General construction of three-dimensional Z2 monopole charge nodal line semimetals and prediction of abundant candidate materials

The interplay between symmetry and topology led to the concept of symmetry-protected topological states, including all noninteracting and weakly interacting topological quantum states. Among them, recently proposed nodal line semimetal states with space-time inversion (PT) symmetry which are classified by the Stiefel-Whitney characteristic class associated with real vector bundles and can carry a nontrivial Z2 monopole charge have attracted widespread attention. However, such three-dimensional (3D) Z2 nodal line semimetals have been poorly studied, such as how to construct them and how to predict them efficiently. We generally construct 3D Z2 nodal line semimetals by stacking of 2D PT-symmetric Dirac semimetals via nonsymmorphic symmetries. Based on our construction scheme, numerous models can be produced. Combined with first-principles calculations, we predict abundant candidate electronic and phononic materials for Z2 nodal line semimetals, namely two kinds of electronic materials: fourteen Si and Ge structures and 108 transition metal dichalcogenides MX2 (M=Cr, Mo, W, X=S, Se, Te) and one kind of phononic material: SiH. Our theoretical construction scheme can be directly applied to metamaterials and circuit systems. Our work greatly enriches the candidate materials and deepens the understanding of Z2 nodal line semimetal states.