Experimental observation of Weyl points

TL;DR

This paper reports the first experimental observation of Weyl points in a 3D double-gyroid photonic crystal, confirming their existence and topological properties in a natural material system.

Contribution

It provides the first experimental evidence of Weyl points in a natural 3D photonic crystal with inversion symmetry breaking.

Findings

Weyl points observed in a 3D photonic crystal

Weyl points exhibit topological monopole characteristics

Confirmation of Weyl physics in photonic systems

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.