Origin of superconductivity of CaC6 and of other intercalated graphites

By intercalation of donor the low conductivity of graphite can be enhanced and a superconducting state can occur at low temperature. The first discovered superconductors were alkali-intercalated compounds, with a critical temperatures (T-c) of the order of I K. In 2005 we learned with surprise that Ca intercalated graphite (CaC6) is a superconductor with the sizable T-c of 11.5 K. Using density functional theory we demonstrate that superconductivity in CaC6 is phonon-mediated with an electron-phonon coupling lambda equal to 0.83 and a phonon-frequency logarithmic-average equal to 25 meV. Superconductivity is mostly due C vibrations perpendicular and Ca vibrations parallel to the graphite layers. A non zero electron-phonon coupling for these modes can not be associated to the Fermi surface of the graphite pi bands but requires the presence of a second Fermi surface associated to the intercalant atoms. This result suggests a general mechanism for the occurrence of superconductivity in intercalated graphite. In order to stabilize a superconducting state it is necessary to have an intercalant Fermi surface since the simple doping of the pi bands in graphite does not lead to a sizeable electron-phonon coupling. This condition occurs if the intercalant band is partially occupied, i.e. when the intercalant is not fully ionized. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.