Quantum mechanics

Theoretical physics

Fermi level

Massless particle

Band gap

Physics

Mathematics

Fermion

Semimetal

Superconductivity

Condensed matter physics

Combinatorics

Electron

Topology (electrical circuits)

Quasiparticle

Fermi Gamma-ray Space Telescope

Weyl semimetal

Science

A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs). Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.

Discovery of a Weyl fermion semimetal and topological Fermi arcs