Detail:

Abstract： 
In 1929, H. Weyl proposed that the massless solution of the Dirac equation represents a pair of new type of particles, the so-called Weyl fermions. However, their existence in particle physics remains elusive after more than eight decades. Recently, significant advances in both topological insulators and topological semimetals have provided an alternative way to realize Weyl fermions in condensed matter as an emergent phenomenon. Here we report the experimental realization of a Weyl semimetal in TaAs by directly observing the bulk Weyl nodes and surface state Fermi arcs using angle-resolved photoemission spectroscopy (ARPES). Our first-principles calculations, matching remarkably well with the experimental results, further confirm that TaAs is a Weyl semimetal.