The normal state electronic structure of cuprate superconductors: a covalent picture

We propose a view of the cuprate superconductors based on strong covalent bonding within the CuO 2 planes. This covalent picture was developed from an alternative analysis of some recently reported spectroscopic and transport properties of several high temperature superconductors. In this view, antiferromagnetic superexchange splits the σ x 2− y 2 orbital, and crystal field splitting leaves the π ∗ xy orbital as the highest occupied molecular orbital (HOMO). In undoped materials, electrons in this orbital are localized, with peak approximately 1.7 eV below the unoccupied half of the σ ∗ x 2− y 2 band. With doping, this orbital broadens into a band where the Fermi energy overlaps the energy of the lowest unoccupied state in the σ x 2− y 2 band. Further, the axial ligand found on all p-type cuprate superconductors raises the energy of the nearly-localized σ ∗ z 2 orbital to an energy slightly below the top of the π ∗ xy band. Excitations of electrons in these three bands yield the combination of properties found in the normal state of the cuprate materials.