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.
ASJC Scopus subject areas
- Physics and Astronomy(all)