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

Abstract

From ab initio calculations on various clusters representing the La_(2-x)Sr_xCu_1O_4 and Y_1Ba_2Cu_3O_7 classes of high-temperature superconductors, it is shown that (i) all copper sites have a Cu^(II)(d^9)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^(II)d 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).

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