Two-Dimensional Node-Line Semimetals in a Honeycomb-Kagome Lattice
  Recently, the concept of topological insulators has been generalized to topological semimetals, including three-dimensional (3D) Weyl semimetals, 3D Dirac semimetals, and 3D node-line semimetals. In particular, several compounds (e.g., certain three-dimensional graphene networks, Cu3PdN, Ca3P2) were discovered to be 3D node-line semimetals, in which the conduction and the valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional (2D) Dirac semimetal (e.g., in graphene), 2D topological semimetals are much less investigated. Here, we propose the new concept of a 2D node-line semimetal and suggest that this state could be realized in a new mixed lattice (we name it as HK lattice) composed by kagome and honeycomb lattices. We find that A3B2 (A is a group-IIB cation and B is a group-VA anion) compounds (such as Hg3As2) with the HK lattice are 2D node-line semimetals due to the band inversion between cation s orbital and anion pz orbital. In the presence of buckling or spin-orbit coupling, the 2D node-line semimetal state may turn into 2D Dirac semimetal state or 2D topological crystalline insulating state. 
J. L. Lu, W. Luo, X. Y. Li, S. Q. Yang, J. X. Cao, X. G. Gong, H. J.   Xiang
cond-mat.mtrl-sci