Discovery of the first Weyl fermion semimetal and topological Fermi arcs in TaAs*

Weyl semimetals have opened a new era in condensed matter physics and materials science. They host Weyl fermions as emergent quasiparticles and admit a topological classification that protects Fermi arc surface states on the boundary. 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. We directly observe the Weyl fermions and the Fermi arcs in a TaAs single crystal and demonstrate its topological character. Our work opens the field for studying of Weyl fermions in table-top experiments.