Anomalous charge and negative-charge-transfer insulating state in cuprate chain compound KCuO2

Using a combination of x-ray absorption spectroscopy (XAS) experiments and first-principles calculations, we demonstrate that insulating KCuO2 contains Cu in an unusually high formal 3+ valence state, and the ligand-to-metal (O-to-Cu) charge-transfer energy is intriguingly negative ( Delta ~ -1.5 eV) and has a dominant (~60%) ligand-hole character in the ground state akin to the high Tc cuprate Zhang-Rice state. Unlike most other formal Cu3+ compounds, the Cu 2p XAS spectra of KCuO2 exhibit pronounced 3d8 (Cu3+) multiplet structures, which account for ~40% of its ground state wave function. Ab initio calculations elucidate the origin of the band gap in KCu02 as arising primarily from strong intracluster Cu 3d-O 2p hybridizations (tpd); the value of the band gap decreases with a reduced value of tpd. Further, unlike conventional negative-charge-transfer insulators, the band gap in KCuO2 persists even for vanishing values of Coulomb repulsion U, underscoring the importance of single-particle band-structure effects connected to the one-dimensional nature of the compound.