Superconductivity of graphite intercalation compounds with alkali-metal amalgams

Superconductivity of the alkali-metal amalgam graphite intercalation compounds of stage 1 (C₄KHg, C₄RbHg) and stage 2 (C₈KHg, C₈RbHg) has been studied as well as that of the pristine amalgams (KHg, RbHg). The transition temperatures are 0.73, 0.99, 1.90, and 1.40 K for C₄KHg, C₄RbHg, C₈KHg, and C₈RbHg, respectively. The critical-field anisotropy ratio Hc2^⊥Hc2^∥ is about 10 for the stage 1 and about 15 to 40 for the stage 2. It is argued that electrons in the intercalant bands rather than the graphitic bands play the main role in the superconductivity. An interesting feature is that the stage-2 compound, which has a lower density of states at the Fermi level, has a higher transition temperature than the corresponding stage-1 compound.