Anisotropic plasmon dispersion and damping in multilayer 8-Pmmn borophene structures

We investigate the collective plasma oscillations theoretically in multilayer 8-Pmmn borophene structures, where the tilted Dirac electrons in spatially separated layers are coupled via the Coulomb interaction. We calculate the energy dispersions and Landau dampings of the multilayer plasmon excitations as a function of the total number of layers, the interlayer separation, and the different orientations. Like multilayer graphene, the plasmon spectrum in multilayer borophene consists of one in-phase optical mode and N - 1 out-of-phase acoustical modes. We show that the plasmon modes possess kinks at the boundary of the interband single-particle continuum and the apparent anisotropic behavior. All the plasmon modes approach the same dispersion at a sufficiently large interlayer spacing in the short-wavelength limit. Especially along specific orientations, the optical mode could touch an energy maximum in the nondamping region, which shows non-monotonous behavior. Our work provides an understanding of the multilayer borophene plasmon and may pave the way for multilayer borophene-based plasmonic devices.