Discovery of a Weyl fermion state with Fermi arcs in niobium arsenide

Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. 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 calculations, identify the Weyl semimetal state in NbAs, which provides a platform to test the potential of Weyltronics.