Anomalous Dirac Plasmons in 1D Topological Electrides

Plasmon opens up the possibility to efficiently couple light and matter at sub-wavelength scales. In general, the plasmon frequency, intensity and damping are dependent of carrier density. These dependencies, however, are disadvantageous for stable functionalities of plasmons and render fundamentally a weak intensity at low frequency, especially for Dirac plasmon (DP) widely studied in graphene. Here we demonstrate a new type of DP, emerged from a Dirac nodal-surface state, which can simultaneously exhibit density-independent frequency, intensity and damping. 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 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 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.