Regensburg 2007 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 13: Symposium “Superconductivity and Magnetism in Lamellar Cobaltates”

TT 13.8: Talk

Tuesday, March 27, 2007, 17:35–17:50, H18

Electronic theory for itinerant in-plane magnetic fluctuations and many-body correlations in Na_xCoO₂ — •Maxim Korshunov^1,2, Ilya Eremin^1,3, Alexey Shorikov⁴, and Vladimir Anisimov⁴ — ¹Max-Planck-Institut für Physik komplexer Systeme, D-01187 Dresden, Germany — ²L.V. Kirensky Institute of Physics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk, Russia — ³Institute für Mathematische und Theoretische Physik, TU Braunschweig, 38106 Braunschweig, Germany — ⁴Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620041 Yekaterinburg GSP-170, Russia

Based on the ab-initio band structure for Na_xCoO₂ · yH₂O we derive the single-electron energies and the effective tight-binding description for the t_2g bands using projection procedure. 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. Correspondingly, 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 dominant itinerant in-plane ferromagnetic fluctuations 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 further analyze the many-body effects on the electronic and magnetic excitations using various approximations applicable for different U/t ratio.