Electronic structure of high-T c superconductors and related compounds

We study a 5-band Hubbard model for the CuO₂ planes in cuprate super-conductors using Hartree-Fock mean-field theory including spiral spin density waves. For the half-filled case we recover a ZSA-like phase diagrambut with an additional new region characterized by strong covalency effects, which we call a covalent insulator region. We also provide a nonperturbative calculation ofJ _eff, the effective in-plane antiferromagnetic interaction, as a function of parameters of the model. We suggest that the high-T _c cuprates are in or very close to the covalent insulator region and within this we show that a consistent explanation of apparently conflicting high energy spectroscopic and magnetic measurements of the high-T _c cuprates can be given.