Theoretical Explanation of Superconductivity in ${\mathrm{C}}_{6}\mathrm{Ca}$

Theoretical Explanation of Superconductivity in $C_{6} Ca$

Matteo Calandra and Francesco Mauri

Phys. Rev. Lett. 95, 237002 – Published 30 November 2005

Abstract

Using density functional theory, we demonstrate that superconductivity in $C_{6} Ca$ is phonon mediated with $λ = 0.83$ and phonon frequency $⟨ ω ⟩_{\log } = 24.7 meV$ . The calculated isotope exponents are $α (Ca) = 0.24$ and $α (C) = 0.26$ . Superconductivity is due mostly to C vibrations perpendicular and Ca vibrations parallel to the graphite layers. Since the electron-phonon couplings of these modes are activated by the presence of an intercalant Fermi surface, the occurrence of superconductivity in graphite intercalated compounds requires a noncomplete ionization of the intercalant.