We calculate the plasmon dispersion relation for Coulomb coupled metallic
armchair graphene nanoribbons and doped monolayer graphene. The crossing of the
plasmon curves, which occurs for uncoupled 1D and 2D systems, is split by the
interlayer Coulomb coupling into a lower and an upper plasmon branch. The upper
branch exhibits a highly unusual behavior with endpoints at finite q.
Accordingly, the structure factor shows either a single or a double peak
behavior, depending on the plasmon wavelength. The new plasmon structure is
relevant to recent experiments, its properties can be controlled by varying the
system parameters, and be used in plasmonic applications.Comment: 5 pages, 3 figures; in press in Phys. Rev. Let
