Experimental realization of a Weyl semimetal phase with Fermi arc surface states in TaAs

A Weyl semimetal is a gapless topological state of matter described by a unique bulk-boundary correspondence in the electronic band structure. The low-energy bulk electrons are linearlydispersing Weyl fermions, while the Weyl nodes are connected on the boundary by Fermi arc surface states. Using angle-resolved photoemission spectroscopy, we show that the inversion-breaking single crystalline compound TaAs hosts an electronic structure with Fermi arc surface states. We directly observe that these Fermi arc surface states terminate into the bulk bands and that these bulk bands disperse linearly, forming Weyl cones. These results are remarkably consistent with recent theoretical predictions of a Weyl semimetal phase in this compound. Our detailed surface and bulk band structure measurements demonstrate that TaAs hosts topologically non-trivial Fermi arcs and Weyl cones, providing the first experimental realization of a Weyl semimetal in condensed matter physics.