# Colloquium: Relativistic Fermions Generated by Square Lattices in Layered Compounds

## Main Content

Recent discoveries of three dimensional (3D) topological semimetals, including Dirac semimetals (DSM), Dirac nodal-line semimetals and Weyl semimetals, have generated immense interests since they represent new topological states of quantum matters. Both Dirac and Weyl semimetals feature relativistic fermions with linearly dispersing excitations. In this talk, I will first give a brief introduction to this emerging area and discuss how to characterize the properties of Dirac/Weyl fermions. Then I will present our recent studies on topological semimetals [1-6]. I will show nearly massless Dirac fermions generated by 2D Sb layers coexists with ferromagnetism in Sr_{1-y}Mn_{1-z}Sb_{2}, which offers a rare opportunity to investigate the effect of time reversal symmetry breaking on the electronic band structure and explore a possible ferromagnetic Weyl state [1-2]. I will also report our recent discovery of two new nodal line semimetals – ZrSiSe and ZrSiTe [4]. The nodal-line semimetals exhibit Dirac cones along a one dimensional line/loop in their electronic structure, contrasted with the Dirac/Weyl cones with discrete nodes in Dirac/Weyl semimetals. We have found signatures of nodal-line fermions from quantum oscillation experiments in these two materials and demonstrated that their atomically thin crystals are accessible via mechanical exfoliation, which raises the possibility of realizing the theoretically predicted 2D topological insulators [7] in monolayers of ZrSiSe and zrSiTe. Finally I will discuss exotic quantum transport behavior of Weyl semimetal YbMnBi_{2} [6], which results from the zeroth Landau Level, a unique property of topological materials.

References:

[1] J. Hu et al., Scientific Report **6**, 18674 (2016)

[2] J.Y. Liu et al., arXiv:1507.07978.

[3] J.Y. Liu et al., Scientific Report 6, 30525 (2016).

[4] J. Hu et al., PRL **117**, 016602 (2016).

[5] J. Hu et al., arXiv: 1604.01567

[6] J. Hu et al., arXiv: 1608.05956

[7] Q. Xu et al., PRB 92, 205310 (2015).