Electronic structure of a delafossite oxide CuAlO₂ in comparison with CuCrO₂

We investigated the electronic structure of a delafossite oxide CuAlO₂ in comparison with CuCrO₂, using vacuum ultraviolet (VUV) and hard x-ray (HX) photoemission spectroscopy (PES) and band structure calculations within the local-density approximation +U scheme. Cu 3p-3d resonant PES and Cu 2p core-level HX-PES give concrete evidence that the Cu valence in CuAlO₂ is monovalent, which makes a sharp contrast to CuCrO₂ that showed contradictory results depending upon probing methods [Yokobori et al., Phys. Rev. B 87, 195124 (2013)]. We further noticed weak but detectable satellite structures due to Cu²⁺ state in the Cu 2p HX-PES spectrum of CuCrO₂, surprisingly, without an observable main peak due to the Cu²⁺ state. Comparisons between the valence-band HX-PES spectra and the band structure calculations revealed a significant amount of the Cu 4s and Al 3s=Cr 4s states in the valence band. The above results can reasonably be explained by the Cu 3d, 4s-Cr 3d charge transfer model that we previously proposed in the above paper, thereby demonstrating the critical role of empty 3d orbitals in the electronic structure of the copper delafossite oxides.