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Discovering massless electrons in 3D

Massless electrons in 3D

Electrons in solids sometimes behave as relativistic massless particles. Interesting properties of such electrons were largely explored in one- and two-dimensional systems (carbon nanotubes, graphene and surfaces of topological insulators). The electronic states with linear, photon-like, dispersions in all three dimensions are now demonstrated experimentally. They appear in a zinc-blende crystal, HgCdTe, at the point of the semiconductor-to-semimetal topological transition. This finding is a result of an international collaboration of researchers from France, Russia, and Germany.
Three-dimensional massless electrons are revealed by the results of optical magneto-spectroscopy experiments, which are supported by the instructive/pertinent theoretical modelling. One of the main observations is the characteristic evolution of optical transitions which energies scale as a function of the square root of the magnetic field, see Fig. 1. Massless electrons in HgCdTe have already been discussed in 1960s, but the structure, suitable for the decisive experiments, has been designed and fabricated only recently, using technique of molecular beam epitaxy.
Massless electrons in HgCdTe are baptized as “massless Kane fermions” to distinguish them from massless particles in Dirac or Weyl semimetals which are two other, distinct and currently searched systems with linear dispersions in all three dimensions. The protected, by symmetry or topology, electronic states of Weyl and Dirac semimetals might be difficult to manipulate. In contrast, the band structure of HgCdTe can be suitably engineered (by tuning the Cd content) to design and fabricate “gapped- at-will“ compounds, including interfaces of materials with massive and massless particles. Scattering processes and dynamics of massless Kane fermions are now being investigated, in a view of new perspectives to use HgCdTe in opto-electronic devices.

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Figure 1: Light absorption in HgCdTe in magnetic field. Energies of individual transitions increase the square root of the magnetic field, which is behavior typical of massless particles.