Charge Transfer and $dd$ excitations in AgF$_{2}$

TL;DR

This study investigates AgF$_2$, a charge transfer insulator similar to cuprates, revealing its electronic structure, covalency, and potential proximity to charge density wave phases, supporting its relevance for high-temperature superconductor research.

Contribution

It provides experimental and theoretical insights into AgF$_2$, demonstrating its similarities to cuprates and highlighting its covalent nature and proximity to charge transfer instability.

Findings

AgF$_2$ has a charge transfer gap of ~3.4 eV, larger than cuprates.

AgF$_2$ exhibits similar $dd$ excitations to La$_2$CuO$_4$.

AgF$_2$ is more covalent and near a charge transfer instability.

Abstract

Charge transfer (CT) insulators are the parent phase of a large group of today's unconventional high-temperature superconductors. Here we study experimentally and theoretically the interband excitations of the CT insulator silver fluoride AgF$_2$, which has been proposed as an excellent analogue of oxocuprates. Optical conductivity and resonant inelastic X-ray scattering (RIXS) on AgF$_2$ polycrystalline sample show a close similarity with that measured on undoped La$_2$CuO$_4$. While the former shows a CT gap $\sim$3.4 eV, larger than in the cuprate, $dd$ excitations are nearly at the same energy in the two materials. DFT and exact diagonalization cluster computations of the multiplet spectra show that AgF$_2$ is more covalent than the cuprate, in spite of the larger fundamental gap. Furthermore, we show that AgF$_2$ is at the verge of a charge transfer instability. The overall resemblance of our data on AgF$_2$ to those published previously on La$_2$CuO$_4$ suggests that the underlying CT insulator physics is the same, while AgF$_2$ could also benefit from a proximity to a charge density wave phase as in BaBiO$_3$. Therefore, our work provides a compelling support to the future use of fluoroargentates for materials' engineering of novel high-temperature superconductors.