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Weyl fermions

Massless yet real

Nature Materials volume 14, page 863 (2015) Cite this article

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The quantum description of spin-1/2 particles is given by the solutions of the Dirac equation. In 1929, one year after Paul Dirac published it, Hermann Weyl reported that, for massless particles, the equation could be split into a system of two equations whose solutions are distinguished by chirality. These massless spin-1/2 particles, so-called Weyl fermions, have never been found in nature. Neutrinos had long been considered to be Weyl fermions, until their tiny mass — which is at least one million times lighter than that of electrons — was detected at the Super-Kamiokande neutrino observatory in Japan in the late 1990s, and then in subsequent experiments elsewhere.

Zahid Hasan and co-authors probed the electronic structure of TaAs with vacuum ultraviolet and soft X-ray angle-resolved photoemission spectroscopy (ARPES). Ultraviolet measurements are sensitive to the surface of the material and revealed the existence of Fermi arcs, whereas soft X-ray ARPES is sensitive to the bulk and showed the existence of the Weyl nodes. The authors demonstrate that the positions of these features in momentum space, and the particular form of the band structure close to the Weyl points, are in good agreement with their theoretical predictions. And the fact that the terminations of the Fermi arcs coincide with the projections of the Weyl nodes provides further evidence for the topological nature of the observations.

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References

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  1. David Ciudad
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Ciudad, D. Massless yet real. Nature Mater 14, 863 (2015). https://doi.org/10.1038/nmat4411

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