  We have investigated electron band structure of epitaxially grown graphene on
an SiC(0001) substrate using angle-resolved photoemission spectroscopy. In
single-layer graphene, abnormal high spectral intensity is observed at the
Dirac energy whose origin has been questioned between in-gap states induced by
the buffer layer and plasmaron bands induced by electron-plasmon interactions.
With the formation of double-layer graphene, the Dirac energy does not show the
high spectral intensity any longer different from the single-layer case. The
inconsistency between the two systems suggests that the main ingredient of the
high spectral intensity at the Dirac energy of single-layer graphene is the
electronic states originating from the coupling of the graphene $\pi$ bands to
the localized $\pi$ states of the buffer layer, consistent with the theoretical
prediction on the presence of in-gap states.
