  Three-dimensional (3D) topological Weyl semimetals (TWSs) represent a novel
state of quantum matter with unusual electronic structures that resemble both a
"3D graphene" and a topological insulator by possessing pairs of Weyl points
(through which the electronic bands disperse linearly along all three momentum
directions) connected by topological surface states, forming the unique
"Fermi-arc" type Fermi-surface (FS). Each Weyl point is chiral and contains
half of the degrees of freedom of a Dirac point, and can be viewed as a
magnetic monopole in the momentum space. Here, by performing angle-resolved
photoemission spectroscopy on non-centrosymmetric compound TaAs, we observed
its complete band structures including the unique "Fermi-arc" FS and linear
bulk band dispersion across the Weyl points, in excellent agreement with the
theoretical calculations. This discovery not only confirms TaAs as the first 3D
TWS, but also provides an ideal platform for realizing exotic physical
phenomena (e.g. negative magnetoresistance, chiral magnetic effects and quantum
anomalous Hall effect) which may also lead to novel future applications.
