Weyl fermions: Massless yet real.

863 news & views and patterning, opens up a wide range of new applications for hot-electron-driven chemistry. The LEE source might also prove a critical tool for obtaining a deeper nanoscale understanding of radiation chemistry and biology, and material degradation by high-energy radiation in general. The quantum description of spin-1/2 particles is given by the solutions of the Dirac equation. In 1929, one year after Paul Dirac published it, Hermann Weyl reported that, for massless particles, the equation could be split into a system of two equations whose solutions are distinguished by chirality. These massless spin-1/2 particles, so-called Weyl fermions, have never been found in nature. Neutrinos had long been considered to be Weyl fermions, until their tiny mass — which is at least one million times lighter than that of electrons — was detected at the Super-Kamiokande neutrino observatory in Japan in the late 1990s, and then in subsequent experiments elsewhere. Theoretical studies have predicted that Weyl fermions, in the form of low-energy excitations or quasiparticles, could be hosted in crystals — called Weyl semimetals — whose conduction properties are determined, as in topological insulators, by the topological properties of the bulk electron wavefunctions. Whereas topological insulators are insulators in the bulk, in Weyl semimetals the conduction and valence bands touch each other at some particular energies (Weyl points, represented as red and blue spheres in the figure), allowing the propagation of electron waves that behave as Weyl quasiparticles in the bulk. And, whereas the surface of a topological insulator displays a metallic behaviour, that of a Weyl semimetal shows a more complex behaviour in momentum space, with spin-polarized surface states (Fermi arcs, shown in green) connecting two bulk Weyl nodes of opposite chirality (blue and red dots). Now, following previous theoretical Massless yet real WEYL FERMIONS predictions 1,2 , a group led by M. Zahid Hasan reports in Science the experimental demonstration that monopnictide TaAs is a Weyl semimetal 3. Zahid Hasan and co-authors probed the electronic structure of TaAs with vacuum ultraviolet and soft X-ray angle-resolved photoemission spectroscopy (ARPES). Ultraviolet measurements are sensitive to the surface of the material and revealed the existence of Fermi arcs, whereas soft X-ray ARPES is sensitive to the bulk and showed the existence of the Weyl nodes. The authors demonstrate that the positions of these features in momentum space, and the particular form of the band structure close to the Weyl …