Electronic theory for itinerant in-plane magnetic fluctuations in Na${}_x$CoO${}_2$

Abstract

Starting from {\it ab-initio} band structure for Na${}_x$CoO${}_2$, we derive the single-electron energies and the effective tight-binding description for the $t_{2g}$ bands using a projection procedure. We find that due to the presence of the next-nearest-neighbor hoppings a local minimum in the electronic dispersion close to the $\Gamma$ point of the first Brillouin zone forms. Therefore, in addition to a large Fermi surface an electron pocket close to the $\Gamma$ point emerges at high doping concentrations. The latter yields the new scattering channel resulting in a peak structure of the itinerant magnetic susceptibility at small momenta. This indicates itinerant in-plane ferromagnetic state above certain critical concentration $x_m$, in agreement with neutron scattering data. Below $x_m$ the magnetic susceptibility shows a tendency towards the antiferromagnetic fluctuations. We estimate the value of $0.58<x_m<0.7$ within the rigid band model and within the Hubbard model with infinite on-site Coulomb repulsion consistent with the experimental phase diagram.

arXiv comment4 pages, 4 figures; LDA calculations were done with Na in the symmetric 2d position contrary to the 6h position in a previous version of this paper

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• The origin of a${}_{1g}$ and e${}_g$' orderings in Na${}_x$CoO${}_2$
• Spin fluctuations, magnetic long-range order and Fermi surface gapping in NaxCoO2
• Role of electronic correlations in the Fermi surface formation of Na${}_x$CoO${}_2$