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

Matteo Calandra and Francesco Mauri

Phys. Rev. B 74, 094507 – Published 18 September 2006

Abstract

Using density functional theory we investigate the occurrence of superconductivity in $A C_{6}$ 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 $Sr C_{6}$ and $Ba C_{6}$ critical temperatures should be substantially enhanced by pressure. On the contrary, for $Ca C_{6}$ 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 $Ca C_{6}$ a continous phase transformation, such as an increase in staging, occurs at finite pressure. Finally we argue that, although $Mg C_{6}$ is unstable, the synthesis of intercalated systems of the kind ${Mg}_{x} {Ca}_{1 - x} C_{y}$ could lead to higher critical temperatures.