Raman response of stage-1 graphite intercalation compounds revisited

Abstract

We present a detailed in situ Raman analysis of stage-1 KC${}_{8}$, CaC${}_{6}$, and LiC${}_{6}$ graphite intercalation compounds to unravel their intrinsic fingerprint. Four main components were found between 1200 cm${}^{\ensuremath{-}1}$ and 1700 cm${}^{\ensuremath{-}1}$ and each of them were assigned to a corresponding vibrational mode. From a detailed line-shape analysis of the intrinsic Fano lines of the $G$- and $D$-line response we precisely determine the position (${\ensuremath{\omega}}_{\mathrm{ph}}$), line width (${\ensuremath{\Gamma}}_{\mathrm{ph}}$), and asymmetry ($q$) from each component. The comparison to the theoretical calculated line width and position of each component allows us to extract the electron-phonon coupling constant of these compounds. A coupling constant ${\ensuremath{\lambda}}_{\mathrm{ph}}<0.06$ was obtained. This highlights that Raman active modes alone are not sufficient to explain the superconductivity within the electron-phonon coupling mechanism in CaC${}_{6}$ and KC${}_{8}$.