title: Disproportionation, metal-insulator transition, and critical interaction strength in Na(1/2)CoO2.
abstract: Charge disproportionation (CD) and spin differentiation in Na(1/2)CoO2 are studied using the correlated band local-density approximation + Hubbard U (LDA+U) approach. The simultaneous CD and gap opening seen previously is followed in detail through a first-order charge disproportionation transition 2Co(3.5+)-->Co3++Co4+. Disproportionation in the Co a(g) orbital results in half of the ions (Co3+) becoming electronically and magnetically dead, transforming the quarter-filled a(g) system into a half-filled subsystem that subsequently undergoes the observed charge ordering or metal-insulator transition. Comparing with data in the x approximately 0.3 regime suggests the system has moved into the multiband regime where the effective Coulomb repulsion U-->U(eff)=U/sqrt[3] strongly lessens correlation effects.
authors.affiliation: Department of Physics, University of California, Davis, California 95616, USA.
authors.forename: K-W
authors.initials: KW
authors.lastname: Lee
authors.forename: J
authors.initials: J
authors.lastname: Kunes
authors.forename: P
authors.initials: P
authors.lastname: Novak
authors.forename: W E
authors.initials: WE
authors.lastname: Pickett