Ultrahigh mobility and giant magnetoresistance in the Dirac semimetal Cd${}_3$As${}_2$

Dirac semimetals and Weyl semimetals are 3D analogs of graphene in which crystalline symmetry protects the nodes against gap formation [1-3]. Na${}_3$Bi and Cd${}_3$As${}_2$ were predicted to be Dirac semimetals [4,5], and recently confirmed to be so by photoemission [6-8]. Several novel transport properties in a magnetic field $\mathbf{H}$ have been proposed for Dirac semimetals [2,9-11]. Here we report an interesting property in Cd${}_3$As${}_2$ that was unpredicted, namely a remarkable protection mechanism that strongly suppresses back-scattering in zero $\mathbf{H}$. In single crystals, the protection results in a very high mobility that exceeds $>{10}^7$ cm${}^2$/Vs below 4 K. Suppression of backscattering results in a transport lifetime 10${}^4\times$ longer than the quantum lifetime. The lifting of this protection by $\mathbf{H}$ leads to an unusual giant $\mathbf{H}$-linear magnetoresistance that violates Kohler’s rule. We discuss how this may relate to changes to the Fermi surface induced by $\mathbf{H}$.