Superconductivity Near Phase Separation in Models of Correlated Electrons

TL;DR

This paper investigates various correlated electron models, revealing that superconductivity often occurs near phase separation, with different pairing symmetries and phases identified through numerical and analytical methods.

Contribution

It demonstrates that multiple models of correlated electrons tend to exhibit superconductivity near phase separation, highlighting the significance of this regime for understanding superconducting phases.

Findings

Superconductivity near phase separation in the 2D t-J model at quarter-filling.

Transition from d_{x^2-y^2} to s-wave pairing as density decreases.

Superconducting correlations also found in the two-band Hubbard model on a chain.

Abstract

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.