▎ 摘　　要

Two-dimensional electrons in graphene axe known to behave as massless fermions with Dirac-Weyl type linear dispersion near the Dirac crossing points. We have investigated the collective excitations of this system in the presence or absence of an external magnetic field. Unlike in the conventional two-dimensional electron system, the v = 1/m fractional quantum Hall state in graphene was found to be most stable in the n = 1 Landau level. In the zero field case, but in the presence of the spin-orbit interaction, an undamped plasmon mode was found to exist in the gap of the single-particle continuum.