Weyl semi-metal is the three dimensional analog of graphene. According to the quantum field theory, the appearance of Weyl points near the Fermi level will cause novel transport phenomena related to chiral anomaly. In the present paper, we report the first experimental evidence for the long-anticipated negative magneto-resistance generated by the chiral anomaly in a newly predicted time-reversal invariant Weyl semi-metal material TaAs. Clear Shubnikov de Haas oscillations (SdH) have been detected starting from very weak magnetic field. Analysis of the SdH peaks gives the Berry phase accumulated along the cyclotron orbits to be π, indicating the existence of Weyl points.When two non-degenerate bands cross in three dimensional momentum space, the crossing points are called Weyl points, which can be viewed as magnetic monopoles(1) or topological defects (2) in band structure, like "knots" on a rope. Near Weyl points, the low energy physics can be described by Weyl equations (3) with distinct chirality (either left-or right-handed), which mimics the relativistic field theory in particle physics. On lattice system, Weyl points always appear in pairs with opposite chirality and are topologically stable against perturbations that keep translational symmetry (4-7). If two weyl points with opposite chirality meet in the momentum space, they will generally annihilate each other, but may also be stabilized as 3D
Dirac points by additional (such as crystalline) symmetry (8-11). For materials withWeyl points located near the Fermi level, called as Weyl semi-metals (WSMs), exotic low energy physics will be expected, such as the Fermi arcs on the surfaces (5,6), and the chiral-anomaly induced quantum transport (12)(13)(14)(15). Recently, 3D Dirac semimetals, Na 3 Bi and Cd 3 As 2 , have been theoretically predicted (9,10) and experimentally confirmed (16)(17)(18)(19)(20), while WSM are still waiting for its experimental verification in spite of various theoretical proposals (5,6,(21)(22)(23)(24)(25).The anomalous DC transport properties are important consequence of the topological band structure (14,26,27). In topological insulators (TI), the transport properties are dominated by the topological surfaces states (SS), where the lack of back scattering caused by the unique spin structure of the SS leads to the weak anti-localization (WAL) behavior. While in Weyl semi-metals, the bulk states are semi-metallic and dominate the DC transport. In relativistic field theory, for a continue system described by Weyl equation, chiral anomaly can be understood as the non-conservation of the particle number with given chirality, which only happens under the presence of
