查看文章

Dirac and Weyl semimetals are 3D analogues of graphene in which crystalline symmetry protects the nodes against gap formation^,,. Na₃Bi and Cd₃As₂ were predicted to be Dirac semimetals^,, and recently confirmed to be so by photoemission experiments^,,. Several novel transport properties in a magnetic field have been proposed for Dirac semimetals^,,,. Here, we report a property of Cd₃As₂ that was unpredicted, namely a remarkable protection mechanism that strongly suppresses backscattering in zero magnetic field. In single crystals, the protection results in ultrahigh mobility, 9 × 10⁶ cm² V⁻¹ s⁻¹ at 5 K. Suppression of backscattering results in a transport lifetime 10⁴ times longer than the quantum lifetime. The lifting of this protection by the applied magnetic field leads to a very large magnetoresistance. We discuss how this may relate to changes to the Fermi surface induced by the applied magnetic field.