Electronic collective excitations in AB-stacked nanographite ribbons

The inter-pi -band electronic excitations of AB-stacked nanographite ribbons are studied within the self-consistent-field approach. Such excitations directly reflect the characteristics of the pi -electronic structure. They depend on the magnitude (q) and the direction (theta) of the transferred momentum, the edge structure, and the stacking sequence. The loss function exhibits several peak structures. The pronounced peak, with frequency larger than 5 eV, is identified as the pi -electronic collective excitations. The pi plasmons exhibit highly anisotropic behavior. They axe almost absent, when the transferred momentum is perpendicular to the graphitic plane (theta = 90 degrees). However, their frequencies at small theta are not sensitive to the change in the stacking sequence.