Observation of unconventional chiral fermions with long Fermi arcs in CoSi

In condensed system, Chiral fermions exist at nodes (that is, robust band-crossing points protected from being gapped by nontrivial band topology) that carry a nonzero Chern number. Recently, we reported two types of unconventional chiral fermions in CoSi, which are threefold-degenerate node at point and fourfold-degenerate node at R point. These nodes at point and R point carry nonzero Chern number and we observed two surface Fermi arcs on surface to connect their projection on (001) surface, which are related by a π rotation about Γ. Further, we show the two helical surface states encircle the projection of Γ point and R point, which can reveal the topological nature of bulk nodes in CoSi.