RT Journal Article SR Electronic T1 Discovery of a Weyl fermion semimetal and topological Fermi arcs JF Science JO Science FD American Association for the Advancement of Science SP 613 OP 617 DO 10.1126/science.aaa9297 VO 349 IS 6248 A1 Xu, Su-Yang A1 Belopolski, Ilya A1 Alidoust, Nasser A1 Neupane, Madhab A1 Bian, Guang A1 Zhang, Chenglong A1 Sankar, Raman A1 Chang, Guoqing A1 Yuan, Zhujun A1 Lee, Chi-Cheng A1 Huang, Shin-Ming A1 Zheng, Hao A1 Ma, Jie A1 Sanchez, Daniel S. A1 Wang, BaoKai A1 Bansil, Arun A1 Chou, Fangcheng A1 Shibayev, Pavel P. A1 Lin, Hsin A1 Jia, Shuang A1 Hasan, M. Zahid YR 2015 UL http://science.sciencemag.org/content/349/6248/613.abstract AB Weyl fermions—massless particles with half-integer spin—were once mistakenly thought to describe neutrinos. Although not yet observed among elementary particles, Weyl fermions may exist as collective excitations in so-called Weyl semimetals. These materials have an unusual band structure in which the linearly dispersing valence and conduction bands meet at discrete “Weyl points.” Xu et al. used photoemission spectroscopy to identify TaAs as a Weyl semimetal capable of hosting Weyl fermions. In a complementary study, Lu et al. detected the characteristic Weyl points in a photonic crystal. The observation of Weyl physics may enable the discovery of exotic fundamental phenomena.Science, this issue p. 613 and 622A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs). Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.