Electronic State of NaxCoO2 Based on the Two Dimensional Triangular Lattice d-p Model

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The electronic state in a CoO2 plane of the layered cobalt oxides NaxCoO2 is investigated by using the 11 band d–p model on a two-dimensional triangular lattice, where the tight-binding parameters are determined so as to fit the LDA band structure. Effects of the Coulomb interaction at a Co site: the intra- and inter-orbital direct terms U and U', the exchange coupling J and the pair-transfer J', are treated within the Hartree–Fock approximation. We also consider the effect of the Na order at x = 0.5, where Na ions form one-dimensional chains, by taking into account an effective one-dimensional potential Δεd on the CoO2 plane. It is found that the Na order enhances the Fermi surface nesting, resulting in the antiferromagnetism (AFM) which is suppressed due to the frustration effect in the case without the Na order. When U and Δεd are varied, we observe three types of AFM: (1) metallic AFM with large density of states NF at the Fermi level for small values of U and Δεd, (2) the semimetallic AFM with tiny NF for large U with small Δεd, and (3) the insulating AFM with a finite energy gap for large values of U and Δεd.
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