Electronic structure of misfit-layered calcium cobaltite

Abstract

We have performed the first-principles calculations on $({\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}{)}_{4}({\mathrm{CoO}}_{2}{)}_{6}$ to understand electronic structures of the misfit-layered calcium cobaltite, $({\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}{)}_{x}{\mathrm{CoO}}_{2},$ within the generalized gradient approximation. The optimized structure, consisting of a triple rocksalt-type ${\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}$ subsystem and a ${\mathrm{CdI}}_{2}$-type ${\mathrm{CoO}}_{2}$ subsystem in which their respective octahedra are significantly distorted, shows good agreement with recent experiment. The calculated electronic structures include two-dimensionally dispersive ${e}_{g}$ bands across the Fermi energy, which yield the p-type conductivity in the rocksalt subsystem, while the Fermi energy lies in the crystal-field gap of the d states in the ${\mathrm{CoO}}_{2}$ subsystem. The spin-polarized calculations show that antiferromagnetic or ferrimagnetic ordering with a small net magnetic moment within the rocksalt subsystem is found to be the ground state, which gives a reasonable explanation of low-temperature behavior of the resistivity and the susceptibility observed in experiment.