Topological matter

A fine line

Phys. Rev. Lett. 117, 136401 (2016)

Dirac semimetals are materials whose electronic band structures are three-dimensional analogues of graphene, featuring electron bands with linear dispersion close to a Dirac point, forming Dirac cones. Bulk cadmium arsenide was thought to fall into this class of material, containing a single pair of Dirac cones that are protected by crystal symmetry, but experiments by Ana Akrap and colleagues suggest that the situation may be a little more complex.

Previous experiments classified the charge carriers in bulk cadmium arsenide as massless symmetry-protected Dirac fermions, which is what you'd expect for a Dirac semimetal. And although magneto-optical experiments by Akrap et al. support the idea that the carriers are massless, for certain energies, they suggest that they are actually massless Kane fermions.

Kane fermions are a type of carrier that can appear in some narrow-gap semiconductors that have conical electronic band dispersions. And although they exhibit several behaviours similar to Dirac fermions, they have quite a different origin, and are not protected by crystal symmetry. Could this material contain both massless Dirac and Kane fermions over different energy scales? The authors think so.

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Fleet, L. A fine line. Nature Phys 12, 992 (2016). https://doi.org/10.1038/nphys3957

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