Raman scattering from the CaC6 superconductor in the presence of disorder

TL;DR

This study uses polarized Raman scattering to analyze phonon modes and disorder effects in CaC6 superconductor, revealing correlations between Raman features, disorder, and superconducting properties.

Contribution

It identifies specific Raman active phonon modes in CaC6 and links disorder levels to Raman spectral features and defect spacing estimates.

Findings

Identification of two Raman active Eg phonon modes at 440 and 1508 cm-1.

Correlation between D and G band intensities and disorder degree.

Observation of a superconducting coherence peak at 24 cm-1 below Tc.

Abstract

Polarized Raman scattering has been performed on CaC6 superconductor. We identify two of the three Raman active Eg phonon modes at 440 and 1508 cm-1 expected for the R-3m space group of CaC6. These first order scattering modes appear along with the D and G bands around 1300 cm-1 and 1600 cm-1 that are similar in origin to the corresponding bands in plain graphite. The intensities of the D and G bands in CaC6 correlate with degree of disorder. The D band arises from the double resonant Raman scattering process; its frequency shifts as a function of excitation energy with approximately 35 cm-1/eV. The double resonant Raman scattering probes phonon excitations with finite wave vector q. We estimate the characteristic spacing of structural defects to be on the scale of about 100 Angstrom by comparing the intensity of the D band and the 1508 cm-1 Eg mode in CaC6 to calibrated intensity ratio of analogous bands in disordered graphites. A sharp superconducting coherence peak at 24 cm-1 is observed below Tc.