Comparative study of the phonons in non-superconducting BaC6 and superconducting CaC6 using inelastic x-ray scattering

TL;DR

This study compares phonon behaviors in non-superconducting BaC6 and superconducting CaC6 using inelastic x-ray scattering, revealing differences in phonon energies and the influence of disorder on additional inelastic intensity.

Contribution

It provides experimental insights into phonon differences between BaC6 and CaC6 and highlights the role of disorder in inelastic scattering signals, suggesting the need for improved theoretical models.

Findings

Phonons in BaC6 are significantly higher in energy than predicted.

Additional inelastic intensity is linked to sample disorder.

Disorder affects phonon measurements and interpretation.

Abstract

The low energy phonons of two different graphite intercalation compounds (GICs) have been measured as a function of temperature using inelastic x-ray scattering (IXS). In the case of the non-superconductor BaC6, the phonons observed are significantly higher (up to 20 %) in energy than those predicted by theory, in contrast to the reasonable agreement found in superconducting CaC6. Additional IXS intensity is observed below 15 meV in both BaC6 and CaC6. It has been previously suggested that this additional inelastic intensity may arise from defect or vacancy modes in these compounds, unpredicted by theory (d'Astuto et al, Phys. Rev. B 81 104519 (2010)). Here it is shown that this additional intensity can arise directly from the large disorder of the available samples. Our results show that future theoretical work is required to understand the relationship between the crystal structure, the phonons and the superconductivity in GICs.