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...

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We use ultra-high resolution, tunable, VUV laser-based, angle-resolved photoemission spectroscopy (ARPES) and temperature and field dependent resistivity and thermoelectric power (TEP) measurements to study the electronic properties of WTe2, a compound that manifests exceptionally large, temperature dependent magnetoresistance. The temperature dependence of the TEP shows a change of slope at T=175 K and the Kohler rule breaks down above 70-140 K range. The Fermi surface consists of two electron pockets and two pairs of hole pockets along the X-Γ-X direction. Upon increase of temperature from 40K, the hole pockets gradually sink below the chemical potential. Like BaFe2As2, WTe2 has clear and substantial changes in its Fermi surface driven by modest changes in temperature. In WTe2, this leads to a rare example of temperature induced Lifshitz transition, associated with the complete disappearance of the hole pockets. These dramatic changes of the electronic structure naturally explain unusual features of the transport data.The discovery of giant magnetoresistance (GMR) in Fe/Cr superlattice [1,2], opened a new era of applications in magnetic field sensors, read heads in high density hard disks, random access memories, and galvanic isolators [3]. In the quest of achieving large MR in crystalline materials, a class of manganese oxides colossal magnetoresistance (CMR) materials was discovered that exhibits a large change in resistance with applied magnetic fields but only at low temperatures [4][5][6] The temperature dependent electrical resistivity, Hall coefficient and thermoelectric power of tungstenditelluride have been known for several decades [13] and a three-carrier semi-metal band model [13,14] was proposed to explain the electrical resistivity. Later on, density-functional based augmented spherical wave (ASW) electronic structure calculations and relatively low resolution ARPES [15] were used to study the electronic properties of WTe 2 and further supported the semimetallic nature of this material. However due to the low resolution of these early experiments, no details about the Fermi surface or band dispersion were obtained. Recent ARPES [11] and quantum oscillation [16] results revealed presence of small electron and hole pockets of roughly similar size. These findings were consistent with carrier compensation mechanism as the primary source of the MR effect [10,12]. Furthermore, this study also reported a change of the size of the Fermi pockets between 20K and 100K. More recently, Jiang et al.[17] claimed observation of strong spin-orbital coupling effect and proposed that the backscattering protection mechanism also may play a role in the large nonsaturating MR of WTe 2 .In this letter, we present the results from temperature dependent transport and ultra-high resolution, tunable VUV laser based ARPES[18] measurements. Our data show that there are two electron pockets and four hole pockets along the X-Γ-X direction and a fully occupied light "hole" band at the center of the Brillouin Zone (BZ) located j...