Resonant inelastic X-ray scattering as a probe of exciton-phonon coupling

QUICK INFORMATION
Type
PhD Defense
Start Date
11-12-2018 14:00
End Date
11-12-2018 16:00
Location
Auditorium, Central Building
Speaker's name
Andrey Geondzhian
Speaker's institute
ESRF Grenoble, France
Contact name
Claudine Roméro
Host name
Nick Brookes & Keith Gilmore
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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. Information about the dispersion of the electron-phonon interaction is highly desirable in the context of understanding anisotropic conventional and unconventional superconductivity.

We considered 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.

This difference leads to quantitative and qualitative deviations from the interpretation of the implied electron-phonon coupling from the standard view expressed in the literature. Thus, our objective is to develop a rigorous approach to quantify electron- phonon coupling within the context of RIXS measurements. The ability to accurately reproduce experimental results from first-principles calculations, without recourse to adjustable parameters, should be viewed as the ultimate test of a proper understanding of the phonon contribution to RIXS.

We start by considering only the core-hole–phonon interaction within the context of X-ray photoemission spectroscopy. We combine an ab initio calculation of the real- space response function with many-body Green’s functions techniques to reproduce the vibrational side-bands in SiX4 (X = H,F) molecules. The approach we developed is suitable for application to crystalline materials.

We next consider the phonon contribution to X-ray absorption spectra. Unlike the charged excitations generated by X-ray photoemission, X-ray absorption creates a neutral excitation that we approximate as a core-hole and an excited electron. We first solved the electronic part of the problem on the level of the Bethe-Salpeter equation and then dressed the resulting 2-particle excitonic quasiparticle with the exciton-phonon interactions using the cumulant ansatz. The viability of this methodology was tested by calculating the N K-edge XAS of the N2 molecule and the O K−edge of acetone. The resulting vibronic spectra agreed favorably with experimental results.

Finally, we construct a hybrid formulation of the RIXS cross section that preserves explicit summation over a small number of final states, but replaces the summation over intermediate states, which might be enormously expensive, with a Green’s function. We develop an expansion of the Green’s function and derive both analytically exact (in the no-recoil limit) and approximate solutions. The formalism was again tested on the O K−edge of acetone and agrees well with the experiment. To provide an outlook towards future work, we discuss application of the developed formalism to crystalline materials.

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