When Paul Dirac introduced his famous equation in 1929, he aimed to describe one well-known particle: the electron. Shortly thereafter, Hermann Weyl pointed out that the equation has another solution when the mass is set to zero. The so-called Weyl fermions described by that solution would be charged, like electrons, but being massless, they would travel faster and with less energy dissipation than electrons; that feature would make them interesting candidates for use in electronic and spintronic devices. No such elementary particle has yet been found; however, in 2015 researchers identified the first Weyl semimetal (WSM), tantalum arsenide, which can host quasiparticles with the properties of Weyl fermions.

A WSM must have a broken symmetry, and in TaAs it was inversion symmetry (IS). However, researchers have continued searching for materials—particularly ferromagnetic materials—that instead rely on broken time-reversal symmetry (TRS), because tying the crystal’s properties to magnetism makes them potentially tunable. The underlying electronic band structure that gives the materials their unique properties is also simpler in TRS-breaking materials, so they are considered prototypes for studying Weyl fermion behavior.

Three new papers examine two different materials to provide experimental evidence for magnetic WSMs. Yulin Chen’s team at Oxford University and Haim Beidenkopf’s team at the Weizmann Institute, together with collaborators, presented studies of Co 3 Sn 2 S 2 , and Zahid Hasan’s group at Princeton University looked at Co 2 MnGa. All three relied on numerical calculations to guide their choice of material and their data analysis.

Confirming the materials as WSMs required investigations of their electronic structures in reciprocal space. The groups looking at Co 3 Sn 2 S 2 studied surface states in its band structure. Paired with theoretical calculations, their measurements support the existence of Weyl nodes—points in the Brillouin zone of the bulk crystal at which the valence and conduction bands touch. In Co 2 MnGa, the researchers paired spectroscopic and electron transport measurements to observe the Weyl nodes directly.