Origin of anisotropy and metallic behavior in delafossite PdCoO₂

The electronic structure of PdCoO₂ has been studied using the full potential linearized augmented plane wave (FP-LAPW) method employing the Perdew-Burke-Ernzerhof generalized-gradient approximations. The role of valence Pd and Co orbitals to the electrical conduction in PdCoO₂ has been analyzed in detail. Based on the electronic structure and electron density distribution, causes for the conducting behavior of the Pd layer and insulating property of CoO₂ layer have been identified. The present results support the proposal that anisotropy in the conductivity behavior can be attributed to metallic interactions between hybridized orbitals derived from mixing of Pd 4d_z², 4d_x²−y², and 4d_xy orbitals. However, a small contribution from Pd-5s orbital located at adjunct sites in the ab plane at the Fermi level was also found. In contrast, the conductivity in the c-direction is limited by the antibonding Pd-O states and the insulating state of CoO₂ layer. Good agreement is obtained for the calculated and observed Co-L₂-L₃ x-ray absorption spectrum confirming only very small contribution of Co density of states at the Fermi level.