RESUMO
The three-band Emery model is applied to study the selected principal features of thed-wavesuperconducting phase in the copper-based compounds. The electron-electron correlations are taken into account by the use of the diagrammatic expansion of the Guztwiller wave function (DE-GWF method). The nodal Fermi velocity, Fermi momentum, and effective mass are all determined in the paired state and show relatively good agreement with the available experimental data, as well as with the corresponding single-band calculations. Additionally, the influence of the next-nearest neighbor oxygen-oxygen hopping and intersite Coulomb repulsion terms on the superconducting phase is analyzed.
RESUMO
We propose a systematic approach to the systems of correlated electrons, the so-called [Formula: see text]-DE-GWF method, based on reciprocal-space ([Formula: see text]-resolved) diagrammatic expansion of the variational Gutzwiller-type wave function for parametrized models of correlated fermions. The present approach, in contrast to either variational Monte-Carlo (VMC), or the recently developed real-space diagrammatic expansion of the Gutzwiller-type wave function (direct-space DE-GWF technique), is applicable directly in the thermodynamic limit and thus is suitable for describing selected singular features of the wave-vector-dependent quantities. We employ the [Formula: see text]-DE-GWF method to extract the non-analytic part of the two leading moments of the fermion spectral-density function across the (two-dimensional) Brillouin zone for the Hubbard model and away from the half-filling. Those moments are used to evaluate the nodal quasiparticle velocities and their spectral weights in the correlated superconducting state. The two velocities determined in that manner exhibit scaling with the electron concentration qualitatively different from that obtained earlier for the excited states of the high-T c cuprates within the projected quasi-particle ansatz, and the results are in a very good quantitative agreement with experimental data if interpreted as those characterizing the spectrum below and above the observed kink. We provide a detailed discussion of the two gaps and two excitation branches (two velocities) appearing naturally within our DE-GWF approach. The two separate sets of characteristics distinguish the renormalized quasiparticle states very close to the Fermi surface from the deeper correlated-state properties. Also, an enhancement of the [Formula: see text]-dependent magnetic susceptibility is shown to contain a spin-fluctuation contribution within our language. Finally, the [Formula: see text]-DE-GWF approach is compared to both the VMC and real-space DE-GWF results for the cases of Hubbard and t-J-U models.
RESUMO
In the Néel phase of La-based cuprates, magnons are pseudo-Goldstones as the rotational symmetry of the spin system is explicitly broken by the small anisotropies. Respecting the approximate SO(3) symmetry we calculate the doping and temperature dependence of the magnon gaps in La2-xSrxCuO4 (LSCO) within the framework of the anisotropic linear σ-model coupled to dipoles representing holes. We show that the temperature behaviour of the reduced magnon gaps depends weakly on the hole concentration and that the relative in-plane gap reduction with doping is insensitive to anisotropies of the parent compound. The obtained magnon gaps agree with experiments on LSCO, in particular the strong in-plane gap reduction with doping is reproduced.
