Quantum anomalies in Weyl semimetals

Quantum anomalies can be naively understood as the breaking of classical symmetries in the Lagranngian of a system due to quantum fluctuations. One of the simplest examples is the non-conservation of chiral charge (difference between number of right and left moving fermions) in a fermionic parabolic band under an electric field. It was realised more three decades ago that relativisitic fermionic field theories in 3+1 dimensions under magnetic fields host chiral Landau levels making them the idea platforms for studying such anomalies. More recently, the discovery of Weyl semimetals whose low-energy excitations are relativisitc Weyl fermions, and the synthesis of such systems in cold-atomic systems with artificial gauge fields has renewed the interest in the physics of quantum anomalies in the context of condensed matter and atomic physics due to the possibility of experimental verifications of such predictions. My interest lies in understanding how the quantum anomalies on lattice Weyl semimetals tie up the quantum field theoretic results and what is the fate of the anomalies on lattices in regimes not accessible in field theories.

Chiral anomalies in Weyl butterflies


The field theory results suggest that the chiral anomaly is linearly proportional to the electric and magnetic field. A natural question to ask on lattice systems is what happens when the magnetic field strength is so large that the cyclotron length becomes equivalent to the lattice spacing, the so called Hofstadter regime. We found that in such a case, the chiral anomaly is no longer linearly proportional to the magnetic field, but is a fractal series of integer slopes [1]. This is directly related to the fractal structure, that is the Weyl butterfly, which is a family of Hofstadter butterfly spectra describing the Weyl semimetal under magnetic fields.

The chiral anomaly as a function of magnetic flux is related to Chern numbers in the gaps of Weyl butterflies

Consistent and covariant anomalies


In addition to external electric and magnetic fields, spatiotemporal strain in Weyl semimetals can generate axial gauge fields which couple to different chiralities of Weyl fermions with different signs. In such a case, not only does the chiral anomaly get additional contributions, it leads to non-conservation of total charge. This seemingly unphysical results is accounted for in field theories via Bardeen counterterms and the price paid then is the current no longer remains gauge invariant. This leads to two versions of the chiral anomaly, known as the chiral and consistent anomaly. However, on a lattice, there is only one right answer. In [2], we investigated this issue and found that the boundaries of the lattice, which host topological Fermi arcs, are key towards understanding the connection between the two anomalies on the lattice.


  1. Sthitadhi Roy, Michael Kolodrubetz, Joel E. Moore, Adolfo G. Grushin, "Chern numbers and chiral anomalies in Weyl butterflies" [Phys. Rev. B 94, 161107(R) (2016)] [arXiv: 1605.08445]

  2. Jan Behrends, Sthitadhi Roy, Michael H. Kolodrubetz, Jens H. Bardarson, Adolfo G. Grushin, "Landau levels, Bardeen polynomials and Fermi arcs in Weyl semimetals: the who's who of the chiral anomaly" [Phys. Rev. B 99, 140201(R) (2019)][arXiv:1807.06615]