Abstract
The low-energy dielectric properties of CaC${}_{6}$---a representative graphite intercalated compound (GIC)---were investigated by $\mathit{ab}$ $\mathit{initio}$ 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 $``\ensuremath{\pi}p$'' 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.
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