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.

No abstract

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...

No abstract