Electron correlation and Fermi surface topology of NaₓCoO₂

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The electronic structure of NaₓCoO₂ revealed by recent photoemission experiments shows important deviations from band theory predictions. The six small Fermi surface pockets predicted by LDA calculations have not been observed as the associated eg′ band fails to cross the Fermi level for a wide range of sodium doping concentration x. In addition, significant bandwidth renormalizations of the t₂g complex have been observed. We show that these discrepancies are due to strong electronic correlations by studying the multi-orbital Hubbard model in the Hartree-Fock and strong-coupling Gutzwiller approximation. The quasiparticle dispersion and the Fermi surface topology obtained in the presence of strong local Coulomb repulsion are in good agreement with experiments.