arXiv subjects
Publications and source records attributed to Ferdinando Mancini.
We draw some rigorous conclusions about the functional properties of the $μ-ρ$ relation in the Hubbard model based on symmetry considerations and unitary transformations. It is shown that the charge susceptibility reaches its local extreme at half-filling. Exact expressions are obtained in two limiting cases.
The addition to the Hubbard Hamiltonian of a t' diagonal hopping term, which is considered to be material dependent for high-Tc cuprate superconductors, is generally suggested to obtain a model capable to describe the physics of high-Tc 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 1-layer compounds NCCO and LSCO. On the other hand, for the 2-layer compound YBCO 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 t-t'-U model was thought to be.
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.
We have studied the t-t'-U model by means of the composite operator method. The effect of the bare diagonal hopping term t' that appears to be material dependent for high-Tc cuprate superconductors is analyzed in detail. In particular, some local quantities are computed and a comprehensive comparison with the data by numerical simulations on finite size lattices is presented. The result show a good agreement with those obtained by Monte Carlo methods.