Polar Kerr-effect measurements of the high-temperature YBa2Cu3O6+x superconductor:: Evidence for broken symmetry near the pseudogap temperature

The polar Kerr effect in the high-T-c 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 similar to 1 mu rad 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.