Comment on Berry phase mechanism for optical gyrotropy in stripe-ordered cuprates

I comment on two recent papers on the 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 Orenstein and Moore [Phys. Rev. B 87, 165110 (2013)], whereas, in Hosur et al. [Phys. Rev. B 87, 115116 (2013)], 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 on experimental grounds in both of these papers.