Dirac semimetals and Weyl semimetals are 3D analogs of graphene in which crystalline symmetry protects the nodes against gap formation [1-3]. Na3Bi and Cd3As2 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 H have been proposed for Dirac semimetals [2, 10, 11, 16]. Here we report an interesting property in Cd3As2 that was unpredicted, namely a remarkable protection mechanism that strongly suppresses back-scattering in zero H. In single crystals, the protection results in ultrahigh mobility, 9 × 10 6 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 a very large magnetoresistance. We discuss how this may relate to changes to the Fermi surface induced by H.

No abstract

Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The most challenging problem faced by current investigations of these materials is the existence of signifi cant bulk conduction. Here we show how the band structure of topological insulators can be engineered by molecular beam epitaxy growth of (Bi 1 − x Sb x ) 2 Te 3 ternary compounds. The topological surface states are shown to exist over the entire composition range of (Bi 1 − x Sb x ) 2 Te 3 , indicating the robustness of bulk Z 2 topology. Most remarkably, the band engineering leads to ideal TIs with truly insulating bulk and tunable surface states across the Dirac point that behave like one-quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new topologically insulating devices based on wellestablished semiconductor technology.

Topological insulators (TI) are a new class of quantum materials with insulating bulk enclosed by topologically protected metallic boundaries 1-3 . The surface states of three-dimensional TIs have spin helical Dirac structure 4-6 , and are robust against time reversal invariant perturbations. This extraordinary property is notably exemplified by the absence of backscattering by nonmagnetic impurities 7-9 and the weak antilocalization (WAL) of Dirac fermions 10-12 . Breaking the time reversal symmetry (TRS) by magnetic element doping is predicted to create a variety of exotic topological magnetoelectric effects 13-18 . Here we report transport studies on magnetically doped TI Cr-Bi 2 Se 3 . With increasing Cr concentration, the low temperature electrical conduction exhibits a characteristic crossover from WAL to weak localization (WL). In the heavily doped regime where WL dominates at the ground state, WAL reenters as temperature rises, but can be driven back to WL by strong magnetic field. These complex phenomena can be explained by a unified picture involving the evolution of Berry phase with the energy gap opened by magnetic impurities. This work demonstrates an effective way to manipulate the topological transport properties of the TI surface states by TRS-breaking perturbations.Bi 2 Se 3 is an ideal three-dimensional TI due to its large bulk energy gap (~ 300meV) and a Dirac point located well outside the bulk bands 19,20 . On the surface of magnetically doped Bi 2 Se 3 single crystals, Angle-resolved photoemission spectroscopy (ARPES) has revealed the opening of an energy gap at the Dirac point 21 and the creation of odd multiples of Dirac

Thin films of magnetically doped topological insulators Cr(0.22) (Bi(x) Sb(1-x) )(1.78) Te(3) are found to possess carrier-independent long-range ferromagnetic order with perpendicular magnetic anisotropy. The anomalous Hall resistance is greatly enhanced, up to one quarter of quantum Hall resistance, by depletion of the carriers. The results demonstrate this material as a promising system to realize the quantized anomalous Hall effect.