Evolution of Co charge disproportionation with Na order in Na$_{x}$CoO$_{2}$

Abstract

$^{59}$Co NMR experiments have been performed on single crystals of the layered cobaltate Na$_{x}$CoO$_{2}$ with x=0.77 which is an antiferromagnet with N\'eel temperature $T_{N}=22$~K. In this metallic phase six Co sites are resolved in the NMR spectra, with distinct quadrupole frequencies $\nu _{Q}$, magnetic shifts $K_{ZZ}$ and nuclear spin lattice relaxation rates $% 1/T_{1}$. Contrary to the $x=1/2$ or $x=2/3$ phases the 3D stacking of the Na planes is not perfect for $x=0.77$ but this does not influence markedly the electronic properties. We evidence that the magnetic and charge properties of the Co sites are highly correlated with each other as $K_{ZZ}$ and $(1/T_{1})^{1/2}$ scale linearly with $\nu _{Q}$. The data analysis allows us to separate the contribution $\nu_{Q}^{latt}$ of the ionic charges to $\nu _{Q}$ from that $\nu _{Q}^{el}$ due to the hole orbitals on the Co sites. We could extend coherently this analysis to all the known phases in the Na cobaltate phase diagram. The variation with $x$ of $\nu _{Q}^{latt}$ is found to fit rather well numerical computations done in a point charge model. The second term $\nu _{Q}^{el}$ allowed us to deduce the hole concentration on the cobalts. These detailed experimental results should stimulate theoretical calculations of the electronic structure involving both the Co orbital configurations and DMFT approaches to take into account the electronic correlations.