Fig. 2: q-EELS measurements of monolayer and bilayer 2H-TaS₂ along the Γ-M direction.

a Bilayer spectra, vertically shifted based on the corresponding q-value set in reciprocal space, as indicated by the coloured disks in the inset. 2D plasmon peaks are observed and fitted with a Drude–Lorentz function to extract the q-dependent plasmon energy, forming a dispersion relation. The black dashed line in (a) serves as a visual guide to estimate the group velocity from the slope of the fitted plasmon dispersion relation. The lowest group velocity observed in the measurement is estimated to be\(\, \sim {10}^{-4}c\) at high momentum. b Fitting of the measured dispersion relation of the monolayer (cyan) and bilayer (blue) TaS₂ with the model based on Eqs. (2) and (4) with the screening length \({\rho }_{0}\) as described in Supplementary Note 5. The data points are the experimentally extracted plasmon energy and momentum values, and the dashed lines are the fitted dispersion relations. c Interband screening component \({\varepsilon }_{{inter}}\) of the dielectric function calculated with screening lengths \({\rho }_{0,{ML}}=28.4\,{{\text{\AA}}}\) for the monolayer and \({\rho }_{0,{BL}}=56.8\,{{\text{\AA}}}\) for the bilayer (see Supplementary Note 5). The solid cyan and blue lines represent the linear q-dependence expected from the Keldysh model of Eq. (4), serving as a visual guide. The black dashed line shows the interband screening calculated ab initio⁹. The x-error bars are calculated from the momentum resolution evaluated in Supplementary Note 3b. The y-error bars include uncertainty in the plasmon energy \(\hslash \omega\) due to the peak fitting described in Supplementary Note 4, the width of the ZLP, and from the effect of finite \(\Delta q\) resolution as discussed in Supplementary Note 3 and 5. From this, the y-error bars for \({\varepsilon }_{{inter}}\) are calculated from Supplementary Eq. (S23) using propagation of uncertainty.