Successive Phase Transitions in Na_{0.5}CoO_{2} based on the Triangular Lattice d-p Model

TL;DR

This study models the electronic states of Na_{0.5}CoO_{2} using a detailed d-p model on a triangular lattice, revealing successive phase transitions including metallic antiferromagnetism and orbital order leading to an insulator.

Contribution

It introduces a realistic 11-band d-p model with Coulomb interactions to explain the successive phase transitions in Na_{0.5}CoO_{2}.

Findings

Metallic antiferromagnetism below T_{c1}.

Orbital and charge order leading to an insulating state below T_{c2}.

Results align with experimental observations of phase transitions.

Abstract

We investigate the electronic states of the CoO_2 plane in the layered cobalt oxides Na_{0.5}CoO_{2} using the realistic 11 band d-p model on a two-dimensional triangular lattice. Effects of 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 J', are treated within the Hartree-Fock approximation. It is found that the metallic antiferromagnetism takes place below T_{c1} and, in addition, the orbital order accompanied by the small charge order takes place below T_{c2} (< T_{c1}) where the system becomes insulator. The obtained results are consistent with the successive phase transitions observed in Na_{0.5}CoO_{2}.