We report the energy-momentum dispersion $\omega$($\mathcal{q}$) of a low-energy intraband $\pi$ plasmon arising from Dirac fermions in the conduction band of a single-layer graphene (SLG), not complicated by couplings with other excitations. For a wide range of $\mathcal{q}$ (0.39 $⩽\mathcal{q}/k_F⩽$ 2.36), where $k_F$ $=$ 0.061 $\AA {}^{-1}$ is the Fermi wave vector, the intraband plasmon survives through the interband single-particle excitation (SPE) region, and linearly disperses beyond the Landau damping $q_c$ $=$ 0.90 $k_F$ asymptotically approaching the boundary of the interband SPE without ever entering the intraband SPE. Such a unique feature is completely distinct from the plasmon of either a multilayer graphene or a normal two-dimensional electron gas, and hence demonstrates another intrinsic nature of Dirac fermions in SLG.

• Received 11 January 2011

DOI:https://doi.org/10.1103/PhysRevB.83.161403