Abstract
Using a combination of x-ray absorption spectroscopy (XAS) experiments and first-principles calculations, we demonstrate that insulating contains Cu in an unusually high formal 3+ valence state, and the ligand-to-metal (O-to-Cu) charge-transfer energy is intriguingly negative ( eV) and has a dominant () ligand-hole character in the ground state akin to the high cuprate Zhang-Rice state. Unlike most other formal compounds, the Cu XAS spectra of exhibit pronounced () multiplet structures, which account for of its ground state wave function. Ab initio calculations elucidate the origin of the band gap in as arising primarily from strong intracluster Cu -O hybridizations (); the value of the band gap decreases with a reduced value of . Further, unlike conventional negative-charge-transfer insulators, the band gap in persists even for vanishing values of Coulomb repulsion , underscoring the importance of single-particle band-structure effects connected to the one-dimensional nature of the compound.
- Received 22 April 2015
- Revised 3 November 2015
DOI:https://doi.org/10.1103/PhysRevB.92.201108
©2015 American Physical Society