Anomalous charge and negative-charge-transfer insulating state in cuprate chain-compound KCuO_2

Using a combination of x-ray absorption spectroscopy (XAS) experiments and first-principles calculations, we demonstrate that insulating ${\mathrm{KCuO}}_{2}$ contains Cu in an unusually high formal 3+ valence state, and the ligand-to-metal (O-to-Cu) charge-transfer energy is intriguingly negative ($\mathrm{\ensuremath{\Delta}}\ensuremath{\sim}\ensuremath{-}1.5$ eV) and has a dominant ($\ensuremath{\sim}60%$) ligand-hole character in the ground state akin to the high ${T}_{c}$ cuprate Zhang-Rice state. Unlike most other formal ${\mathrm{Cu}}^{3+}$ compounds, the Cu $2p$ XAS spectra of ${\mathrm{KCuO}}_{2}$ exhibit pronounced $3{d}^{8}$ (${\mathrm{Cu}}^{3+}$) multiplet structures, which account for $\ensuremath{\sim}40%$ of its ground state wave function. Ab initio calculations elucidate the origin of the band gap in ${\mathrm{KCuO}}_{2}$ as arising primarily from strong intracluster Cu $3d$-O $2p$ hybridizations (${t}_{\mathrm{pd}}$); the value of the band gap decreases with a reduced value of ${t}_{\mathrm{pd}}$. Further, unlike conventional negative-charge-transfer insulators, the band gap in ${\mathrm{KCuO}}_{2}$ 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.