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The progress in exploiting new electronic materials and devices has been a major driving force in solid-state physics. As a new state of matter, a Weyl semimetal (WSM), particularly a type-II WSM, hosts Weyl fermions as emergent quasiparticles and may harbor novel electrical transport properties because of the exotic Fermi surface. Nevertheless, such a type-II WSM material has not been experimentally observed in nature. In this work, by performing systematic magneto-transport studies on thin films of a predicted material candidate WTe2, we observe notable angle-sensitive (between the electric and magnetic fields) negative longitudinal magnetoresistance (MR), which can likely be attributed to the chiral anomaly in WSM. This phenomenon also exhibits strong planar orientation dependence with the absence of negative longitudinal MR along the tungsten chains (a axis), which is consistent with the distinctive feature of a type-II WSM. By applying a gate voltage, we demonstrate that the Fermi energy can be tuned through the Weyl points via the electric field effect; this is the first report of controlling the unique transport properties in situ in a WSM system. Our results have important implications for investigating simulated quantum field theory in solid-state systems and may open opportunities for implementing new types of electronic applications, such as field-effect chiral electronic devices., Comment: accepted by Nature Communications; 27 pages, 3 figures, 9 supplementary figures; submitted on May 1st, 2016

Three-dimensional topological Dirac semimetals have hitherto stimulated unprecedented research interests as a new class of quantum materials. Breaking certain types of symmetries has been proposed to enable the manipulation of Dirac fermions; and that was soon realized by external modulations such as magnetic fields. However, an intrinsic manipulation of Dirac states, which is more efficient and desirable, remains a significant challenge. Here, we report a systematic study of quasi-particle dynamics and band evolution in Cd3As2 thin films with controlled Chromium (Cr) doping by both magneto-infrared spectroscopy and electrical transport. For the first time, we observe square-root-B relation of inter-Landau-level resonance in undoped Cd3As2 Dirac semimetal, an important signature of ultra-relativistic Dirac state inaccessible in previous optical experiments. A crossover from quantum to quasi-classical behavior makes it possible to directly probe the mass of Dirac fermions. Importantly, Cr doping allows for a Dirac mass acquisition and topological phase transition enabling a desired dynamic control of Dirac fermions. Corroborating with the density-functional theory calculations, we show that the mass generation is essentially driven by explicit C4 rotation symmetry breaking and the resultant Dirac gap engineering through Cr substitution for Cd atoms. The manipulation of the system symmetry and Dirac mass in Cd3As2 thin films provides a tuning knob to explore the exotic states stemming from the parent phase of Dirac semimetals.