PT - JOURNAL ARTICLE AU - Xu, Su-Yang AU - Belopolski, Ilya AU - Alidoust, Nasser AU - Neupane, Madhab AU - Bian, Guang AU - Zhang, Chenglong AU - Sankar, Raman AU - Chang, Guoqing AU - Yuan, Zhujun AU - Lee, Chi-Cheng AU - Huang, Shin-Ming AU - Zheng, Hao AU - Ma, Jie AU - Sanchez, Daniel S. AU - Wang, BaoKai AU - Bansil, Arun AU - Chou, Fangcheng AU - Shibayev, Pavel P. AU - Lin, Hsin AU - Jia, Shuang AU - Hasan, M. Zahid TI - Discovery of a Weyl fermion semimetal and topological Fermi arcs AID - 10.1126/science.aaa9297 DP - 2015 Aug 07 TA - Science PG - 613--617 VI - 349 IP - 6248 4099 - http://science.sciencemag.org/content/349/6248/613.short 4100 - http://science.sciencemag.org/content/349/6248/613.full SO - Science2015 Aug 07; 349 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.