Experimental observation of Weyl points

TLDR

Weyl points are topological band degeneracies with linear dispersion in 3D momentum space, predicted for massless fermions, neutrinos, and condensed‑matter excitations, and proposed for photons in photonic crystals, yet have not been experimentally observed. This study aims to report the first experimental observation of Weyl points in a photonic crystal. The authors achieved this by realizing inversion‑breaking 3D double‑gyroid photonic crystals that preserve time‑reversal symmetry and measuring the resulting band structure.

Abstract

In 1929, Hermann Weyl derived the massless solutions from the Dirac equation - the relativistic wave equation for electrons. Neutrinos were thought, for decades, to be Weyl fermions until the discovery of the neutrino mass. Moreover, it has been suggested that low energy excitations in condensed matter can be the solutions to the Weyl Hamiltonian. Recently, photons have also been proposed to emerge as Weyl particles inside photonic crystals. In all cases, two linear dispersion bands in the three-dimensional (3D) momentum space intersect at a single degenerate point - the Weyl point. Remarkably, these Weyl points are monopoles of Berry flux with topological charges defined by the Chern numbers. These topological invariants enable materials containing Weyl points to exhibit a wide variety of novel phenomena including surface Fermi arcs, chiral anomaly, negative magnetoresistance, nonlocal transport, quantum anomalous Hall effect, unconventional superconductivity[15] and others [16, 17]. Nevertheless, Weyl points are yet to be experimentally observed in nature. In this work, we report on precisely such an observation in an inversion-breaking 3D double-gyroid photonic crystal without breaking time-reversal symmetry.