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

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

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

We report measurements of phonon dispersion in CaC$_6$ 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 $(1 1 1)$ 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.

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$_6$.. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2010, 81 (10), pp.104519. ⟨10.1103/PhysRevB.81.104519⟩. ⟨hal-00431291v4⟩