Low-energy plasmonic structure in CaC ₆

The low-energy dielectric properties of CaC ₆ -a representative graphite intercalated compound (GIC)-were investigated by ab initio time-dependent density functional theory calculations with full inclusion of local field effects. The calculations predict the existence of several kinds of plasmons in CaC ₆ with energy below 10 eV. The mode with the largest energy is a conventional ''πp" mode strongly dispersing in the hexagonal basal plane and almost nondispersing in the perpendicular direction. In the 2.3-3 eV energy range, we find a long-lived intraband plasmon with negative (positive) dispersion with momentum transfer in (perpendicular to) the basal plane. In the 0-1.5 eV energy range, a mode with linear soundlike dispersion along all three high-symmetry directions is observed. All the three modes present strong anisotropy originated from the band structure. The physical origin of these excitation modes is discussed in terms of intra- and interband transitions. The crucial role of local field effects in the propagation of the two lowest-energy modes at large momentum transfers and in the determination of its dispersion over extended momentum-transfer region is analyzed.