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VOLUME 84 (2006) | ISSUE 12 | PAGE 769
Electronic theory for itinerant in-plane magnetic fluctuations in NaxCoO2
PACS: 31.15.Ar, 71.10.-w, 74.70.-b, 75.40.Cx
Abstract
Starting from ab-initio band structure for NaxCoO2, we derive the single-electron energies and the effective tight-binding description for the t2g 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 Γ point of the first Brillouin zone forms. Therefore, in addition to a large Fermi surface an electron pocket close to the Γ 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 xm, in agreement with neutron scattering data. Below xm the magnetic susceptibility shows a tendency towards the antiferromagnetic fluctuations. We estimate the value of 0.56 < xm < 0.68 within the rigid band model and within the Hubbard model with infinite on-site Coulomb repulsion consistent with the experimental phase diagram.