Consecutive topological transitions of helical Fermi arcs at saddle points in CoSi

The CoSi family hosts unconventional topological nodes with nonzero Chem numbers. The nontrivial topology is manifested by conspicuous surface Fermi arcs connecting surface projections of the nodes. Here, using angle-resolved photoemission spectroscopy, we have systematically investigated the (001) surface states of pristine and Ni-doped CoSi. The surface states form saddle-like band structures at/near the time-reversal invariant point near the Fermi level. The Fermi arcs undergo consecutive Lifshitz transitions at the saddle points (X) over bar, leading to changes of the Fermi arc configuration. As the density of states has a van Hove singularity at the saddle points, exotic many-body physical phenomena may emerge accompanied by the topological transitions of surface Fermi arcs.