title: Charge and spin ordering in insulating Na0.5CoO2: effects of correlation and symmetry.
abstract: Ab initio band theory including correlations due to intra-atomic repulsion is applied to study charge disproportionation and charge and spin ordering in insulating Na0.5CoO2. Various ordering patterns (zigzag and two striped) for four-Co supercells are analyzed before focusing on the observed "out-of-phase stripe" pattern of antiferromagnetic Co4+ spins along charge-ordered stripes. This pattern relieves frustration and shows distinct analogies with the cuprate layers: a bipartite lattice of antialigned spins, with axes at 90degrees angles. Substantial distinctions with cuprates are also discussed, including the tiny gap of a new variant of "charge-transfer" type within the Co 3d system.
authors.affiliation: Department of Physics, University of California, Davis, California 95616, USA.
authors.forename: K-W
authors.initials: KW
authors.lastname: Lee
authors.forename: W E
authors.initials: WE
authors.lastname: Pickett