  We propose a new type of photoresponse induced in asymmetric Weyl semimetals
in an external magnetic field. In usual symmetric Weyl semimetals in a magnetic
field, the particles and holes produced by an incident light in different Weyl
cones have opposite helicities and hence move in opposite directions, canceling
each others's contributions to the photocurrent. However this cancelation does
not occur if the Weyl semimetal possesses both a broken particle-hole symmetry
and a broken spatial inversion symmetry. We call the resulting generation of
photocurrent the helical magnetic effect because it is induced by the helicity
imbalance in a magnetic field. We find that due to the large density of states
in a magnetic field, the helical magnetic effect induces a remarkable large
photocurrent for incident THz frequency light. This suggests a potential
application of asymmetric Weyl semimetals for creating THz photosensors.
