in the P4/nmm structure*

Since its first synthesis and structure determination in 1975, KCoO2 in the P4/nmm structure has not received any further attention apart from a recent thesis exploring a novel synthesis route. The layered structure features Co on the corners of a square with O in the center of each square edge similar to the Cu-O arrangement in cuprates. Co is situated at the center of a square pyramid with one O tightly bound to the Co perpendicular to the layers and a K atom opposite. This unique structure combined with the d⁶ configuration of Co³⁺ leads to interesting electronic structure, which is here investigated for the first time. We performed quasiparticle self-consistent GW calculations including spin polarization. Analysis of the partial wave decomposition of the bands explains the occurrence of a large magnetic moment of 4 Bohr magneton. In the ferromagnetic form, the valence band consists entirely of majority spin while the conduction band minimum has rather flat bands of minority spin with a large gap or about 4 eV.  The spins are found to order antiferromagnetically along the [110] direction and this can be ascribed to superexchange.  Calculations of the optical dielectric function using the Bethe Salpeter equation for spin-conserving transitions indicate large exciton binding energies.