Superconductivity in compounds of sodium-intercalated graphite

The discovery of superconductivity in CaC6 with a critical temperature (T-c) of 11.5 K reignited much interest in exploring high-temperature superconductivity in graphite intercalation compounds (GICs). Here we identify a GIC NaC4, discovered by ab initio evolutionary structure search, as a superconductor with a computed T-c of 41.2 K at 5 GPa. This value is eight times higher than that of the synthesized GIC NaC2 and possesses the highest T-c among available GICs. The remarkable superconductivity of GIC NaC4 mainly arises from the coupling of pi electrons in graphene with the low-frequency vibrations involving both Na and C atoms. These findings suggest that Na GICs may hold great promise as high-T-c superconductors.