Abstract:A Weyl semimetal is a crystal which 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 the first Weyl semimetal, 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 nodes, consistent with their topological character.Our work opens the field for the experimental study of Weyl fermions in physics and materials science.NoteAdded: This experimental discovery (Science 2015) is based on our earlier 2014 theoretical discovery/prediction reported at [Huang et al., Nature Commun. 6:7373 (2015)

Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. Whereas Dirac fermions have been known for decades, the latter two have not been observed as any fundamental particle in high-energy physics, and have emerged as a much-sought-out treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. It has received worldwide interest and is believed to open the next era of condensed matter physics after graphene and three-dimensional topological insulators. However, experimental research has been held back because Weyl semimetals are extremely rare in nature. Here, we present the experimental discovery of the Weyl semimetal state in an inversion-symmetry-breaking single-crystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft X-ray and ultraviolet photoemission spectroscopy, we systematically study both the surface and bulk electronic structure of NbAs. We experimentally observe both the Weyl cones in the bulk and the Fermi arcs on the surface of this system. Our ARPES data, in agreement with our theoretical band structure calculations, identify the Weyl semimetal state in NbAs, which provides a real platform to test the potential of Weyltronics. W eyl semimetals have received significant attention in recent years because they extend the classification of topological phases beyond insulators, host exotic Fermi arc surface states, demonstrate unusual transport phenomena and provide an emergent condensed matter realization of Weyl fermions, which do not exist as fundamental particles in the standard model 1-21 . Such kind of topologically non-trivial semimetals are believed to open a new era in condensed matter physics. In contrast to topological insulators, where only the surface states are interesting, a Weyl semimetal features unusual band structure in the bulk and on the surface, leading to novel phenomena and potential applications. This opens up unparalleled research opportunities, where both bulk-and surface-sensitive experimental probes can measure the topological nature and detect quantum phenomena. In the bulk, a Weyl semimetal has a band structure with band crossings, Weyl nodes, which are associated with definite chiral charges. Unlike the two-dimensional Dirac points in graphene, the surface-state Dirac point of a threedimensional topological insulator or the three-dimensional Dirac points in the bulk of a Dirac semimetal, the degeneracy associated with a Weyl node does not require any symmetry for its protection, other than the translation symmetry of the crystal lattice. The low-energy quasiparticle excitations of a Weyl semimetal are chiral fermions described by the Weyl equation, well known in highenergy physics, which gives rise to a condensed matter analogue of the chiral anomaly associated with a negative magnetoresistance in transport [16][17][18][19][20][21] . On the surface, the non-trivial topology guarantees the existence of surface states in the f...

Weyl semimetals provide the realization of Weyl fermions in solid-state physics. Among all the physical phenomena that are enabled by Weyl semimetals, the chiral anomaly is the most unusual one. Here, we report signatures of the chiral anomaly in the magneto-transport measurements on the first Weyl semimetal TaAs. We show negative magnetoresistance under parallel electric and magnetic fields, that is, unlike most metals whose resistivity increases under an external magnetic field, we observe that our high mobility TaAs samples become more conductive as a magnetic field is applied along the direction of the current for certain ranges of the field strength. We present systematically detailed data and careful analyses, which allow us to exclude other possible origins of the observed negative magnetoresistance. Our transport data, corroborated by photoemission measurements, first-principles calculations and theoretical analyses, collectively demonstrate signatures of the Weyl fermion chiral anomaly in the magneto-transport of TaAs.