Strong phonon-plasmon coupled modes in the graphene/silicon carbide heterosystem

We report on strong coupling of the charge carrier plasmon $\omega_{PL}$ in graphene with the surface optical phonon $\omega_{SO}$ of the underlying SiC(0001) substrate with low electron concentration ($n=1.2\times 10^{15}$ $cm^{-3}$) in the long wavelength limit ($q_\parallel \rightarrow 0$). Energy dependent energy-loss spectra give for the first time clear evidence of two coupled phonon-plasmon modes $\omega_\pm$ separated by a gap between $\omega_{SO}$ ($q_\parallel \rightarrow 0$) and $\omega_{TO}$ ($q_\parallel >> 0$), the transverse optical phonon mode, with a Fano-type shape, in particular for higher primary electron energies ($E_0 \ge 20eV$). A simplified model based on dielectric theory is able to simulate our energy - loss spectra as well as the dispersion of the two coupled phonon-plasmon modes $\omega_\pm$. In contrast, Liu and Willis [1] postulate in their recent publication no gap and a discontinuous dispersion curve with a one-peak structure from their energy-loss data.