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 AgF2, which has been proposed as an excellent analogue of oxocuprates. Optical conductivity and resonant inelastic X-ray scattering (RIXS) on AgF2 polycrystalline sample show a close similarity with that measured on undoped La2CuO4. While the former shows a CT gap ∼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 AgF2 is more covalent than the cuprate, in spite of the larger fundamental gap. Furthermore, we show that AgF2 is at the verge of a charge transfer instability. The overall resemblance of our data on AgF2 to those published previously on La2CuO4 suggests that the underlying CT insulator physics is the same, while AgF2 could also benefit from a proximity to a charge density wave phase as in BaBiO3. Therefore, our work provides a compelling support to the future use of fluoroargentates for materials' engineering of novel high-temperature superconductors.
@article{arxiv.2105.08862,
title = {Charge Transfer and $dd$ excitations in AgF$_{2}$},
author = {Nimrod Bachar and Kacper Koteras and Jakub Gawraczynski and Waldemar Trzcinski and Józef Paszula and Riccardo Piombo and Paolo Barone and Zoran Mazej and Giacomo Ghiringhelli and Abhishek Nag and Ke-Jin Zhou and José Lorenzana and Dirk van der Marel and Wojciech Grochala},
journal= {arXiv preprint arXiv:2105.08862},
year = {2022}
}