Fig. 6: Collective mode dispersions.

a–c Collective mode dispersions of Bi₂Te₃ slabs of different thickness d, rendered using the false color maps of Im{r_p}. The parameters of the calculation are μ = 0.2 eV and a d = 118 nm; b d = 76 nm; c d = 39 nm. In the plots, the Fermi velocity is v_F = 0.5 × 10⁸ cm/s as for Bi₂Te₃ from ref. ⁴⁸, the electron scattering rate is γ_e = 1 THz as in ref. ⁴, the vacuum and substrate permittivity are ε₀ = 1, and ε_sub = 6.3. The white symbols correspond to the experimental data of Fig. 5d.