Coexistence of topological nodal lines, Weyl points, and triply degenerate points in TaS

We theoretically propose that the single-crystal formed TaS is a new type of topological metal, hosting ring-shaped gapless nodal lines and triply degenerate points (TDPs) in the absence of spin-orbit coupling (SOC). In the presence of SOC, each TDP splits into four TDPs along the high-symmetric line in momentum space, and one of the nodal rings remains closed due to the protection of the mirror reflection symmetry, while another nodal ring is fully gapped and transforms into six pairs of Weyl points (WPs) carrying opposite chirality. The electronic structures of the projected surfaces are also discussed, the unique Fermi arcs are observed, and the chirality remains or vanishes depending on the projection directions. On the (010) projected surface, one may observe a Lifshitz transition. The new type of topological metal TaS is stable and experimentally achievable, and the coexistence of topological nodal lines, WPs, and TDPs in TaS makes it a potential candidate to study the interplay between different types of topological fermions.