Elementary Excitations in a Carbon Nanotube

A carbon nanotube exhibits the decoupled π plasmons of different angular momenta (L’s). The magnetic flux is expected to be useful in identifying them. All the L-decoupled plasmons have strong dispersion relations with momentum (q). The L # 0 plasmons belong to optical plasmons, while the L = 0 plasmon could not exist a,t q→ 0. The former, respectively, behave as the plasmons in three- and two-dimensional electron gases at small and large q’s. The plasmon frequencies would be affected by radius, chiral angle, and intertube interaction. To obtain the actual plasmon frequencies of a multi-shell nanotube, the intertube interactions from the neighboring shells have to be taken into account. The calculated results are consistent with the experimental measurements. A detailed comparison between a carbon nanotube and a graphite layer is also made. These two related systems may exhibit similar π plasmons, when L = 0 in a nanotube. Furthermore, the chirality effect of the former corresponds to the anisotropy effect of the latter. The predicted q-, L-, and geometry-dependence of the π plasmons needs further experimental verifications.