Elementary excitations in cylinder bundles

Elementary excitations in a cylinder bundle, which is formed by packing the same hollow tubules into a two-dimensional triangular lattice, are studied. The dielectric response function is calculated within the linear response approximation. The electronic excitations of the cylinder bundle are the superposition of those of all tubules. Due to the intertube Coulomb interactions, their characteristics quite differ from those of an isolated tubule. The plasmon, which corresponds to the coherent oscillations of all carriers, could exhibit a prominent peak in excitation spectrum. The excitation properties strongly depend on the tubular radius, the effective electron mass, and the magnitude and the direction of the momentum transfer (q). At small momentum, there exists a simple relation between excitation properties and polar angles (theta's). Both intensity of excitation spectrum and plasmon frequency decrease as theta increases. The collective plasma oscillations parallel and perpendicular to the tubular axis contrast sharply with each other, which clearly illustrates the anisotropic behavior. The ratio between these two plasmon frequencies is found to be smaller than root 2.