Possibility of superconductivity in graphite intercalated with alkaline earths investigated with density functional theory

Using density functional theory we investigate the occurrence of superconductivity in AC(6) with A=Mg,Ca,Sr,Ba. We predict that at zero pressure, Ba and Sr should be superconducting with critical temperatures (T-c) 0.2 and 3.0 K, respectively. We study the pressure dependence of T-c assuming the same symmetry for the crystal structures at zero and finite pressures. We find that the SrC6 and BaC6 critical temperatures should be substantially enhanced by pressure. On the contrary, for CaC6 we find that in the 0-5 GPa region, T-c weakly increases with pressure. The increase is much smaller than that shown in several recent experiments. Thus we suggest that in CaC6 a continous phase transformation, such as an increase in staging, occurs at finite pressure. Finally we argue that, although MgC6 is unstable, the synthesis of intercalated systems of the kind MgxCa1-xCy could lead to higher critical temperatures.