  Dirac and Weyl materials possess chiral fermions, which are characterized by
nontrivial topology and large Berry curvature. Chiral fermions have nontrivial
interactions with magnetic fields and light. In this work, we propose three
different mechanisms for magnetic photocurrents caused by chiral fermions, with
different requirements on the symmetry group of the crystal. We also study
quantum oscillations of the anomalous current in the presence of a magnetic
field, showing it has a different phase from the Ohmic current. We formulate a
mechanism for THz emission observed in TaAs in response to ultrafast pulses. We
propose a strain-induced anomalous current. We also show that an external
magnetic field can create a difference between left and right handed fermions,
which can be controlled by changing the field.
