PHASE-SEPARATION AND VALENCE INSTABILITIES IN CUPRATE SUPERCONDUCTORS- EFFECTIVE ONE-BAND-MODEL APPROACH

We study the Cu-O valence instability (VI) and the related phase separ ation (PS) driven by Cu-O nearest-neighbor repulsion U-pd, using an ef fective extended one-band Hubbard (H-eff) obtained from the extended t hree-band Hubbard, through an appropriate low-energy reduction. N-eff is solved by exact diagonalization of a square cluster with 10 unit ce lls and also within a slave-boson mean-field theory. Its parameters de pend on doping for U(pd)double dagger 0 and on-site 0 repulsion U-p do uble dagger 0. The results using both techniques coincide in that ther e is neither VI nor PS for doping values levels x < 0.5 if U-pd greate r than or equal to 2 eV. The PS region begins for U-pd greater than or equal to 2 at large doping x > 0.6 and increases with increasing U-pd . The PS also increases with increasing on-site Cu repulsion U-d.