Testing Interpretations of Electron-Phonon Coupling as Measured by RIXS

Electron-phonon coupling plays a key role in many condensed matter phenomena, notably superconductivity. However, this quantity can be difficult to measure, particularly in many unconventional superconductors for which large, single crystals cannot be grown. Even in these challenging cases resonant inelastic X-ray scattering (RIXS) is sufficiently sensitive to probe both the phonon dispersion and electron-phonon coupling strength. Correctly quantifying the latter remains contentious. We turn to simple organic molecules, such as acetone, to perform quantitative tests on the interpretation of electron-phonon coupling as probed by RIXS. We use density functional theory and excited-state calculations to obtain parameters for typical model Hamiltonians and test whether the models, with calculated, non-adjustable parameters, can explain the experimental RIXS data. We apply our improved understanding of RIXS modeling to explain recent experimental data on strontium titanate and STO/LAO multilayers.