Loss spectra of graphite-related systems: A multiwall carbon nanotube, a single-wall carbon nanotube bundle, and graphite layers

The π-electron excitations are studied for a multiwall carbon nanotube, a single-wall carbon nanotube bundle with finite nanotubes, and graphite layers. The loss spectra of the nanotube systems exhibit several plasmon peaks. The most prominent one is the π plasmon, and the others are the interband plasmons. The latter are absent in graphite layers. The π plasmon depends on the number of carbon nanotubes or graphite layers (N), the transferred momentum (q), and the transferred angular momentum (L). The intertube or interlayer Coulomb interactions clearly enhance π-plasmon frequency and oscillator strength as N increases. A multiwall carbon nanotube can exhibit L-decoupled π plasmons. For the $L=0\mathrm{}\pi$ plasmon, the multiwall nanotube behaves like graphite layers, but not like a single-wall carbon nanotube bundle. The radius dependence is negligible for a multiwall nanotube, while it is strong for a single-wall nanotube bundle. Furthermore, the former exhibits stronger collective excitations, higher π-plasmon frequency, and relatively rapid increase of plasmon frequency with q. The calculated results are compared with experimental measurements.