Protection mechanism

Nature Mater. http://doi.org/xkv (2014)

Much of the excitement surrounding graphene and the wider class of so-called Dirac materials arises from their unusual electronic structure, in which low-energy electrons effectively mimic the motion of relativistic high-energy particles known as Dirac fermions. A notable recent addition to this family has been that of three-dimensional Dirac semimetals, bulk 'analogues' of graphene that have been realized in materials such as Na₃Bi and Cd₃As₂.

The wide variety of phenomena associated with graphene is well known, and it is natural to ask if the physics of three-dimensional Dirac semimetals is equally rich. In fact, Cd₃As₂ has been studied for decades because of its high carrier mobility, but without a clear understanding of its full band structure. Recent theoretical studies also predict several novel transport properties.

Tian Liang and colleagues now report an unexpected characteristic of Cd₃As₂. In addition to a giant magnetoresistance, they observed a suppression of the backscattering of electrons in zero magnetic field, which resulted in an exceptional mobility rivalling the record values measured in epitaxially-grown semiconductors. They attributed this to a topological protection mechanism inherent to the system's band structure, which suggests further surprises may be just around the corner.