Ab initio molecular orbital calculations of effective exchange integrals for transition metal oxides and halides: Strong superexchange interactions and high T c superconductivity

Abstract

Ab initio unrestricted Hartree-Fock (UHF) calculations involving the correlation corrections by the Møller-Plesset (MP) perturbation method are carried out for K 2NiF 4-type transition-metal halides and related copper oxides. The magnitude of effective exchange integrals (Jab) obtained by the spin-projected UHF MP method is found to be particularly large for copper oxides, in agreement with recent experiments. Implications of these results are discussed in relation to spin-mediated theories for the mechanism of high-T c superconductivity of copper oxides.