D. Villani

The superconducting gap in the two-dimensional Hubbard model

Abstract Superconductivity with singlet d -wave pairing has been studied for the two-dimensional single-band Hubbard model by means of the composite operator method. Results are given for the one-particle energy gap as a function of wave vector.

INCOMMENSURATE MAGNETISM IN CUPRATE MATERIALS

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.

Incommensurate spin fluctuations in the two-dimensional t-t ' -U model

Abstract 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 LSCO and YBCO compounds.

Fermi surface and density of states in the two-dimensional t-t′-U model

Abstract The two-dimensional t - t ′- U model has been studied by means of the composite operator method. We have found that the intensity of the t ′ parameter controls the topology of the Fermi surface, the structure of the density of states and the position of the van Hove singularity.

LOCAL MAGNETIC MOMENT IN THE TWO-DIMENSIONAL REDUCED P-D MODEL

Abstract A two-dimensional reduced p - d model describing CuO 2 sheets in high- T c oxides is studied by use of the composite operator approach. A set of self-consistent equations for the Green function is derived in a generalized mean-field approximation for the paramagnetic state. The results show a good agreement with the exact results obtained by numerical simulations on finite size lattices.

Dynamical spin magnetic susceptibility in the 2D Hubbard model

Magnetic properties of the two-dimensional Hubbard model are investigated by studying the imaginary part of the dynamical spin magnetic susceptibility as a function of momentum and doping. The calculations are performed by means of the composite operator method in the static approximation. It is shown that the results are in good qualitative agreement with the experimental data for LaSrCuO compounds.

Magnetic properties of the two-band singlet-hole model for the copper-oxide plane

A two-band Hubbard-like model with highly asymmetric hopping parameters is studied by means of composite operator method. In particular, the spin magnetic susceptibility for the paramagnetic case is computed and the results are in qualitative agreement with the experimental situation in La2−xSrxCuO4 systems.

Dynamical incommensurability in the 2D Hubbard model

Abstract The recent discovery of incommensurate fluctuations in yttrium compounds has reopened the problem of finding new appropriate frameworks to describe the variety of experimental features presented by cuprate superconductors. According to this, the imaginary part of the dynamical spin magnetic susceptibility of the Hubbard model has been studied by means of the composite operator method. By varying the frequency, the crossover between the two antiferromagnetic regimes has been found. The incommensurability issue has been studied and discussed.

Specific heat of the two-dimensional Hubbard model

By means of the composite operator method we calculate the specific heat of the 2D Hubbard model. Our results show that the model gives a detailed description of the experimental situation in the normal state of cuprate high Tc superconductors. The observed behavior is consistent with a Fermi liquid picture where the unusual properties are described in terms of the van Hove scenario. There is experimental evidence, based on the data for the spin magnetic susceptibility and specific heat, that this scenario is related to the overdoped region where superconductivity is depressed.Abstract In this paper we study the normal state electronic specific heat of the 2D Hubbard model by means of the composite operator method. The results are in good qualitative agreement with the experimental data for cuprate high T c superconductors and confirm the “van Hove scenario”.

Electronic states in the t-J model

Abstract Doping dependence of electronic states in the t-J model is studied by use of the composite operator method. Spin-coupled composite electron operators are introduced to study spin effects from the nearest neighbor electrons. A change from a half-zone structure at half-filling to a flat band structures at low doping is obtained.