Vibronic excitations in the resonant inelastic x-ray scattering spectra of spin-orbit Mott insulators

Resonant inelastic x-ray scattering (RIXS) spectroscopy is a powerful tool to investigate elementary excitations in spin-orbit Mott insulators. Recent RIXS data of cubic Ru
⁴
+ and Ir
⁵
+ antifiluorites show puzzling features that cannot be explained by spin-orbit theories: the relevance of the dynamic JahnTeller effect was mentioned, whereas the evidence and its fingerprints in the RIXS spectra are unknown. Here, we address these issues based on ab initio calculations. We simulated the RIXS spectra fully taking account of the dynamic Jahn- Teller effect, which shows a good agreement with the experimental spectra, confirming the presence of the dynamic Jahn- Teller effect in the excited states. We revealed that the experimental RIXS spectra have several peaks that come from the dynamic Jahn- Teller effect rather than the standard crystal-field splitting. Our results suggest that the dynamic Jahn- Teller effect is an important factor in adequately interpreting the RIXS spectra of spin-orbit Mott insulators.