Geometry-diversified Coulomb excitations in trilayer AAB stacking graphene

The lower-symmetry trilayer AAB-stacked graphene exhibits rich electronic properties and diverse Coulomb excitations. Three pairs of unusual valence and conduction bands create nine available interband excitations for the undoped case, in which the imaginary (real) part of the polarizability shows one-dimensional square root asymmetric peaks and two-dimensional shoulder structures (pairs of antisymmetric peaks and logarithm-type symmetric peaks). The low-frequency acoustic plasmon, being revealed as a prominent peak in the energy-loss spectrum, can survive in a narrow-gap system with the large density of states from the valence band. This type of plasmon mode is similar to that in a narrow-gap carbon nanotube. However, the decisive mechanism governing this plasmon is the intraband conduction state excitations. Its frequency, intensity and critical momentum exhibit a nonmonotonic dependence on the Fermi energy. The well-defined electron-hole excitation boundaries and the higher frequency optical plasmons are transformed by varying the Fermi energy. Many substantial differences between the electronic properties of AAB and other trilayer graphene stackings are discussed.