A First-Principles Insight into the Superconductivity of Graphite Intercalation Compounds

Experimental evidences have estabilished that the recently discovered superconductivity in graphite-intercalation compounds (GICs) CaC$_6$ and YbC$_6$ is due to electron-phonon ($e-ph$) coupling. First-principles calculations predict for CaC$_6$ an intermediate $e-ph$ coupling ($\lambda \sim 0.83$), resulting from intercalant in-plane ($I_{xy}$) and carbon out-of-plane ($C_z$) vibrations. Whereas the softening of the $I_{xy}$ modes explains increase of T$_c$ with pressure [1], the presence of the $C_z$ peak is due to an interaction which is ``dormant'' in pure graphite. A simple analysis of the band structure of the GICs also permits to rule out the possibility of plasmon-meadiated superconductivity[1]. \newline \newline [1] J. S. Kim, L. Boeri, R. K. Kremer, and F. S. Razavi Phys. Rev B, in press and Phys. Rev. Lett. 96, 217002 (2006).