1Department of Physics, Xiangtan University, Xiangtan 411105 2Key Laboratory of Computational Physical Sciences (Ministry of Education), State Key Laboratory of Surface Physics, and Department of Physics, Fudan University, Shanghai 200433 3Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093
Abstract：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 (NLSs). In particular, several compounds (e.g., certain 3D graphene networks, Cu$_{3}$PdN, Ca$_{3}$P$_{2}$) were discovered to be 3D NLSs, in which the conduction and valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional (2D) Dirac semimetal (e.g., graphene), 2D topological semimetals are much less investigated. Here we propose a new concept of a 2D NLS and suggest that this state could be realized in a new mixed lattice (named 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 2D NLSs due to the band inversion between the cation Hg-$s$ orbital and the anion As-$p_{z}$ orbital with respect to the mirror symmetry. Since the band inversion occurs between two bands with the same parity, this peculiar 2D NLS could be used as transparent conductors. In the presence of buckling or spin-orbit coupling, the 2D NLS state may turn into a 2D Dirac semimetal state or a 2D 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 2D NLS at room temperature. Our work suggests a new route to design topological materials without involving states with opposite parities.
Lv B Q, Muff S, Qian T, Song Z D, Nie S M, Xu N, Richard P, Matt C E, Plumb N C, Zhao L X, Chen G F, Fang Z, Dai X, Dil J H, Mesot J, Shi M, Weng H M and Ding H 2015 Phys. Rev. Lett.115 217601
Lv B Q, Weng H M, Fu B B, Wang X P, Miao H, Richard J Ma P, Huang X C, Zhao L X, Chen G F, Fang Z, Dai X, Qian T and Ding H 2015 Phys. Rev. X 5 031013
[9]
Xu N, Weng H M, Lv B Q, Matt C, Park J, Bisti F, Strocov V N, Gawryluk D, Pomjakushina E, Conder K, Plumb N C, Radovic M, Autès G, Yazyev O V, Fang Z, Dai X, Aeppli G, Qian T, Mesot J, Ding H and Shi M 2016 Nat. Commun.7 11006
[10]
Xu S Y, Belopolski I, Daniel S S, Zhang C, Chang G, Guo C, Bian G, Yuan Z, Lu H, Chang T R, Shibayev P P, Prokopovych M L, Alidoust N, Zheng H, Lee C C, Huang S M, Sankar R, Chou F C, Hsu C H, Jeng H T, Bansil A, Neupert T, Strocov V N, Lin H, Jia S and Hasan M Z 2015 Sci. Adv.1 e1501092
[11]
Huang S M, Xu S Y, Belopolski I, Lee C C, Chang G, Wang B, Alidoust N, Bian G, Neupane M, Zhang C, Jia S, Bansil A, Lin H and Hasan M Z 2015 Nat. Commun.6 7373
[12]
Liu Z K, Zhou B, Zhang Y, Wang Z J, Weng H M, Prabhakaran D, Mo S K, Shen Z X, Fang Z, Dai X, Hussain Z and Chen Y L 2014 Science343 864
Liu Z K, Jiang J, Zhou B, Wang Z J, Zhang Y, Weng H M, Prabhakaran D, Mo S K, Peng H, Dudin P, Kim T, Hoesch M, Fang Z, Dai X, Shen Z X, Feng D L, Hussain Z and Chen Y L 2014 Nat. Mater.13 677
Neupane M, Xu S Y, Sankar R, Alidoust N, Bian G, Liu C, Belopolski I, Chang T R, Jeng H T, Lin H, Bansil A, Chou F and Hasan Z M 2014 Nat. Commun.5 3786
Bian G, Chang T R, Sankar R, Xu S Y, Zheng H, Neupert T, Chiu C K, Huang S M, Chang G, Belopolski I, Sanchez D S, Neupane M, Alidoust N, Liu C, Wang B K, Lee C C, Jeng H T, Bansil A, Chou F, Lin H and Hasan M Z 2015 arXiv:1505.03069v1
[29]
Bian G, Chang T R, Zheng H, Velury S, Xu S Y, Neupert T, Chiu C K, Huang S M, Sanchez D S, Belopolski I, Alidoust N, Chen P J, Chang G, Bansil A, Jeng H T, Lin H and Hasan M Z 2016 Phys. Rev. B 93 121113(R)
2017, Vol. 34 
