Phonon dispersion and low energy anomaly in CaC$_6$

TL;DR

This study investigates phonon dispersion in CaC$_6$ using inelastic scattering techniques, revealing unexpected mode interactions and clarifying previous misassignments, with implications for understanding its lattice dynamics.

Contribution

It provides the first detailed phonon dispersion measurements in CaC$_6$, identifying an unexpected anti-crossing and clarifying mode assignments through combined experimental and theoretical analysis.

Findings

Discovery of an anti-crossing in longitudinal phonon modes at 11 meV.

Observation of unexpected intensity in in-plane phonon directions.

Clarification that the mode anomaly is unlikely due to plasmons, suggesting defect-related origins.

Abstract

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.