  Chiral fermions in solid state feature "Fermi arc" states, connecting the
surface projections of the bulk chiral nodes. The surface Fermi arc is a
signature of nontrivial bulk topology. Unconventional chiral fermions with an
extensive Fermi arc traversing the whole Brillouin zone have been theoretically
proposed in CoSi. Here, we use scanning tunneling microscopy / spectroscopy to
investigate quasiparticle interference at various terminations of a CoSi single
crystal. The observed surface states exhibit chiral fermion-originated
characteristics. These reside on (001) and (011) but not (111) surfaces with
pi-rotation symmetry, spiral with energy, and disperse in a wide energy range
from ~-200 to ~+400 mV. Owing to the high-energy and high-space resolution, a
spin-orbit coupling-induced splitting of up to ~80 mV is identified. Our
observations are corroborated by density functional theory and provide strong
evidence that CoSi hosts the unconventional chiral fermions and the extensive
Fermi arc states.
