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A range of exotic quantum effects have been spotted in class of materials called “topological chiral crystals”. The materials were studied by a team of physicists let by Zahid Hasan at Princeton University, who predicted and then measured properties including the emergence of magnetic monopoles.

Chiral crystals have unit cells that cannot be superimposed onto their mirror images – a property that can lead to very interesting and potentially useful magnetic, optical, and electronic properties.

In a previous theoretical study, Hasan’s team predicted the collective behaviour of electrons in nonmagnetic chiral crystals. They found that the materials should contain particle-like electronic excitations called “Weyl fermions”. These quasiparticles resemble electrons, but are massless and have chiral properties

Now, Hasan and colleagues have used the group theory of crystals to look deeper into the properties of these materials. Their calculations predict the existence of a special types of Weyl fermion, which they have dubbed “chiral fermions”. Furthermore, their calculations suggest that these materials should be called “topological chiral crystals” because they are expected to have an intriguing set of electronic, optical, and topological behaviours.

One particularly striking feature predicted by the calculations is the existence of large, distinctive “Fermi arcs” in the momentum-space representation of electrons on the surface of the materials. These arcs are indicative of the presence of Weyl fermions within a material. The calculations also suggest that the collective behaviour of spins will create quasiparticles that behave like magnetic monopoles.

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