  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.
