In their experiments, Hasan and colleagues and Ding and colleagues used angle-resolved photoemission spectroscopy (ARPES) to detect the Fermi arcs, characteristic of Weyl nodes, on the surface of TaAs. ARPES is an ideal tool for such a purpose. The technique involves shining light on a surface and measuring the energy and momentum of ejected electrons. This allows for the explicit determination of both bulk nodes and the Fermi-arc surface states. Ding’s team used an interesting strategy to identify a Fermi arc and distinguish it from a more conventional closed Fermi surface (Fig. 1). They defined a closed contour in the momentum space spanned by their measurements and investigated how many times surface states at the Fermi energy crossed this contour. Such a contour will intersect a regular Fermi surface an even number of times. But it will intersect a Fermi arc an odd number of times if the arc encloses the projection of a Weyl point, thus providing a clean signature.

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