Kerr effect measurements in the pseudo-gap regime of LBCO and Pb-BSCO using high resolution Sagnac

Polar Kerr effect in several high-Tc superconductors systems was measured at zero magnetic field with high precision using a cryogenic Sagnac fiber interferometer with zero-area. We observed non-zero Kerr rotations of order $\sim 1 \mu$rad appearing near the pseudogap temperature $T^*$, and marking what appears to be a true phase transition. In this talk we will review our work on $\mathrm{YBa_2Cu_3O}_{6+x}$, $\mathrm{La_{1.875}Ba_{0.125}CuO_4}$ and $\mathrm{Pb_{0.55}Bi_{1.5} Sr_{1.6}La_{0.4}CuO_{6+\delta}}$. In particular, in Pb-BSCO we observe an emergence of Kerr signal that coincides with ARPES data showing an abrupt change at $T^*$ from a relatively simple one- band metal into a state with profoundly-altered electronic structure.