Raman scattering from theCaC6superconductor in the presence of disorder

Abstract

Polarized Raman scattering has been performed on ${\text{CaC}}_{6}$ superconductor. We identify two of the three Raman-active ${E}_{g}$ phonon modes at 440 and $1508\text{ }{\text{cm}}^{\ensuremath{-}1}$ expected for the $R\overline{3}m$ space group of ${\text{CaC}}_{6}$. These first-order scattering modes appear along with the $D$ and $G$ bands around 1300 and $1600\text{ }{\text{cm}}^{\ensuremath{-}1}$ that are similar in origin to the corresponding bands in plain graphite. The intensities of the $D$ and $G$ bands in ${\text{CaC}}_{6}$ 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 $\ensuremath{\sim}35\text{ }{\text{cm}}^{\ensuremath{-}1}/\text{eV}$. The double resonant Raman scattering probes phonon excitations with finite wave vector $\mathbit{q}$. We estimate the characteristic spacing of structural defects to be on the scale of about $100\text{ }\text{\AA{}}$ by comparing the intensity of the $D$ band and the $1508\text{ }{\text{cm}}^{\ensuremath{-}1}$ ${E}_{g}$ mode in ${\text{CaC}}_{6}$ to calibrated intensity ratio of analogous bands in disordered graphites. A sharp superconducting coherence peak at $24\text{ }{\text{cm}}^{\ensuremath{-}1}$ is observed below ${T}_{c}$.