Plasmon excitations in trilayer AAB-stacked graphene

We report on the fascinating electronic properties and distinct plasma excitations for low-symmetry trilayer AAB-stacked graphene. For the undoped structure, there are three pairs of unusual valence and conduction subbands which give rise to nine allowed interband excitations, for which the imaginary (real) part of the polarizability exhibits 1D square root asymmetric peaks and 2D shoulder structures (pairs of antisymmetric peaks and logarithm type symmetric peaks). The low frequency acoustic plasmon, appearing as a prominent peak in the energy loss spectrum, can survive in a narrow gap system with large density-of-states from the valence band. This type of plasmon mode is similar to that in a narrow gap carbon nanotube. The mechanism responsible for this plasmon is the intraband conduction state excitations. Its frequency, intensity and critical momentum exhibit a non-monotonic dependence on the Fermi energy.