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=1.2x10(15) cm(-3)) in the long-wavelength limit (q(parallel to)-> 0). Energy-dependent energy-loss spectra give clear evidence of two coupled phonon-plasmon modes omega(+/-) separated by a gap between omega(SO)(q(parallel to)-> 0) and omega(TO)(q(parallel to)>> 0), the transverse-optical-phonon mode, in particular, for higher primary electron energies (E-0 >= 20 eV). 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(+/-). In contrast, Liu and Willis [Phys. Rev. B 81, 081406(R) (2010)] postulate in their recent publication no gap and a discontinuous dispersion curve with a one-peak structure from their energy-loss data.