Computational characterization of the RIXS Raman-to-fluorescence crossover in BaFe₂As₂

Resonant inelastic X-ray scattering (RIXS) studies have significantly enhanced our understanding of correlated materials. However, experimental and computational efforts have largely focused on insulating materials. We recently collected RIXS data on metallic BaFe₂As₂ at the Fe L₃ edge, which exhibits a Raman-to-fluorescence crossover as the absorption threshold is traversed. By combining core and valence level Bethe-Salpeter solvers, we evaluate the RIXS cross section of BaFe₂As₂ from first principles. Our calculations capture the Raman-to-fluorescence crossover as well as the main loss features observed in the experiment. By i) considering additionally the absorption and non-resonant emission spectra, ii) invoking the threshold singularity theory of Nozières and Abrahams [1], iii) recognizing the role of the intermediate state lifetime, and iv) decomposing the orbital character of the intermediate and final excitonic states, we are able to quantitatively and qualitatively separate simpler band structure contributions from more complex many-body effects in the RIXS spectra of BaFe₂As₂. This analysis is applicable to other strongly correlated metals.

[1] Nozières and Abrahams, Phys. Rev. B 10, 3099 (1974)