Disparate Exciton-Phonon Couplings for Zone-Center and Boundary Phonons in Solid-State Graphite.

The exciton-phonon coupling in highly oriented pyrolytic graphite is studied using resonant inelastic x-ray scattering (RIXS) spectroscopy. With ∼70 meV energy resolution, multiple low energy excitations associated with coupling to phonons can be clearly resolved in the RIXS spectra. Using resonance dependence and the closed form for RIXS cross section without considering the intermediate state mixing of phonon modes, the dimensionless coupling constant g is determined to be 5 and 0.35, corresponding to the coupling strength of 0.42 eV+/-20 meV and 0.20 eV+/-20 meV, for zone center and boundary phonons, respectively. The reduced g value for the zone-boundary phonon may be related to its double resonance nature