D3z2-r2 orbital in high-Tc cuprates: Excitonic spectrum, metal-insulator phase diagram, optical conductivity, and orbital character of doped holes

The single-site dynamical mean-field approximation is used to solve a model of high-T_c cuprate superconductors, which includes both d _x2-y2 and d_3z2-r2 orbitals on the Cu as well as the relevant oxygen states. Both T (with apical oxygen) and T^′ (without apical oxygen) crystal structures are considered. In both phases, inclusion of the d_3z2-r2 orbital is found to broaden the range of stability of the charge-transfer insulating phase. For equal charge-transfer energies and interaction strengths, the T^′ phase is found to be less strongly correlated than the T phase. For both structures, d-d excitons are found within the charge-transfer gap. However, for all physically relevant dopings, the Fermi surface is found to have only one sheet and the admixture of d_3z2-r2 into the ground-state wave function remains negligible (