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

TL;DR

This study reveals that KCuO_2 is an unusual negative-charge-transfer insulator with Cu in a high formal valence state, featuring significant ligand-hole character and a band-gap driven by strong hybridization, challenging conventional understanding.

Contribution

It demonstrates the existence of a negative-charge-transfer insulating state in KCuO_2 with high Cu valence and ligand-hole character, supported by combined spectroscopy and first-principles calculations.

Findings

KCuO_2 has Cu in a formal +3 valence state.

The ligand-to-metal charge transfer energy is negative (~ -1.5 eV).

The band-gap arises mainly from Cu 3d - O 2p hybridization.

Abstract

Using a combination of X-ray absorption spectroscopy experiments with first principle calculations, we demonstrate that insulating KCuO_2 contains Cu in an unusually-high formal-3+ valence state, 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 Cu^{3+} compounds, the Cu 2p XAS spectra of KCuO_2 exhibits pronounced 3d^8 (Cu^{3+}) multiplet structures, which accounts for ~40% of its ground state wave-function. Ab-initio calculations elucidate the origin of the band-gap in KCuO_2 as arising primarily from strong intra-cluster Cu 3d - O 2p hybridizations (t_{pd}); the value of the band-gap decreases with reduced value of t_{pd}. Further, unlike conventional negative charge-transfer insulators, the band-gap in 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.