Charge-Transfer and dd excitations in AgF2

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 ${\mathrm{AgF}}_{2}$, which has been proposed as an excellent analog of oxocuprates. Optical conductivity and resonant inelastic x-ray scattering on ${\mathrm{AgF}}_{2}$ polycrystalline sample show a close similarity with that measured on undoped ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$. While the former shows a charge-transfer gap $\ensuremath{\sim}3.4$ 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 ${\mathrm{AgF}}_{2}$ is more covalent than the cuprate, in spite of the larger fundamental gap. Furthermore, we show that ${\mathrm{AgF}}_{2}$ is at the verge of a charge-transfer instability. The overall resemblance of our data on ${\mathrm{AgF}}_{2}$ to those published previously on ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ suggests that the underlying charge-transfer insulator physics is the same, while ${\mathrm{AgF}}_{2}$ could also benefit from a proximity to a charge density wave phase as in ${\mathrm{BaBiO}}_{3}$. Therefore, our work provides a compelling support to the future use of fluoroargentates for materials' engineering of novel high-temperature superconductors.