arXiv subjects

A. Greco

Publications and source records attributed to A. Greco.

62 records · Page 4Linked to original sources

arXiv1999

Electronic correlations, electron-phonon interaction, and isotope effect in high-Tc cuprates

Using a large-N expansion we present and solve the linearized equation for the superconducting gap for a generalized t-J model which also contains phonons within a Holstein model. The leading Tc has d-wave symmetry with phonons giving a positive contribution to Tc. The corresponding isotope coefficient is very small at optimal doping and increases towards the classical value 1/2 with increasing dopings similar as in many cuprates.

cond-mat.supr-con↗

arXiv1999

First-Order Lagrangians and Path-Integral Quantization in the t-J Model

By using the supersymmetric version of the Faddeev-Jackiw symplectic formalism, a family of first-order constrained Lagrangians for the t-J model is found. In this approach the Hubbard ${\hat X}$-operators are used as field variables. In this framework, we first study the spinless fermion model which satisfies the graded algebra spl(1,1). Later on, in order to satisfy the Hubbard ${\hat X}$-operators commutation rules satisfiying the graded algebra spl(2,1), the number and kind of constraints that must be included in a classical first-order Lagrangian formalism for the t-J model are found. This model is also analyzed in the context of the path- integral formalism, and so the correlation generating functional and the effective Lagrangian are constructed.

cond-mat↗

arXiv1998

Dynamical Model and Path Integral Formalism for Hubbard Operators

In this paper, the possibility to construct a path integral formalism by using the Hubbard operators as field dynamical variables is investigated. By means of arguments coming from the Faddeev-Jackiw symplectic Lagrangian formalism as well as from the Hamiltonian Dirac method, it can be shown that it is not possible to define a classical dynamics consistent with the full algebra of the Hubbard $X$-operators. Moreover, from the Faddeev-Jackiw symplectic algorithm, and in order to satisfy the Hubbard $X$-operators commutation rules, it is possible to determine the number of constraint that must be included in a classical dynamical model. Following this approach it remains clear how the constraint conditions that must be introduced in the classical Lagrangian formulation, are weaker than the constraint conditions imposed by the full Hubbard operators algebra. The consequence of this fact is analyzed in the context of the path integral formalism. Finally, in the framework of the perturbative theory, the diagrammatic and the Feynman rules of the model are discussed.

cond-mat.str-el↗

arXiv1998

Effective interactions and superconductivity in the t-J model in the large-N limit

The feasibility of a perturbation expansion for Green's functions of the t-J model directly in terms of X-operators is demonstrated using the Baym- Kadanoff functional method. As an application we derive explicit expressions for the kernel of the linearized equation for the superconducting order parameter in leading order of a 1/N expansion. The linearized equation is solved numerically on a square lattice. We find that a reasonably strong instability occurs only for even frequency pairing with d-wavelike symmetry. Results for the transition temperature and the effective interaction are given as a function of doping.

cond-mat.supr-con↗

arXiv1997

On the Path Integral Representation for Spin Systems

We propose a classical constrained Hamiltonian theory for the spin. After the Dirac treatment we show that due to the existence of second class constraints the Dirac brackets of the proposed theory represent the commutation relations for the spin. We show that the corresponding partition function, obtained via the Fadeev-Senjanovic procedure, coincides with the one obtained using coherent states. We also evaluate this partition function for the case of a single spin in a magnetic field.

cond-mat↗

arXiv1996

Stability of correlated electronic systems under the influence of the electron-phonon interaction

We have used an exact diagonalization technique to study the stability of the $t-J$-Holstein and Hubbard-Holstein models under the influence of the electron-phonon interaction. Previous results have been obtained using frozen-phonon technique or introducing only a few dynamical phonon modes due to the large Hilbert space. To check these results we have done exact diagonalization in a small cluster (four sites) including all the phonon modes allowed by symmetry. We compare our results with those obtained by using the adiabatic approximation.

cond-mat↗

arXiv1995

Exact diagonalization study of the two-dimensional t-J-Holstein model

We study by exact diagonalization the two-dimensional t-J-Holstein model near quarter filling by retaining only few phonon modes in momentum space. This truncation allows us to incorporate the full dynamics of the retained phonon modes. The behavior of the kinetic energy, the charge structure factor and other physical quantities, show the presence of a transition from a delocalized phase to a localized phase at a finite value of the electron-phonon coupling. We have also given some indications that the e-ph coupling leads in general to a suppression of the pairing susceptibility at quarter filling.

cond-mat↗

arXiv1993

Polaronic Self-Localization Effects in the Three Band Peierls Hubbard Model

We have studied the three-band Peierls-Hubbard model describing the Cu-O layers in high-T$_c$ superconductors by using Lanczos diagonalization and assuming infinite mass for the ions. When the system is doped with one hole, and when the electron-lattice coupling is greater than a critical value, we found that the oxygens around one Cu contract and the hole self-traps forming a lattice and electronic small polaron. The self-trapped hole forms a local singlet analogous to the Zhang-Rice singlet in the undeformed lattice. We also studied the single-particle spectral function and the optical conductivity. We have found that the spectral weight, in general, is similar to that found in previous studies where the coupling with the lattice was absent. There is an anomalous transfer of spectral but, contrary to those studies, it goes to these localized polaronic states. However, this polaronic shift does not seem enough by itself to explain the pinning of the chemical potential observed in real materials. We compare our results to those obtained in inhomogeneous Hartree-Fock calculations and we discuss their relation with experiments.

cond-mat↗