Comments on Kerr effect and gyrotropic order in cuprates

TL;DR

This paper critically examines recent claims linking Kerr effect observations to gyrotropic order in cuprates, highlighting issues with wave vector assumptions and the robustness of symmetry arguments, leaving the origin of the Kerr effect unresolved.

Contribution

It provides a critical analysis of recent theoretical and experimental claims about gyrotropic order and Kerr effect in cuprates, questioning their validity.

Findings

Wave vector $k_{z}$ is not a good quantum number in cuprates.

Arguments against time reversal symmetry breaking are strong.

The origin of Kerr effect in cuprates remains unresolved.

Abstract

I comment on two recent papers on Kerr effect as evidence of gyrotropic order in cuprates, and I suggest that the arguments may not be sound. The difficulty is that in practically all cases the wave vector $k_{z}$ perpendicular to the copper-oxygen plane is not a good quantum number. This appears to be problematic for arXiv:1212.2698, whereas in arXiv:1212.2274 the symmetry arguments may turn out to be robust, but the microscopic picture is wanting. Thus, the Kerr effect in cuprates remains a puzzle, as there is little doubt that the arguments presented against time reversal symmetry breaking appear to be rather strong in both of these papers on experimental grounds.