Electronic Structure and Valence-Bond Band Structure of Cuprate Superconducting Materials

Abstract

From ab initio calculations on various clusters representing the La_2-xSr_xCu₁O₄ and Y₁Ba₂Cu₃O₇ 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 pπhole at these oxidized sites is ferromagnetically coupled to the adjacent Cu^IId 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).