Ultrahigh mobility and giant magnetoresistance in the Dirac Semimetals Cd$_{3}$As$_{2}$ and Na$_{3}$Bi*

Dirac semimetals and Weyl semimetals are 3D analogs of graphene in which crystalline symmetry protects the nodes against gap formation. Na$_{3}$Bi and Cd$_{3}$As$_{2}$ were predicted to be Dirac semimetals, and recently confirmed to be so by photoemission. Several novel transport properties in a magnetic field H have been proposed for Dirac semimetals. 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 H. In single crystals, the protection results in a very high mobility, 10$^{7}$ cm$^{2}$/Vs at 5 K. Suppression of backscattering results in a transport lifetime 10$^{4} $longer than the quantum lifetime. The lifting of this protection by H leads to very large magnetoresistance with a striking H-linear profile. I will also report transport results on Na$_{3}$Bi and compare them with results in Cd$_{3}$As$_{2}$. I discuss how this may relate to changes to the Fermi surface induced by H.\\[4pt] Coauthors: Tian Liang, Jun Xiong, Quinn Gibson, Minhao Liu, Satya Kushwaha, Jason Krizan Maz Ali, and R. J. Cava