The polar Kerr effect in the high-Tc superconductor YBa2Cu3O6+x was measured at zero magnetic field with high precision using a cyogenic Sagnac fiber interferometer. We observed nonzero Kerr rotations of order ~1 µrad appearing near the pseudogap temperature T* and marking what appears to be a true phase transition. Anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature.
Xia, J., Schemm, E., Deutscher, G., Kivelson, S. A., Bonn, D. A., Hardy, W. N., ... Kapitulnik, A. (2008). Polar Kerr-effect measurements of the high-temperature YBa2Cu3O6+x superconductor: Evidence for broken symmetry near the pseudogap temperature. Physical review letters, 100(12), 127002-. https://doi.org/10.1103/PhysRevLett.100.127002