Hidden kagome-lattice picture and origin of high conductivity in delafossite PtCoO2

We study the electronic structure of delafossite PtCoO2 to elucidate its extremely small resistivity and high mobility. The band exhibits steep dispersion near the Fermi level despite the fact that it is formed mainly by Pt 𝑑 orbitals that are typically localized. We propose a picture based on two hidden kagome-lattice-like electronic structures: one originating from Pt 𝑠+𝑝𝑥/𝑝𝑦 orbitals, and the other from Pt 𝑑3⁢𝑧2−𝑟2+𝑑𝑥⁢𝑦/𝑑𝑥2−𝑦2 orbitals, each placed on the bonds of the triangular lattice. In particular, we find that the underlying Pt 𝑠+𝑝𝑥/𝑝𝑦 bands actually determine the steepness of the original dispersion, so that the large Fermi velocity can be attributed to the large width of the Pt 𝑠+𝑝𝑥/𝑝𝑦 band. In addition, the kagome-like electronic structure gives rise to “orbital-momentum locking” on the Fermi surface, which reduces the electron scattering by impurities. We conclude that the combination of the large Fermi velocity and the orbital-momentum locking is likely to be the origin of the extremely small resistivity in PtCoO2.