Raman scattering from the CaC6 superconductor in the presence of disorder

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 3̄ m space group of CaC6. These first-order scattering modes appear along with the D and G bands around 1300 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 ∼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 Å 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.