A detailed study of the elementary excitations of an electron gas confined to a tubule system is presented. The system could consist either of a single cylindrical tubule or of several tubules sharing a common axis. Graphene tubules with a radius as small as 11 have been recently realized. Essential features revealed from this study are expected to be common to the graphene tubules. The dielectric function of the quasi-one-dimensional (1D) tubular system has been evaluated exactly within the random-phase approximation, where both the intrasubband and the intersubband excitations are included. The angular momentum (L) is conserved in the tubule system. The excitations, e.g., plasmons, of different L's are thus mutually decoupled. At any given L, only a small number of plasmon branches exist, every one of which can be studied systematically. Intertubule interaction for coaxial tubules has been included. The coupling among coaxial tubules adds unique features that distinguish the tubules from other quasi-1D systems.
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