ELEMENTARY EXCITATIONS IN A CARBON NANOTUBE

A carbon nanotube exhibits the decoupled pi plasmons of different angu lar momenta (L's). The magnetic Aux is expected to be useful in identi fying them. All the L-decoupled plasmons have strong dispersion relati ons with momentum (q). The L not equal 0 plasmons belong to optical pl asmons, while the L = 0 plasmon could not exist at q --> 0. The former , respectively, behave as the plasmons in three- and two-dimensional e lectron gases at small and large q's. The plasmon frequencies would be affected by radius, chiral angle, and intertube interaction. To obtai n the actual plasmon frequencies of a multi-shell nanotube, the intert ube interactions from the neighboring shells have to be taken into acc ount. The calculated results are consistent with the experimental meas urements. A detailed comparison between a carbon nanotube and a graphi te layer is also made. These two related systems may exhibit similar p i plasmons, when L = 0 in a nanotube. Furthermore, the chirality effec t of the former corresponds to the anisotropy effect of the latter. Th e predicted q-, L-, and geometry-dependence of the pi plasmons needs f urther experimental verifications.