Where the Weyl Things Are

In 1929, the mathematician Hermann Weyl proposed a simplified version of the Dirac equation for relativistic particles, whose solution predicted massless fermions with a definite handedness (or chirality) [1]. Weyl’s equation was intended as a model of elementary particles, but in nearly 80 years, no candidate Weyl fermions have ever been observed in high-energy experiments. Now, analogs of the particles have been discovered in the completely different setting of solids, as reported by three independent research groups. Two teams, one led by Zahid Hasan of Princeton University, New Jersey [2], and the other by Hong Ding of the Chinese Academy of Sciences in Beijing [3], have shown that electron excitations in the semimetal tantalum arsenide (TaAs) behave like Weyl fermions. In a concurrent paper, Marin Soljačić of the Massachusetts Institute of Technology, Cambridge, and his colleagues have observed Weyl-like states in a photonic crystal [4]. Weyl particles are associated with a number of unexpected phenomena, and the authors’ findings may lead to applications in optics or electronics.