Relaxation time of the fermions in the magnetic field
Dirac semimetal has a massless Dirac fermion protected by discrete symmetries. Under the external magnetic field, the chiral magnetic effect shows a peculiar behavior induced by the quantum anomaly. In strong magnetic field and exact massless limit, the intra-corn transition does not occur because of the helicity conservation. In exact massless limit, the transport property is dictated by the inter-cone transition which is highly dependent on the lattice structure.
We study the chiral magnetic effect for fermions with a small mass, which could be realized in slightly distorted Dirac semimetals. We give a formula for the intra-cone transition and find that the small mass allows for the helicity flipping in intra-cone transition. We also find that under the weaker magnetic field intra-cone transition through the excited states in Landau levels provides a new mechanism of helicity flipping, which exists even in massless limit.