Charge Transfer and $dd$ excitations in AgF${}_2$

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.

14 pages, 9 Figures (including SI)