Condensed-Matter Physics & Materials Science Seminar

Phonons contribute to resonant inelastic X-ray scattering (RIXS) as a consequence of the coupling between electronic and lattice degrees of freedom. Unlike other techniques that are sensitive to electron-phonon interactions, RIXS can give access to momentum dependent coupling constants. This Information is highly desirable in the context of understanding anisotropic conventional and unconventional superconductivity. In my talk, I will consider the phonon contribution to RIXS from the theoretical point of view. In contrast to previous studies, we emphasize the role of the core-hole lattice coupling. Our model, with parameters obtained from first principles, shows that even in the case of a deep core-hole, RIXS probes exciton-phonon coupling rather than a direct electron-phonon coupling. Further, to address the needs of predictive approach and overcome limitations of the model studies we developed a Green's function formalism to capture electron-phonon contributions to RIXS and other core-level spectroscopies (X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS)). Our approach is based on the cumulant expansion of the Green's function combined with many-body theory calculated vibrational coupling constants. In the case of the XAS and RIXS, we use a two-particle exciton Green's function, which accounts implicitly for particle-hole interference effects that have previously proved difficult. Finally, to demonstrate the methodology, we successfully applied our formalism to small molecules, for which unambiguous experimental data exist.