  A large negative magnetoresistance is anticipated in topological semimetals
in the parallel magnetic and electric field configuration as a consequence of
the nontrivial topological properties. The negative magnetoresistance is
believed to demonstrate the chiral anomaly, a long-sought high-energy physics
effect, in solid-state systems. Recent experiments reveal that Cd3As2, a Dirac
topological semimetal, has the record-high mobility and exhibits positive
linear magnetoresistance in the orthogonal magnetic and electric field
configuration. However, the negative magnetoresistance in the parallel magnetic
and electric field configuration remains unveiled. Here, we report the
observation of the negative magnetoresistance in Cd3As2 microribbons in the
parallel magnetic and electric field configuration as large as 66% at 50 K and
even visible at room temperatures. The observed negative magnetoresistance is
sensitive to the angle between magnetic and electrical field, robust against
temperature, and dependent on the carrier density. We have found that carrier
densities of our Cd3As2 samples obey an Arrhenius's law, decreasing from
3.0x10^17 cm^-3 at 300 K to 2.2x10^16 cm^-3 below 50 K. The low carrier
densities result in the large values of the negative magnetoresistance. We
therefore attribute the observed negative magnetoresistance to the chiral
anomaly. Furthermore, in the perpendicular magnetic and electric field
configuration a positive non-saturating linear magnetoresistance up to 1670% at
14 T and 2 K is also observed. This work demonstrates potential applications of
topological semimetals in magnetic devices.
