  Trilayer graphene (TLG) exhibits rich novel electronic properties and
extraordinary quantum Hall phenomena owning to enhanced electronic interactions
and tunable chirality of its quasiparticles. Here, we report direct observation
of chirality transition of quasiparticles at stacking solitons of TLG via
spatial-resolved Landau level spectroscopy. The one-dimensional stacking
solitons with width of the order of 10 nm separate adjacent Bernal-stacked TLG
and rhombohedral-stacked TLG. By using high field tunneling spectra of scanning
tunneling microscopy, we measured Landau quantization in both the
Bernal-stacked TLG and the rhombohedral-stacked TLG and, importantly, we
observed evolution of quasiparticles between the chiral degree l = 1&2 and l =
3 across the stacking domain wall solitons. Our experiment indicates that such
a chirality transition occurs smoothly, accompanying the transition of the
stacking orders of TLG, around the domain wall solitons. This result
demonstrates the important and hitherto neglected relationship between the
crystallographic stacking order and the chirality of quasiparticles in graphene
systems.
