Low-temperature specific heats of graphite intercalation compounds with potassium and cesium

Low-temperature specific-heat measurements in the temperature range between 1.5 and 5 K are reported for graphite intercalation compounds with potassium and cesium as follows: ${\mathrm{C}}_8$K, ${\mathrm{C}}_{24}$K, ${\mathrm{C}}_{36}$K; ${\mathrm{C}}_8$Cs, ${\mathrm{C}}_{24}$Cs, ${\mathrm{C}}_{36}$Cs, and ${\mathrm{C}}_{48}$Cs. With increasing the alkali-metal concentration, the electronic-specific-heat coefficient $\gamma$ increases for both the ${\mathrm{C}}_x\mathrm{K}$ and ${\mathrm{C}}_x\mathrm{Cs}$ compounds, showing the lack of dependence on the solute species such as K or Cs. For the graphite-rich stage compounds ($x\ge 24$), the $\gamma$ value can be reasonably well approximated in terms of the rigid-band model based on pure graphite. In ${\mathrm{C}}_8$K and ${\mathrm{C}}_8$Cs compounds, however, the rigid-band approach breaks down and the presence of the alkali-metal sheets, withholding an appreciable amount of conduction electrons, should be taken into account. A Schottky-type anomalous specific heat was observed in all compounds with cesium, but it was not obvious in compounds with potassium.