In the present paper we study some correlation effects in a generalized Hubbard model with correlated hopping within low-temperature region using a generalized mean-field approximation. It is shown that in a series of cases the model leads to consequences deviating essentially from those of the Hubbard model. We consider the possibility of applying the result to interpret the peculiarities of p...

The magnetism of the t − t ′ Hubbard model Abstract The magnetic properties of the t − t ′ Hubbard model in the two dimensional square lattice are studied within an unrestricted Hartree Fock approximation in real space. The interplay between antiferromagnetism, ferromagnetism, phase separation and in-homogeneous magnetic textures is studied. It is shown that, at sufficiently large values of t ′...

Weak–coupling calculation of the gap structure of doped n–leg Hubbard ladders Abstract In weak coupling, the spin gap in doped, even, n–leg periodic Hubbard ladders reflects the energy to break a pair into separate quasiparticles. Here we investigate the structure of the gap within a spin–fluctuation exchange approximation. We also calculate the amplitude for removing a singlet pair from two la...

The Kondo divergences owing to interaction of current carriers with local moments in highly correlated electron systems are considered within the Hubbard and s-d exchange models with infinitely strong onsite interaction, the many-electron Hubbard representation being used. The picture of density of states containing a peak at the Fermi level is obtained. Various forms of the self-consistent app...

We present a systematic study of various forms of renormalization that can be applied in the calculation of the self-energy of the Hubbard model within the T-matrix approximation. We compare the exact solutions of the attractive and repulsive Hubbard models, for linear chains of lengths up to eight sites, with all possible taxonomies of the T-matrix approximation. For the attractive Hubbard mod...

Degenerate Hubbard models are studied using the Generalized-GutzwillerApproximation. It is found that the metal-insulator transition occurs at a finite correlation Uc when the average number of electrons per lattice site is an integer. The critical Uc depends sensitively on both the band degeneracy N and the filling x. A derivation is presented for the general expression of Uc(x,N) which reprod...

We use the Random Dispersion Approximation (RDA) to study the Mott-Hubbard transition in the Hubbard model at half band filling. The RDA becomes exact for the Hubbard model in infinite dimensions. We implement the RDA on finite chains and employ the Lanczos exact diagonalization method in real space to calculate the ground-state energy, the average double occupancy, the charge gap, the momentum...

We present a numerical solution of the parquet approximation, a conserving diagrammatic approach which is self-consistent at both the single-particle and the two-particle levels. The fully irreducible vertex is approximated by the bare interaction thus producing the simplest approximation that one can perform with the set of equations involved in the formalism. The method is applied to the Hubb...

Correlation effects play an important role in the electronic structure of half-metallic (HM) magnets. In particular, they give rise to nonquasiparticle states above (or below) the Fermi energy at finite temperatures that reduce the spin polarization and, as a consequence, the efficiency of spintronics devices. Employing the constrained random-phase approximation (cRPA) within the full-potential...

A new theoretical analysis of the current-charge and charge-charge propagators is presented for the Hubbard model, using the static approximation for the Composite Operator Method. This approximation manifestly preserves a sum rule which governs the single-site dynamics. We compare our results with those obtained by numerical analysis.