Electronic structure of misfit-layered calcium cobaltite

We have performed the first-principles calculations on (Ca₂CoO₃)₄(CoO₂)₆ to understand electronic structures of the misfit-layered calcium cobaltite, (Ca₂CoO₃)_xCoO₂, within the generalized gradient approximation. The optimized structure, consisting of a triple rock-salt-type Ca₂CoO₃ subsystem and a CdI₂-type CoO₂ subsystem in which their respective octahedra are significantly distorted, gives good agreement with recent experiment. The calculated electronic structures show two-dimensionally dispersive e_g bands across the Fermi energy, which yield a p-type conductivity in the rock-salt subsystem, while the Fermi energy lies in the crystal field gap of the d states in the CoO₂ subsystem. The calculated density of states is consistent with features of the measured X-ray photoemission spectra. The spin-polarized calculations show that ferrimagnetic ordering with a small net magnetic moment within the rock-salt subsystem is found to be the ground state, which gives a reasonable explanation to low-temperature behavior of the resistivity and the susceptibility observed in experiment.