Charge-Transfer and dd excitations in AgF_{2}

Abstract

Charge-transfer 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 charge-transfer insulator silver fluoride AgF_{2}, which has been proposed as an excellent analog of oxocuprates. Optical conductivity and resonant inelastic x-ray scattering on AgF_{2} polycrystalline sample show a close similarity with that measured on undoped La_{2}CuO_{4}. While the former shows a charge-transfer gap ∼3.4 eV, larger than in the cuprate, dd excitations are nearly at the same energy in the two materials. Density functional theory 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 charge-transfer 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.