Electronic structure and valence-bond band structure of cuprate superconducting materials

From ab initio calculations on various clusters representing the La _2-xSr_xCu₁O₄ and Y₁Ba ₂Cu₃O₇ classes of high-temperature superconductors, it is shown that (i) all copper sites have a Cu^II (d⁹) oxidation state with one unpaired spin that is coupled antiferromagnetically to the spins of adjacent Cu^II sites; (ii) oxidation beyond the cupric (Cu^II) state leads not to Cu ^III but rather to oxidized oxygen atoms, with an oxygen pπ hole bridging two Cu^II sites; (iii) the oxygen pπ hole at these oxidized sites is ferromagnetically coupled to the adjacent Cu^II 4 electrons despite the fact that this is opposed by the direct dd exchange; and (iv) the hopping of these oxygen pπ holes (in CuO sheets or chains) from site to site is responsible for the conductivity in these systems (N-electron band structures are reported for the migration of these localized charges).