Origin of the electron disproportionation in the metallic sodium cobaltates

Abstract

Recently the unusual metallic state with a substantially non-uniform distribution of a charge and mag\-ne\-tic density in CoO${}_2$ planes was found experimentally in the Na${}_x$CoO${}_2$ compound with $x>0.6$. We have investigated an origin of such electron disproportionation in the lamellar sodium cobaltates by calculating the ion states as a function of a strength of the electron correlations in the $d$(Co)-shells within the GGA+U approximation for the system with a realistic crystal structure. It was found that the nonuniformity of spin and charge densities are induced by an ordering of the sodium cations and enhanced correlations. Two important magnetic states of cobalt lattice competing with each other at realistic values of the correlation parameter were found~---~low spin hexagons (LS) and higher spin kagomé lattice (HS-KSL). In the heterogeneous metallic HS-KSL phase magnetic Co ions form a kagomé structure. In LS phase the kagomé pattern is decomposed into hexagons and Co ions possess minimal values of their spin. Coexistence of these states could explain the emergence of the disproportionation with the peculiar kagomé structure experimentally revealed in previous studies of the cobaltates.

References in corpus (22)

• Restoring the density-gradient expansion for exchange in solids and surfaces
• Generalized gradient approximation for solids and their surfaces
• Strong electronic correlations from Hund's coupling
• Fermi surface and quasiparticle dynamics of Na(x)CoO2 {x=0.7} investigated by Angle-Resolved Photoemission Spectroscopy
• Electron correlation and Fermi surface topology of Na${}_x$CoO${}_2$
• Doping effects on the electronic and structural properties of CoO2: An LSDA+U study
• Evidence of a single nonmagnetic Co3+ state in the Na1CoO2 cobaltate
• The Fermi surface of Na${}_x$CoO${}_2$
• Na Induced Correlations in Na${}_x$CoO${}_2$
• Quasiparticle coherence and the nature of the metal-insulator phase transition in Na${}_x$CoO${}_2$
• Disproportionation Transition at Critical Interaction Strength: Na${}_{1/2}$CoO${}_2$
• Na ordering imprints a metallic kagome lattice onto the Co planes of Na2/3CoO2
• An ab-initio evaluation of the local effective interactions in the superconducting compound $Na\_0.35CoO\_2-1.3H\_2O$
• Na atomic order, Co charge disproportionation and magnetism in Na${}_x$CoO${}_2$ for large Na contents
• Magnetic properties of spin-orbital polarons in lightly doped cobaltates
• Itinerant in-plane magnetic fluctuations and many-body correlations in Na${}_x$CoO${}_2$
• The origin of strong correlations and superconductivity in Na${}_x$CoO${}_2$
• Geometrical, electronic and magnetic properties of Na${}_{0.5}$CoO${}_2$ from first principles
• Na order and Co charge disproportionation in Na${}_x$CoO${}_2$
• Spin polaron theory for the photoemission spectra of layered cobaltates
• Electron-phonon interaction in the lamellar cobaltate Na${}_x$CoO${}_2$
• From Hubbard bands to spin-polaron excitations in the doped Mott material Na${}_x$CoO${}_2$