Electronic structure of high-$T_c$ superconductors and related compounds

The authors study a 5-band Hubbard model for the $CuO_2$ planes in cuprate superconductors using Hartree-Fock mean-field theory including spiral spin density waves. For the half-filled case they recover a ZSA-like phase diagram but with an additional new region characterized by strong covalency effects, which they call a covalent insulator region. They also provide a nonperturbative calculation of $J_{eff}$, the effective in-plane antiferromagnetic interaction, as a function of parameters of the model. They suggest that the high-$T_c$ cuprates are in or very close to the covalent insulator region and within this they show that a consistent explanation of apparently conflicting high energy spectroscopic and magnetic measurements of the high-$T_c$ cuprates can be given