Quantifying vibronic coupling with resonant inelastic X-ray scattering

Electron-phonon interactions are fundamental to the behavior of chemical and physical systems. Various methods exist to quantify these interactions, however, none are entirely satisfactory for crystalline materials with dispersive phonons. In recent years, resonant inelastic X-ray scattering (RIXS) has been proposed as a new technique that can probe momentum-dependent electron-phonon interactions in crystalline materials with better resolution with respect to the phonon mode and momentum as well as the electronic orbital and momentum. We first summarize theoretical progress on understanding and interpreting RIXS measurements of vibronic coupling, and then outline a path toward eventual predictive first-principles calculations of the phonon contribution to RIXS spectra in the case of dispersive phonons. Particular attention is given to the relation between the coupling constant measured by RIXS, which relates to exciton-phonon scattering, and the standard electron-phonon coupling probed by transport measurements. We discuss first-principles calculation of this exciton-phonon coupling parameter. Example calculations are provided for crystalline MgO.