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-Tc superconductor YBa2Cu3O_6+x was measured at zero magnetic field with high precision using a cyogenic Sagnac fiber interferometer. We observed nonzero Kerr rotations of order ~1 urad 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.