  The effect of edge modification of armchair graphene nanoribbons (AGNRs) on
the collective excitations are theoretically investigated. The tight-binding
method is employed in conjunction with the dielectric function. Unconventional
plasmon modes and their association with the flat bands of the specially
designed AGNRs are thoroughly studied. We demonstrate the robust relationship
between the novel collective excitations and both the type and period of the
edge modification. Additionally, we reveal that the main features displayed in
the (momentum, frequency)-phase diagrams for both single-particle and
collective excitations of AGNRs can be efficiently tuned by edge-extended
defects. Our obtained plasmon modes are found to be analogous to
magnetoplasmons associated with collective excitations of Landau-quantized
electrons. This work provides a unique way to engineer discrete
magnetoplasmon-like modes of AGNRs in the absence of magnetic field.
