  Plasmon opens up the possibility to efficiently couple light and matter at
sub-wavelength scales. In general, the plasmon frequency is dependent of
carrier density. This dependency, however, renders fundamentally a weak plasmon
intensity at low frequency, especially for Dirac plasmon (DP) widely studied in
graphene. Here we demonstrate a new type of DP, excited by a Dirac
nodal-surface state, which exhibits an anomalously density-independent
frequency. Remarkably, we predict realization of anomalous DP (ADP) in 1D
topological electrides, such as Ba3CrN3 and Sr3CrN3, by first-principles
calculations. The ADPs in both systems have a density-independent frequency and
high intensity, and their frequency can be tuned from terahertz to mid-infrared
by changing the excitation direction. Furthermore, the intrinsic weak
electron-phonon coupling of anionic electrons in electrides affords an added
advantage of ultra-low phonon-assisted damping and hence a long lifetime of the
ADPs. Our work paves the way to developing novel plasmonic and optoelectronic
devices by combining topological physics with electride materials.
