Phonon dispersion and low energy anomaly in CaC$_6$.

Matteo d'Astuto (1) , Matteo Calandra (1) , Nedjma Bendiab (2) , G. Loupias (1) , Francesco Mauri (1) , Shuyun Zhou (3, 4) , Jeff Graf (3, 4) , Alessandra Lanzara (3, 4) , N. Emery (5) , C. Herold (5) , P. Lagrange (5) , D. Petitgrand (6) , Moritz Hoesch (7)

1 IMPMC - Institut de minéralogie et de physique des milieux condensés
2 CNRS - Centre National de la Recherche Scientifique
3 Materials Science Division [LBNL Berkeley]
4 Department of Physics [Berkeley]
5 LCSM - Laboratoire de chimie du solide minéral
6 LLB - UMR 12 - Laboratoire Léon Brillouin
7 ESRF - European Synchrotron Radiation Facility

We report measurements of phonon dispersions in CaC$_6$ using inelastic X-ray and neutron scattering. We found good overall agreement, particularly in the 50 meV energy region, between the data and the first-principles density-functional-theory calculations. However, on the longitudinal dispersion along the c-axis of the hexagonal representation, we found 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. This result could help to explain the incorrect interpretation of previous measurements of phonon dispersion in CaC$_6$, which assign a longitudinal character 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 it is probably due to defects or vacancies present in the sample.

Mots clés: Superconducting binary compounds; Phonon-electron and phonon-phonon interactions; Density functional theory; Inelastic X-ray scattering

Domaines: Supraconductivité [cond-mat.supr-con]

Matteo d'Astuto, Matteo Calandra, Nedjma Bendiab, G. Loupias, Francesco Mauri, et al.. Phonon dispersion and low energy anomaly in CaC$_6$.. 2009. ⟨hal-00431291v1⟩