Evidence for symmetry breaking in the pseudogap phase of the single-layer Cuprate Pb-Bi2201

We use time-resolved optical spectroscopy, combined with angle resolved photoemission, and polar Kerr effect measurements, to study the single-layer Cuprate superconductor Pb$_{0.55}$Bi$_{1.5}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+\delta }$ (Pb-Bi2201). Near optimal doping this material has convenient temperature scales with a $T_{c}$ of 38K and $T^{\ast }$ of 130K, allowing signals associated with the superconducting and pseudogap phases to be clearly separated in the raw data. The unusual time dependence of the pseudogap signal is suggestive of a coherent nonlinear optical process which is sensitive to changes in the electronic point group symmetry. This nonlinear signal turns on at $T^{\ast }$ and persists to low temperature. Angle resolved photoemission and polar Kerr effect measurements performed on the same batch of samples reveal the opening of a particle-hole asymmetric gap and the onset of Kerr rotation, both with strikingly similar temperature dependence to the nonlinear optical signal.