Phase separation and valence instabilities in cuprate superconductors. Effective one-band model approach

TL;DR

This study investigates phase separation and valence instabilities in cuprate superconductors using an effective one-band Hubbard model derived from a three-band model, revealing the conditions under which phase separation occurs.

Contribution

It introduces an effective one-band model approach to analyze valence instability and phase separation in cuprates, connecting these phenomena to specific interaction parameters.

Findings

No valence instability or phase separation for doping x<0.5 if U_{pd} ≤ 2 eV.

Phase separation begins at U_{pd} ≥ 2 eV for doping x>0.6.

Phase separation increases with higher U_{pd} and U_d.

Abstract

We study the Cu-O valence instability (VI) and the related phase separation (PS) driven by Cu-O nearest-neighbor repulsion $U_{pd}$, using an effective extended one-band Hubbard model ($H_{eff}$) obtained from the extended three-bandHubbard model, through an appropriate low-energy reduction. $H_{eff}$ is solved by exact diagonalization of a square cluster with 10 unit cells and also within a slave-boson mean-field theory. Its parameters depend on doping for $U_{pd}\neq 0$ or on-site O repulsion $U_p\neq 0$. The results using both techniques coincide in that there is neither VI nor PS for doping levels $x<0.5$ if $U_{pd}\lesssim 2$ eV. The PS region begins for $U_{pd}\gtrsim 2$ eV at large doping $x>0.6$ and increases with increasing $U_{pd}$. The PS also increases with increasing on-site Cu repulsion $U_d$.