Plasmons in graphite and stage-1 graphite intercalation compounds

The pi-electronic excitations df graphite layers are studied within the random-phase approximation. They principally reflect the pi-band characteristics, the strong wave-vector dependence, the anisotropic behavior, and the special symmetry. The pi plasmons in graphite have strong dispersion relations with the transferred momentum (q). They behave as an optical plasmon in a three-dimensional electron gas at small q. Moreover, the anisotropic behavior at the plane is apparent at large q. For a single graphite layer, the pi plasmons would disappear at very small q, and their frequencies are obviously reduced. The absence of interlayer Coulomb interactions is the main reason for this. The stage-1 graphite intercalation compounds (GIC's), as compared with graphite, exhibit the richer excitation spectra and the lower pi-plasmon frequencies. They have the intraband plasmon as well as the interband pi plasmon. These two kinds of plasmons are quite different from each other in certain respects, e.g., the cause of the plasmon. The enhanced interlayer distances could effectively reduce the pi-plasmon frequency, but not the transferred charges. The calculated plasmon frequencies are consistent with the experimental measurements on graphite and stage-1 GIC's.