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 A C6 with A=Mg,Ca,Sr,Ba. We predict that at zero pressure, Ba and Sr should be superconducting with critical temperatures (Tc) 0.2 and 3.0 K, respectively. We study the pressure dependence of Tc assuming the same symmetry for the crystal structures at zero and finite pressures. We find that the Sr C6 and Ba C6 critical temperatures should be substantially enhanced by pressure. On the contrary, for Ca C6 we find that in the 0-5 GPa region, Tc weakly increases with pressure. The increase is much smaller than that shown in several recent experiments. Thus we suggest that in Ca C6 a continous phase transformation, such as an increase in staging, occurs at finite pressure. Finally we argue that, although Mg C6 is unstable, the synthesis of intercalated systems of the kind Mgx Ca1-x Cy could lead to higher critical temperatures. © 2006 The American Physical Society.