Electron correlation and fermi surface topology of NaxCoO2.

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 local-density approximation calculations have not been observed as the associated e'(g) band fails to cross the Fermi level for a wide range of sodium doping concentration x. In addition, significant bandwidth renormalizations of the t(2g) complex have been observed. We show that these discrepancies are due to strong electronic correlations by studying the multiorbital 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.