Observation of intrinsic intraband π-plasmon excitation of a single-layer graphene

We report the energy-momentum dispersion omega(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 q (0.39 <= q/k(F) <= 2.36), where k(F) = 0.061 angstrom(-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.