Emergence of topological nodal lines and type-II Weyl nodes in the strong spin-orbit coupling system InNbX2 (X=S,Se)

Using first-principles density functional calculations, we systematically investigate electronic structures and topological properties of layered materials InNbX2 (X=S, Se). In the absence of spin-orbit coupling (SOC), both compounds show nodal lines protected by mirror symmetry. Including SOC, the Dirac rings in InNbS2 split into two Weyl rings. This unique property is distinguished from other discovered nodal-line materials, which normally require the absence of SOC. On the other hand, SOC breaks the nodal lines in InNbSe2, and the compound becomes a type-II Weyl semimetal with 12 Weyl points in the Brillouin zone. Using a supercell slab calculation, we study the dispersion of Fermi arc surface states in InNbSe2; we also utilize a coherent potential approximation to probe their tolerance to the surface disorder effects. The quasi-two-dimensionality and the absence of toxic elements make these two compounds an ideal experimental platform for investigating novel properties of topological semimetals.