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

TL;DR

This study uses density functional theory to predict superconductivity in alkaline earth intercalated graphite compounds, showing pressure can enhance T_c in some cases and proposing new synthesis routes for higher-temperature superconductors.

Contribution

It provides first-principles predictions of superconductivity in AC6 compounds with A=Mg,Ca,Sr,Ba, and explores pressure effects and potential new materials.

Findings

Ba and SrC6 are predicted to be superconducting at low temperatures.

Pressure can significantly increase T_c in SrC6 and BaC6.

MgC6 is unstable, but mixed compounds may yield higher T_c.

Abstract

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 K 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 SrC$_6$ and BaC$_6$ critical temperatures should be substantially enhanced by pressure. On the contrary, for CaC$_6$ we find that in the 0 to 5 GPa region, T$_c$ weakly increases with pressure. The increase is much smaller than what shown in several recent experiments. Thus we suggest that in CaC$_6$ a continous phase transformation, such as an increase in staging, occurs at finite pressure. Finally we argue that, although MgC$_6$ is unstable, the synthesis of intercalated systems of the kind Mg$_x$Ca$_{1-x}$C$_y$ could lead to higher critical temperatures.