with the critical temperature Tc obtained from HFA. We also determine the temperature Tm at which minimal gap between two quasiparticle bands vanishes in the eta-phase. Our results for repulsive J are contrasted with those found earlier for the PK model with attractive J (i.e., with J > 0).

We study a simple effective model for description of charge orderings in narrow band materials, i.e. the spinless fermion model with repulsive intersite interaction W . The analysis is concentrated on the problem of phase separations and the effects of next-nearest neighbor hopping t2 on the charge ordered states in this system. The cases of d-dimensional (d ≥ 2) hypercubic lattices are considered for arbitrary particle concentration (0 < n < 1). Within the broken symmetry Hartree-Fock approximation the phase diagrams as a function of W and n are evaluated for representative cases. The results for t 2 = 0 are compared with those found for the case with nearest neighbor hopping only.

We study the basic thermodynamic and electromagnetic properties of the superconductor described by the negative-U Hubbard model ( gap parameter ∆, critical temperature T C , London penetration depth λ, thermodynamic critical field H C and Ginzburg-Landau correlation length ξ G-L ).