Plasmons in Narrow-Gap Carbon Toroids

The electronic excitations of narrow-gap carbon toroids are studied within the self-consistent-field approach. The low-frequency excitation spectrum exhibits many peak structures, including a prominent one, which is identified as the collective excitations. This plasmon is due to the coherent carrier oscillation on a toroid surface along the longitudinal direction. Whether the low-frequency plasmon exists in a carbon toroid is determined by the energy gap and the threshold excitation energy. It is very sensitive to the changes in longitudinal angular momentum transfer and the toroid geometry (radius, width, and chiral angle).