Collective modes of doped graphene and a standard two-dimensional electron gas in a strong magnetic field: Linear magnetoplasmons versus magnetoexcitons

A doped graphene layer in the integer quantum-Hall regime reveals a highly unusual particle-hole excitation spectrum, which is calculated from the dynamical polarizability in the random-phase approximation. We find that the elementary neutral excitations in graphene in a magnetic field are unlike those of a standard two-dimensional electron gas: in addition to the upper-hybrid mode, the particle-hole spectrum is reorganized in linear magnetoplasmons that disperse roughly parallel to omega=v(F)q, instead of the usual horizontal (almost dispersionless) magnetoexcitons. These modes could be detected in an inelastic light-scattering experiment.