The Emery model for the HTS cuprates: oxygen role reexamined

Mean field slave boson approximation is performed analytically on the extended Emery model for the CuO2 conducting plane. The model is parameterized by Cu–O charge transfer energy Δpd, copper–oxygen overlap t0, oxygen–oxygen overlap t′ and Coulomb interaction U on the copper site taken as infinite. Special emphasis is placed on the role of t′ in the renormalization trends of the effective band parameters Δpf and t, replacing Δpd and t0, at small doping δ. It is shown that small, negative t′ expands the range of stability of the metallic phase, changing, through the higher order corrections in t′, the nature of the metal–insulator transition point. In the non-perturbative limit, t′ modifies qualitatively the renormalization of Δpf, making it saturate at the value of 4|t′|. Finite doping suppresses the insulating state approximately symmetrically with respect to its sign. The regime Δpf≈4|t′| fits very well the ARPES spectra of Y123, Bi2212 and LSCO and also explains, in the latter case, the evolution of the FS with doping accompanied by the spectral weight transfer from the oxygen to the resonant band.