The rise and fall of the chiral photocurrent quantization in topological semimetals

The chiral photocurrent or circular photogalvanic effect (CPGE) is a photocurrent that depends on the sense of circular polarization. It is a nonlinear effect that is consistently observed in materials without inversion symmetry. In Weyl and multi-Weyl semimetals without inversion and mirror symmetries the injection contribution to the CPGE is approximately quantized in terms of the fundamental constants. The reason for the quantization is that the CPGE directly measures the topological charge of Weyl or multi-Weyl points. It turns out that, unlike the quantum Hall effect in gapped phases or the chiral anomaly in field theories, the inclusion of interactions generically breaks the quantization of the CPGE. The interaction corrections have the structure similar to that of the (nontopological) linear optical conductivity of graphene, and have a potentially observable frequency dependence.