Electronic structure of misfit-layered calcium cobaltite

We have performed the first-principles calculations on $\left({\mathrm{Ca}}_2{\mathrm{CoO}}_3{)}_4\right({\mathrm{CoO}}_2{)}_6$ 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.