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Initially, the „ordinary“ Weyl fermion also existed only in theory. For a long time it was assumed that neutrinos might be Weyl fermions, but then researchers found out that they have an, albeit tiny, mass (this year’s Nobel prize in Physics was awarded for its discovery). Still, physicists continued to believe that Weyl fermions could occur as quasiparticles (i.e., as collective states of many electrons interacting with the atomic nuclei in a crystal) in so-called semimetals.
Finally, in July of this year the massless fermions were actually detected experimentally in Princeton, where Hermann Weyl had once theoretically predicted them, and simultaneously in Beijing. In a tantalum arsenide crystal the researchers found quasiparticles that had exactly the properties expected for Weyl fermions. Their lack of a mass means, among other things, that they can move extremely fast and without being disturbed by obstacles in the crystal. Scientists are already speculating that this could open up entirely new possibilities for electronic devices.
The Particle That Shouldn’t Exist
The type-2 Weyl fermions now found theoretically differ from those conventional Weyl fermions in one crucial aspect: they shouldn’t really exist at all. At least not if, as did Hermann Weyl, one assumes that fermions have to obey the rules of Albert Einstein’s special theory of relativity and, hence, that they have to conserve the so-called Lorentz symmetry. “In the wild”, particles do, indeed, have to abide by those rules. In the artificial universe of a crystal, however, the Lorentz symmetry can be broken, which causes the possible energy states of electrons to differ distinctly from those that lead to the formation of ordinary Weyl fermions.
These peculiar energy states, in turn, could endow materials containing type-2 Weyl fermions with strange and possibly useful properties – for instance the ability to conduct electric current only in certain directions under the influence of a magnetic field. Whether those properties prove to be viable in practice remains to be seen. “For now, the new Weyl fermion is a physical oddity”, Soluyanov concedes, “but a very exciting one – and there is certainly potential for applications”. In any case, the scientists have now, after decades, finally completed Hermann Weyl’s research.
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