We report measurements of phonon dispersion in CaC6 using inelastic X-ray and neutron scattering. We find good overall agreement, particularly in the 50 meV energy region, between experimental data and first-principles density-functional-theory calculations. However, on the longitudinal dispersion along the (111) axis of the rhombohedral representation, we find an unexpected anti-crossing with an additional longitudinal mode, at about 11 meV. At a comparable energy, we observe also unexpected intensity on the in-plane direction. These results resolve the previous incorrect assignment of a longitudinal phonon mode to a transverse mode in the same energy range. By calculating the electron susceptibility from first principles we show that this longitudinal excitation is unlikely to be due to a plasmon and consequently can probably be due to defects or vacancies present in the sample.
@article{arxiv.0911.2239,
title = {Phonon dispersion and low energy anomaly in CaC$_6$},
author = {Matteo D'Astuto and Matteo Calandra and N. Bendiab and G. Loupias and Francesco Mauri and Shuyun Zhou and Jeff Graf and Alessandra Lanzara and N. Emery and C. Herold and P. Lagrange and D. Petitgrand and Moritz Hoesch},
journal= {arXiv preprint arXiv:0911.2239},
year = {2013}
}