Oxygen-Related Band-Features of the Extended Emerymodel for the Hts Cuprates

Abstract

We examine the band properties of the extended Emery model, parameterized by Cu-O charge transfer energy Δ_pd, copper-oxygen overlap t₀, oxygen-oxygen overlap t′ and Coulomb interaction U on the copper site. In the charge-transfer limit U ≫ Δ_pd > t, t′, it retains realistic three band structure, unlike the usually used Hubbard model. Neglecting the ordering of spin degrees of freedom, infinite U slave boson mean field approximation is used to calculate at small doping δ the renormalization trends of the effective band parameters Δ_pf and t, replacing Δ_pd and t₀, while t′, related exclusively to oxygens, remains unchanged. 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 thermodynamical nature of the metal-insulator transition point. In the nonperturbative limit, t′ modifies qualitatively the renormalization of Δ_pf at zero doping, making it saturate at the value of 4|t′| and keeping thus the system in the regime of strong correlations. Finite doping δ suppresses the insulating t = 0 state approximately symmetrically with respect to its sign. The regime δ_pf ≈ 4|t′| fits very well the ARPES spectra of LSCO and also explains the evolution of the FS with doping accompanied by the spectral weight- transfer from the oxygen to the resonant band. Additionally, the values of the band- parameters Δ_pf /t′ and t/t′ are allowed to vary freely, in order to describe the ARPES data on LSCO, thus providing the direct insight in the role of oxygens, underestimated in the magnetic t-J limit of the large U Emery model. The important role in this theory is played by the effective Cu-O overlap t, very small in the underdoped HTS cuprates and evolving to the values of the order of t′ at larger doping.