Plasmons in metallic monolayer and bilayer transition metal dichalcogenides

We study the collective electronic excitations in metallic single-layer and bilayer transition metal dichalcogenides (TMDCs) using time dependent density functional theory in the random phase approximation. For very small momentum transfers (below q≈0.02 {}^-1), the plasmon dispersion follows the √q behavior expected for free electrons in two dimensions. For larger momentum transfer, the plasmon energy is significantly redshifted due to screening by interband transitions. At around q≈0.1 {}^-1, the plasmon enters the dissipative electron-hole continuum and the plasmon dispersions flatten out at an energy around 0.6--1.1 eV, depending on the material. Using bilayer NbSe₂ as example, we show that the plasmon modes of a bilayer structure take the form of symmetric and antisymmetric hybrids of the single-layer modes. The spatially antisymmetric mode is rather weak with a linear dispersion tending to zero for $q=0$, while the energy of the symmetric mode follows the single-layer mode dispersion with a slight blue shift.