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Recently, the concept of topological insulators has been generalized to topological semimetals, including threedimensional(3 D) Weyl semimetals, 3 D Dirac semimetals, and 3 D node-line semimetals(NLSs). In particular,several compounds(e.g.,certain 3 D graphene networks, Cu_3 PdN,Ca_3 P_2) were discovered to be 3 D NLSs, in which the conduction and valence bands cross at closed lines in the Brillouin zone.Except for the two-dimensional(2 D)Dirac semimetal(e.g.,graphene), 2 D topological semimetals are much less investigated. Here we propose a new concept of a 2 D NLS and suggest that this state could be realized in a new mixed lattice(nawed as HK lattice)composed by Kagome and honeycomb lattices. It is found that A_3 B_2(A is a group-ⅡB cation and B is a group-VA anion) compounds(such as Hg_3 As_2) with the HK lattice are 2 D NLSs due to the band inversion between the cation Hg-s orbital and the anion As-p_z orbital with rcespect to the mirror symmetry, Since the band inversion occurs between two bands with the same parity, this peculiar 2 D NLS could be used as transparent conductors.In the presence of buckling or spin-orbit coupling, the 2 D NLS state may turn into a 2 D Dirac semimetal state or a 2 D topological crystalline insulating state. Since the band gap opening due to buckling or spin-orbit coupling is small, Hg_3 As_2 with the HK lattice can still be regarded as a 2 D NLS at room temperature.Our work suggests a new route to design topological materials without involving states with opposite parities.
Recently, the concept of topological insulators has been generalized to topological semimetals, including threedimensional (3 D) Weyl semimetals, 3 D Dirac semimetals, and 3 D node-line semimetals (NLSs). In particular, several compounds (eg, certain 3 D graphene networks, Cu_3 PdN, Ca_3 P_2) were discovered to be 3 D NLSs, where the conduction and valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional (2 D) Dirac semimetal (eg, graphene) , 2 D topological semimetals are much less investigated. Here we propose a new concept of a 2 D NLS and suggest that this state could be realized in a new mixed lattice (nawed as HK lattice) composed by Kagome and honeycomb lattices. It is found that A_3 B_2 (A is a group -ⅡB cation and B is a group-VA anion) compounds (such as Hg_3 As_2) with the HK lattice are 2 D NLSs due to the band inversion between the cation Hg-s orbital and the anion As-p_z orbital with rcespect to the mirror symmetry, Since the band inversion occurs between two bands with the same parity, this peculiar 2 D NLS could be used as a transparent conductor. In the presence of a buckling or spin-orbit coupling, the 2 D NLS state may turn into a 2 D Di sem semital state or a 2 D topological crystalline insulating state. Since the band gap opening due to buckling or spin-orbit coupling is small, Hg_3 As_2 with the HK lattice can still be regarded as a 2 D NLS at room temperature. Our work suggests a new route to design topological materials without involving states with opposite parities