Fig. 4: Plasmon dispersion along the momentum q_∣∣ parallel to the layers for several q_⊥ perpendicular to the layers (squares) together with calculations within the framework of the Fetter model (solid lines).

The q_⊥ values 0, 0.1, 0.2, 0.3, and 0.4 Å⁻¹ are marked by purple, blue, green, yellow, and red color symbols, respectively (see Fig. 3). We have added also the data from the optical plasmon dispersion derived in our previous publication (dark purple diamonds)³⁸. The horizontal error bars originate from the finite momentum resolution while the vertical ones are related to the finite spectral resolution of the EEL spectrometer (the fitting error is small in comparison). The region marked in gray corresponds to the susceptibility χ₀ calculated from a tight binding band structure (see below and Methods). The excitations in the continuum range are marked by open symbols. The colored solid (dashed) curves correspond to theoretical data considering (neglecting) finite momentum resolution, i.e., integration within the EEL collection aperture.