Dirac cones are features in the band structure of a 2D material where the conduction and valence bands meet in a single point at the Fermi level. The bands approach this point in a linear way, which means that the effective kinetic energies of the conduction electrons (and holes) are directly proportional to their momenta. This unusual relationship is normally only seen for photons, which are massless, because the energies of electrons and other particles of matter at non-relativistic velocities usually depend on the square of their momenta. The result is that the electrons in Dirac cones behave as though they are relativistic particles with no rest mass, travelling through the material at extremely high speeds – a property that could be exploited to make ultrafast transistors.

Until now, Dirac cones had only been seen in graphene (and more recently "graphynes"), which has two such (unequal) cones, but Shuang Tang and Mildred Dresselhaus have now found that single Dirac cones can exist in 2D bismuth-antinomy films. “Not only that, but we expect that the single cone found in bismuth-antimony can do all the things that the graphene bi-Dirac-cones can do, and better!” said Tang. “For example, the Dirac cones in graphene are 'isotropic', so the variety of devices that can be made from this material is limited. However, Dirac cones with a wide range of anisotropies can be constructed in bismuth-antimony films, something that could increase the types of potential devices that might be fabricated.”

Thermoelectrics

Bismuth-antimony films with Dirac cones conduct electricity extremely well while having a low thermal conductivity, two properties that make them promising thermoelectric materials. Tang and Dresselhaus say that they could now make quasi-Dirac ones with different bandgaps, which would greatly increase the entropy carried per charge carrier in the material without notably destroying the electrical conductivity. “Basically, for thermoelectrics you need to have a temperature difference across a sample if you want to produce an electric current,” explained Tang. “In this respect, bismuth-antimony films could be especially interesting for applications in space stations and satellites where electricity could be generated by exploiting the difference between the spacecraft's sun-facing and shaded sides.”

Electronics applications

According to Tang, the films could also form the base material for next-generation electronic devices. “Electron speeds in devices made of bismuth-antimony would be hundreds of times greater than those in current silicon devices,” he told nanotechweb.org. “At the same time, the fact that different anisotropies of cones can be elaborated here means that different devices could be made out of the same class of material, which would greatly save on manufacturing costs.”

The MIT group's calculations have been published in Nano Letters.