Plasmons and excitons in graphene We study collective excitations in graphene. When carriers are doped in graphene, plasmons appear, whose energy and optical absorption strongly depend on the carrier density. Without carrier doping, plasmons do not exist while excitonic states appear at energy below the lower edge of one-particle excitation continuum. (Left) Energy dispersion relations of plasmons in doped graphene for various values of a parameter (alpha) for Coulomb interactions. Solid lines denote energy of plasmons, dashed lines indicate analytic energy of plasmons in the long-wave-length limit, and dotted lines are boudaries of regions for one-particle excitation continuums. Colored regions indicate one-particle excitation continuums in a one-particle model. Energy and wave vector are plotted in the units of the Fermi energy and the Fermi wave vector, respectively. (Right) Energy dispersion relations of excitonic collective states in nondoped graphene for various values of a parameter (alpha) for Coulomb interactions. Solid lines denote energy of excitonic states and dotted lines indicate are boudaries of regions for one-particle excitation continuums. Colored regions indicate one-particle excitation continuums in a one-particle model. Energy and wave vector are plotted in the units of a cutoff energy and a cutoff wave vector, respectively. Back
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