Topological nodal lines and hybrid Weyl nodes in YCoC₂

Based on first-principles calculations and effective model analysis, we propose that the noncentrosymmetric superconductor YCoC₂ in normal state is a topological semimetal. In the absence of spin-orbit coupling (SOC), it can host two intersecting nodal rings protected by two mirror planes, respectively. One ring is composed of type-I nodes, where the two crossing bands have opposite slope sign in their dispersions. The other ring consists of both type-I and type-II nodes (the slope signs of the two bands are the same in certain direction). In the presence of SOC, the former nodal ring is gapped totally while the latter one evolves into ten pairs of Weyl nodes, with two of them being type-I and eight being type-II. The type-II Weyl nodes are further classified into two kinds with different velocity matrices when described in Weyl equation near the nodes. Fermi arcs from topological surface states are observed in the surface projected energy dispersions. It is notable that YCoC₂ has been reported as a superconductor with a critical temperature T_c of 4.2 K. This makes it very attractive since including superconducting into a topological semimetal state might result in topological superconductivity and be used to synthesize Majorana zero modes.