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

Abstract

We study the Cu-O valence instability (VI) and the related phase separation (PS) driven by Cu-O nearest-neighbor repulsion ${\mathit{U}}_{\mathit{pd}}$, using an effective extended one-band Hubbard (${\mathit{H}}_{\mathrm{eff}}$) obtained from the extended three-band Hubbard, through an appropriate low-energy reduction. ${\mathit{H}}_{\mathrm{eff}}$ 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 ${\mathit{U}}_{\mathit{pd}}$\ensuremath{\ne}0 and on-site O repulsion ${\mathit{U}}_{\mathit{p}}$\ensuremath{\ne}0. The results using both techniques coincide in that there is neither VI nor PS for doping values levels x0.5 if ${\mathit{U}}_{\mathit{pd}}$\ensuremath{\gtrsim}2 eV. The PS region begins for ${\mathit{U}}_{\mathit{pd}}$\ensuremath{\gtrsim}2 at large doping x\ensuremath{\gtrsim}0.6 and increases with increasing ${\mathit{U}}_{\mathit{pd}}$. The PS also increases with increasing on-site Cu repulsion ${\mathit{U}}_{\mathit{d}}$. \textcopyright{} 1996 The American Physical Society.