▎ 摘　　要

The pi-plasmon dispersion in graphene was scrutinized by momentum-resolved electron energy-loss spectroscopy with an improved momentum q resolution and was found to display the square root of the q dispersion characteristic of the collective excitation of two-dimensional electron systems, in contrast to previous experimental and theoretical studies which reported a linear q dispersion. Our theoretical elaborations on the q-dependent spectra affirm this square root of q relation and further unveil an in-plane electronic anisotropy. The physical property of the pi plasmon is thoroughly compared to that of the two-dimensional plasmon due to carriers of the Dirac fermions. A clear distinction between the pi plasmon and the two-dimensional Dirac plasmon is demonstrated, clarifying the common notion about correlating the linearly dispersed Dirac cones with the linear dispersion of the pi plasmon previously reported.