Theory of dispersive optical phonons in resonant inelastic x-ray scattering experiments

The community currently lacks a complete understanding of how resonant inelastic x-ray scattering (RIXS) experiments probe the electron-phonon interaction in solids. For example, most theoretical models of this process have focused on dispersionless Einstein phonons. Using a recently developed momentum average variational approximation for computing RIXS spectra of band insulators, we examine the influence of both electron and phonon dispersion in the intermediate state of the scattering process. We find that the inclusion of either, and their mutual interplay, introduces significant momentum variations in the RIXS intensity, even for momentumindependent electron-phonon coupling. The phonon dispersion also induces nontrivial changes in the excitation line shapes, which can have a quantitative impact on the data analysis. These results highlight the considerable challenges of interpreting RIXS data in actual materials.

Automated summary

The community currently lacks a complete understanding of how RIXS experiments probe the electron-phonon interaction in solids. In this study, we use a variational approximation method to calculate RIXS spectra in band insulators and investigate the influence of electron and phonon dispersion in the intermediate state of the scattering process. Our results show that even for momentum-independent electron-phonon coupling, the inclusion of electron and phonon dispersion and their interplay lead to significant momentum variations in RIXS intensity and nontrivial changes in the excitation line shapes. These findings highlight the challenges in interpreting RIXS data in real materials.