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

The pi-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 pi plasmon, and the others are the interband plasmons. The latter are absent in graphite layers. The pi 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 pi-plasmon frequency and oscillator strength as N increases. A multiwall carbon nanotube can exhibit L-decoupled pi plasmons. For the L = 0 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 pi-plasmon frequency, and relatively rapid increase of plasmon frequency with q. The calculated results are compared with experimental measurements.