Plasmon-phonon strongly coupled mode in epitaxial graphene

We report the dispersion measurements, using angle-resolved reflection electron-energy-loss spectroscopy, on two-dimensional (2D) plasmons in single and multilayer graphene which couple strongly to surface optical phonon (FK-phonon) modes of silicon carbide substrate. The coupled modes show discrete dispersion behaviors in the single and bilayer graphene. With increasing graphene layers on SiC(0001), a transition from plasmonlike dispersion to phononlike dispersion is observed. For plasmonlike modes, the dispersion is strongly damped by electron-hole pair excitations at entering single-particle continuum, while phononlike mode is undamped. In the region free of coupling, the graphene 2D plasmon exhibits acoustic behavior with linear dispersion with slope and damping determined by the Fermi-surface topology.