Topological semimetal Sr$_{\mathrm{1-y}}$Mn$_{\mathrm{1-Z}}$Sb$_{\mathrm{2}}$

Recent discoveries of topological Weyl semimetals in noncentrosymmetric monopnictides TX (T$=$Ta/Nb, X$=$As/P) [1-4] and photonic crystals$^{\mathrm{\thinspace }}$[5] have generated immense interests since they represent new topological states of quantum matter. Time reversal symmetry (TRS) breaking Weyl semimetal was also recently reported in YbMnBi2 [6]. In this talk, we report a new type of topological semimetal phase arising from two-dimensional Sb layers in Sr$_{\mathrm{1-}}_{y}$Mn$_{\mathrm{1-}}_{z}$Sb$_{\mathrm{2}}$ ($y$, $z $\textless 0.1), which coexists with ferromagnetism. Through quantum transport measurements on this material, we reveal remarkable signatures of relativistic fermions, including light effective quasiparticle mass, high carrier mobility, a $\pi $ Berry phase and valley polarized interlayer conduction. Given Sr$_{\mathrm{1-}}_{y}$Mn$_{\mathrm{1-}}_{z}$Sb$_{\mathrm{2}}$ shows ferromagnetism, it offers a wonderful opportunity to explore the TRS breaking Weyl state. [1]. H. Weng \textit{et al}., \textit{Phys. Rev. X} \textbf{5}, 011029, (2015). [2]. S.M. Huang \textit{et al.}, \textit{Nature Commun.} \textbf{6}, (2015). [3]. S.Y. Xu\textit{ et al., Science } \textbf{349}, 613-617 (2015). [4]. B.Q. Lv\textit{ et al.,} \textit{Phys. Rev. X} \textbf{5}, 031013, (2015). [5]. L. Lu \textit{et al}., \textit{Science} \textbf{349}, 622-624 (2015). [6] S. Borisenko \textit{et al.,} \textit{arXiv:1507.04847}, (2015). [7] J.Y. Liu et al., \textit{arXiv:1507.07978}, (2015).