Low-energy collective electronic excitations inLiC6,SrC6, andBaC6

Abstract

We present a first-principles study of the electronic band structure and low-energy dielectric response properties of a representative group of graphite intercalated compounds---${\mathrm{LiC}}_{6},{\mathrm{SrC}}_{6}$, and ${\mathrm{BaC}}_{6}$. Our results obtained from time-dependent density functional theory calculations reveal the presence of different plasmons in these compounds at energies below 12 eV. The presence of these collective electronic excitations is discussed in terms of intra- and interband transitions. In addition to the bulk plasmon, we find the $\ensuremath{\pi}$- and intraband plasmons. However, their properties vary greatly from one material to another. In ${\mathrm{LiC}}_{6}$ and ${\mathrm{BaC}}_{6}$, the $\ensuremath{\pi}$ plasmon is a two-dimensional excitation, whereas in ${\mathrm{SrC}}_{6}$ it has a three-dimensional character. As for the intraband plasmon, it is observed in all momentum-transfer symmetry directions, with rather strong anisotropy in the energy position between momentum transfers in the basal carbon plane and perpendicular to it. Also, we discuss the appearance in ${\mathrm{SrC}}_{6}$ and ${\mathrm{BaC}}_{6}$ of a low-energy collective electronic mode with the characteristic soundlike dispersion in terms of the energy bands crossing the Fermi level with different group velocities. We find a correlation between the appearance of such low-energy electronic modes and the occupation of the graphite interlayer band.