Antiferromagnetic, Charge and Orbital Order in Na0.5CoO2 Based on the Two Dimensional 11 Bands d-p Model

The electronic state of the CoO2 plane in the layered cobalt oxide Na0.5CoO2 is investigated on the basis of 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 and an effective one dimensional potential is added so as to include the effect of the Na order. We treat the Coulomb interaction on a Co site: the intra- and inter-orbital direct terms U and U', the exchange coupling J and the pair-transfer, within the Hartree-Fock approximation. It is found that the metallic antiferromagnetic order takes place for T-c2 < T < T-c1 and the coexistent state of the antiferromagnetic, orbital and charge orders takes place for T < T-c2 where the system becomes insulator. The obtained results are consistent with the successive phase transitions observed in Na0.5CoO2