Low-temperature specific heat of the graphite intercalation compounds K${\mathrm{C}}_8$, Cs${\mathrm{C}}_8$, Rb${\mathrm{C}}_8$, and their parent highly oriented pyrolytic graphite

The specific heat of the graphite intercalation compounds K${\mathrm{C}}_8$, Cs${\mathrm{C}}_8$, and Rb${\mathrm{C}}_8$ and their parent material, highly oriented pyrolytic graphite (HOPG), has been determined between 0.48 and 90 K. The results are analyzed in terms of electronic and lattice contributions. The electronic contribution to the specific heat of these compounds is enhanced by at least 29 times above that for HOPG. Comparisons are made to theoretical band-structure calculations. The lattice contribution shows deviations from a Debye model for $T>\mathrm{}5\mathrm{}$ K, which can be mathematically fitted by an Einstein specific-heat function. While our results for Rb${\mathrm{C}}_8$ are unique, comparison with previous results for powder-based K${\mathrm{C}}_8$ and Cs${\mathrm{C}}_8$ samples over a limited temperature range suggest that the type of parent graphite and/or details of preparation technique may influence the physical properties of these compounds.