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

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 U pd , using an effective extended one-band Hubbard model (H ef f) obtained from the extended three-band Hubbard model, through an appropriate low-energy reduction. H ef f 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 = 0 or on-site O repulsion U p = 0. The results using both techniques coincide in that there is neither VI nor PS for doping levels x < 0.5 if U pd 2 eV. The PS region begins for U pd 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 .