A. Moreo — Page 3 | Searcharxiv

arXiv1994

Is there spin-charge separation in the 2D Hubbard and t-J models at low electronic densities?

The spin and density correlation functions of the two-dimensional Hubbard model at low electronic density $ $ are calculated in the ground state by using the power method, and at finite temperatures by using the quantum Monte Carlo technique. Both approaches produce similar results, which are in close agreement with numerical and high temperature expansion results for the two-dimensional ${\rm t-J}$ model. Using perturbative approximations, we show that the examination of the density correlation function alone is not enough to support recent claims in the literature that suggested spin and charge separation in the low electronic density regime of the ${\rm t-J}$ model.

cond-mat↗

arXiv1993

Superconductivity Near Phase Separation in Models of Correlated Electrons

Numerical and analytical studies of several models of correlated electrons are discussed. Based on exact diagonalization and variational Monte Carlo techniques, we have found strong indications that the two dimensional t-J model superconducts near phase separation in the regime of quarter-filling density, in agreement with previous results reported by Dagotto and Riera (Phys. Rev. Letters 70, 682 (1993)). At this density the dominant channel is d_{x^2-y^2}, but a novel transition to s-wave superconductivity is observed decreasing the electronic density. In addition, the one band t-U-V model has also been studied using the mean-field approximation that accurately described the spin density wave phase of the repulsive Hubbard model at half-filling. Finally, the two band Hubbard model on a chain is also analyzed. Superconducting correlations near phase separation exist in this model, as it occurs in the t-J model. Based on these nontrivial examples it is $conjectured$ that electronic models tend to have superconducting phases in the vicinity of phase separation.Reciprocally, if it is established that a model that does not phase separate, then its chances of presenting a superconducting phase are considerably reduced.

cond-mat↗