The two-dimensional Hubbard model is analyzed in the framework of the two-pole expansion. It is demonstrated that several theoretical approaches, when considered at their lowest level, are all equivalent and share the property of satisfying the conservation of the first four spectral momenta. It emerges that the various methods differ only in the way of fixing the internal parameters and that it exists a unique way to preserve simultaneously the Pauli principle and the particle-hole symmetry. A comprehensive comparison with respect to some general symmetry properties and the data from quantum Monte Carlo analysis shows the relevance of imposing the Pauli principle.

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We show how to resolve coherent low-energy features embedded in a broad high-energy background by use of a fully self-consistent calculation for composite particle operators. The method generalizes the formulation of Roth, which linearizes the dynamics of composite operators at any energy scale. Self-consistent equations are derived and analyzed in the case of the single-impurity SU(N) Kondo model.

Magnetic properties of the two-dimensional t-t ′ -U model are investigated by studying the static spin magnetic susceptibility as a function of momentum for various temperatures. The calculations are performed by means of the Composite Operator Method in the static approximation. By increasing the value of the t ′ parameter the magnetic scattering in the reciprocal space evolves to an isotropic structure. It is shown that the results are in qualitative agreement with the experimental situation observed in 71.10.Fd. The persistence of antiferromagnetic fluctuations in the superconducting state of high-T c cuprates is one of the most striking features in these materials. Indeed, it is widely accepted that in cuprate materials there is a close relation between the unusual magnetic properties and the occurrence of high temperature superconductivity, and that a comprehension of the magnetic correlations in the normal state may be an important step in the understanding the microscopic mechanism of pairing.The knowledge of the wave-vector and energy dependencies of the spin excitation spectrum is of the most importance in the attempt to build up an appropriate theory for high-T c superconductivity [1]. The dynamical spin susceptibility for cuprate materials has been investigated by inelastic neutron scattering and NMR techniques. Neutron scattering data on La 2−x (Ba, Sr) x CuO 4 have shown [2-9] that away from half-filling the magnetic Bragg peak in the dynamical structure factor S (k, ω) broadens and develops a structure with four peaks located at [(1 ± δ)π, π] and In a previous paper [12], in the context of a single-band Hubbard model, we advanced a theoretical prediction; namely, it was claimed a close relation between superconductivity and incommensurate magnetism in some high-T c cuprates due to the reported proportionality between the calculated amplitude of incommensurability and the experimental superconducting critical temperature for La 2−x Sr x CuO 4 over the whole phase diagram. In the present experimental context, where the incommensurability seems to be a common feature for all cuprate superconductors, it results natural to revisit the analysis of the spin fluctuations spectrum by adding a finite diagonal hopping term t ′ to the original Hubbard Hamiltonian. Infact, the addition of a t ′ bare parameter has often been suggested to handle the complexity of the experimental situation for the cuprates [13,14]. Moreover, the next nearest neighbor hopping parameter t ′ emerges from various reduction procedures as the single parameter, which carries, at the level of the single-band description, the information about the crystal structure outside the CuO 2 planes and thus differentiates between the various cuprates [15,16].In this letter, we focus the attention on the momentum dependence of the static spin magnetic susceptibility χ (k), because this quantity provides strict information about the spatial range of the magnetic correlations. We show that the t-t ′ -U model presents an incommensurate phase at fi...

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A rational representation for the self-energy is explored to interpolate the solution of the Anderson impurity model in general orbitally degenerate case. Several constrains such as the Friedel's sum rule, positions of the Hubbard bands as well as the value of quasiparticle residue are used to establish the equations for the coefficients of the interpolation. We employ two fast techniques, the slaveboson mean-field and the Hubbard I approximations to determine the functional dependence of the coefficients on doping, degeneracy and the strength of the interaction. The obtained spectral functions and self-energies are in good agreement with the results of numerically exact quantum Monte Carlo method.

In the low doping region an incommensurate magnetic phase is observed in LSCO. By means of the composite operator method we show that the single-band 2D Hubbard model describes the experimental situation. In the higher doping region, where experiments are not available, the incommensurability is depressed owing to the van Hove singularity near the Fermi level. A proportionality between the incommensurability amplitude and the critical temperature is predicted, suggesting a close relation between superconductivity and incommensurate magnetism.Comment: 4 pages, 5 figures in one Postscript file, RevTe

The addition to the Hubbard Hamiltonian of a t' diagonal hopping term, which is considered to be material dependent for high-T c cuprate superconductors, is generally suggested to obtain a model capable to describe the physics of high-T c cuprate materials. In this line of thinking, the two-dimensional t-t'-U model has been studied by means of the Composite Operator Method, which allows to determine the dynamics in a fully self-consistent way by use of symmetry requirements, as the ones coming from the Pauli principle. At first, some local quantities have been calculated to be compared with quantum Monte Carlo data. Then, the structure of the energy bands, the shape of the Fermi surface and the position of the van Hove singularity have been computed as functions of the model parameters and studied by the light of the available experimental data. The results of our study show that there exists two sets of parameters that allows the model to describe the relevant features of the 1-layer compounds Nd2-xCexCuO4 and La2-xSrxCuO4. On the other hand, for the 2-layer compound YBa2Cu3O 7 - δ is not possible to find a reasonable set of parameters which could reproduce the position of the van Hove singularity as predicted by ARPES experiments. Hence, it results questionable the existence of an unique model that could properly describe the variety of cuprate superconductors, as the two-dimensional t-t'-U model was thought to be