Superconductivity of graphite intercalation compounds with alkali-metal amalgams

Superconductivity of the alkali-metal amalgam graphite intercalation compounds of stage 1 (${\mathrm{C}}_4$KHg, ${\mathrm{C}}_4$RbHg) and stage 2 (${\mathrm{C}}_8$KHg, ${\mathrm{C}}_8$RbHg) 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 ${\mathrm{C}}_4$KHg, ${\mathrm{C}}_4$RbHg, ${\mathrm{C}}_8$KHg, and ${\mathrm{C}}_8$RbHg, respectively. The critical-field anisotropy ratio $\frac{H_{c2\mathrm{}}^{\perp }}{H_{c2\mathrm{}}^{\parallel }}$ 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.