Experimental discovery of Weyl semimetal TaAs

In 1929, H. Weyl proposed that the massless solution of the Dirac equation represents a pair of a new type of particles, the so-called Weyl fermions. However, their existence as a fundamental particle remains evasive after more than eight decades. Neutrinos were once considered to be Weyl fermions until its mass was found. Recently, it was proposed that Weyl fermions can be hosted in a class of exotic condensed matter, so called Weyl semimetal (WSM), as quasiparticles of its low-energy excitation when two non-degenerate bands cross near the Fermi level in three-dimensional momentum space. The crossing point is called Weyl node. A Weyl node is a singularity point of the Berry curvature, which can be viewed as a magnetic monopole in momentum space. These WSMs possess pairs of Weyl nodes with opposite chirality in the bulk. One of the hallmarks in a WSM is the emergence of surface Fermi arcs, which connect the projections of Weyl nodes with opposite chiral charge on the surface.