  ABA-stacked trilayer graphene (TLG), the simplest system consisting of both
massless and massive Dirac fermions, is expected to exhibit many interesting
broken-symmetry quantum Hall states and interaction-induced phenomena. However,
difficulties in microscopically identifying the stacking order of the TLG and
limited spectroscopic resolution have stymied experimental probes of these
interesting states and phenomena in scanning tunneling microscopy (STM)
studies. Here we studied the detailed features of the electronic structure in
the ABA TLG by using high-resolution STM measurements. Landau-level (LL)
crossings of the massless and massive Dirac fermions, and effective mass
renormalization of the massive Dirac fermions were observed, indicating strong
electron-electron interactions in the ABA TLG. Most unexpectedly, we observed
unconventional splittings of the lowest LLs for both the massless and massive
Dirac fermions in high perpendicular magnetic fields. These splittings of the
LLs, which are beyond the description of tight-binding calculations, reveal
unexplored broken-symmetry quantum Hall states in the ABA TLG induced by
many-body effects.
