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Although not an intrinsic superconductor, it has been long--known that, whenintercalated with certain dopants, graphite is capable of exhibitingsuperconductivity. Of the family of graphite--based materials which are knownto superconduct, perhaps the most well--studied are the alkali metal--graphiteintercalation compounds (GIC) and, of these, the most easily fabricated is theC${}_8$K system which exhibits a transition temperature $\bm{T_c\simeq 0.14}$K. By increasing the alkali metal concentration (through high pressurefabrication techniques), the transition temperature has been shown to increaseto as much as $\bm 5 $K in C${}_2$Na. Lately, in an important recentdevelopment, Weller \emph{et al.} have shown that, at ambient conditions, theintercalated compounds \cyb and \cca exhibit superconductivity with transitiontemperatures $\bm{T_c\simeq 6.5} $K and $\bm{11.5} $K respectively, in excessof that presently reported for other graphite--based compounds. We explore thearchitecture of the states near the Fermi level and identify characteristics ofthe electronic band structure generic to GICs. As expected, we find that chargetransfer from the intercalant atoms to the graphene sheets results in theoccupation of the $\bm\pi$--bands. Yet, remarkably, in all those -- and onlythose -- compounds that superconduct, we find that an interlayer state, whichis well separated from the carbon sheets, also becomes occupied. We show thatthe energy of the interlayer band is controlled by a combination of itsoccupancy and the separation between the carbon layers.Comment: 4 Figures. Please see accompanying experimental manuscript  "Superconductivity in the Intercalated Graphite Compounds C6Yb and C6Ca" by  Weller et a

The role of the interlayer state in the electronic structure of superconducting graphite intercalated compounds