中文摘要：

近日,清华大学物理系研究人员及合作者在Weyl半金属研究中取得新进展：首次在实验上观测到第二类Weyl半金属（MoTe2）上的拓扑费米弧,在真实材料中证实了破坏洛伦兹不变性的第二类Weyl费米子的存在。相关文章于2016年9月6日在Nature Physics上发表。这是继去年第一类Weyl费米子得到证实之后凝聚态物理研究上的又一重大突破。什么是第二类Weyl费米子？它与第一类Weyl费米子有何区别？作者通过本文来做一个简单的介绍。

英文摘要：

Recently, researchers from the Department of Physics at Tsinghua University and collaborators have made important progress in the study of Weyl semimetals. They have reported the first experimental observation of topological Fermi arcs in a type-Ⅱ semimetal Mo Te2, thus confirming the violation of Lorentz invariance in type-Ⅱ Weyl fermions in a real material. The results were published online in Nature Physics on Sept. 6, 2016. This is a major progress in the field of condensed matter physics, after the realization of type-Ⅰ Weyl fermions. What are type-Ⅱ Weyl fermions？ And how is it different from type-Ⅰ Weyl fermions？ Here we shall give a brief introduction.