Monday, August 19, 2013

A distinct experimental signature of the chiral anomaly

There is a nice preprint
Chiral Anomaly and Classical Negative Magnetoresistance of Weyl Metals
by Dam  Son [a string theorist!] and Boris Spivak

This clearly shows a distinct experimental signature of the chiral anomaly associated with Weyl metals.
[Aside: I think that Weyl should not get his name on this for the same reasons discussed in this post.]

The key physics is summarised in the Figure below.

The Dirac cones [another misnomer?] associated with the chiral anomaly must come in pairs.
Consider the case where the magnetic field and electric field are parallel [and in the z-direction]. The magnetic field induces an anomalous charge current that destroys charge at one Dirac point and creates it at the other. This leads to an anomalous current that is proportional to the square of the magnetic field strength. Thus, the resistance decreases with increasing magnetic field, i.e. (classical) negative magnetoresistance. This is in distinct contrast to traditional classical orbital magnetoresistance which is normally positive.

Might this occur in a real material?
For the case of pyrochlore iridates [with 24 Dirac cones!] this magnetoresistance was discussed by Vivek Aji in a PRB.

I thank Boris Spivak for helpful discussions about this work.


  1. "Destroys charge at one Dirac point and creates it at another" sounds like a trip through a wormhole in k-space. I wonder if there is an analogous configurational space effect for diabolical points on related B-O surfaces? Does the charge flow distinguish pairs of cones in the case under discussion (i.e. if you have many dirac cones, will the flow occur only between distinct pairs that do not interchange)?

  2. First question.
    An important difference from the Born-Oppenheimer case is that here you have a filled Fermi/Dirac sea of electrons.

    Second question.
    The pairs of Dirac cones are naturally paired in the full band structure. That determines where the charge reappears.