  Chiral anomaly, a non-conservation of chiral charge pumped by the topological
nontrivial gauge fields, has been predicted to exist in Weyl semimetals.
However, until now, the experimental signature of this effect exclusively
relies on the observation of negative longitudinal magnetoresistance at low
temperatures. Here, we report the field-modulated chiral charge pumping process
and valley diffusion in Cd3As2. Apart from the conventional negative
magnetoresistance, we observe an unusual nonlocal response with negative field
dependence up to room temperature, originating from the diffusion of valley
polarization. Furthermore, a large magneto-optic Kerr effect generated by
parallel electric and magnetic fields is detected. These new experimental
approaches provide a quantitative analysis of the chiral anomaly phenomenon
which is inaccessible previously. The ability to manipulate the valley
polarization in topological semimetal at room temperature opens up a brand-new
route towards understanding its fundamental properties and utilizing the chiral
fermions.
