Neutron scattering study of the high energy graphitic phonons in superconducting CaC$_6$

TL;DR

This neutron scattering study investigates high-energy phonons in superconducting CaC$_6$, comparing experimental data with DFT calculations to clarify lattice dynamics and address previous discrepancies in the literature.

Contribution

The paper provides a detailed experimental and theoretical analysis of phonon modes in CaC$_6$, highlighting the accuracy of DFT in describing lattice dynamics in this superconductor.

Findings

DFT accurately describes general phonon features in CaC$_6$

Experimental data show good agreement with theoretical predictions

Discrepancies in previous studies are not due to phonon frequency inaccuracies

Abstract

We present the results of a neutron scattering study of the high energy phonons in the superconducting graphite intercalation compound CaC$_6$. The study was designed to address hitherto unexplored aspects of the lattice dynamics in CaC$_6$, and in particular any renormalization of the out-of-plane and in-plane graphitic phonon modes. We present a detailed comparison between the data and the results of density functional theory (DFT). A description is given of the analysis methods developed to account for the highly-textured nature of the samples. The DFT calculations are shown to provide a good description of the general features of the experimental data. This is significant in light of a number of striking disagreements in the literature between other experiments and DFT on CaC$_6$. The results presented here demonstrate that the disagreements are not due to any large inaccuracies in the calculated phonon frequencies.