  The wavefunction of massless Dirac fermions is a two-component spinor. In
graphene, a one-atom-thick film showing two-dimensional Dirac-like electronic
excitations, the two-component representation reflects the amplitude of the
electron wavefunction on the A and B sublattices. This unique property provides
unprecedented opportunities to image the two components of massless Dirac
fermions spatially. Here we report atomic resolution imaging of the
two-component Dirac-Landau levels in a gapped graphene monolayer by scanning
tunnelling microscopy and spectroscopy. A gap of about 20 meV, driven by
inversion symmetry breaking by the substrate potential, is observed in the
graphene on both SiC and graphite substrates. Such a gap splits the n = 0
Landau level (LL) into two levels, 0+ and 0-. We demonstrate that the amplitude
of the wavefunction of the 0- LL is mainly at the A sites and that of the 0+ LL
is mainly at the B sites of graphene, characterizing the internal structure of
the spinor of the n = 0 LL. This provides direct evidence of the two-component
nature of massless Dirac fermions.
