Hidden Order in the Pseudogap Region of High-Tc Superconductors
The temperature versus doping phase diagram of the cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low energy electronic excitations. Identifying the complete set of symmetries underlying the pseudogap region is imperative to understanding its microscopic nature and relationship to superconductivity. In this talk I will describe the results of highly symmetry sensitive optical second harmonic generation experiments on YBa2Cu3Oy across a wide temperature and doping range using a recently developed nonlinear optical rotational anisotropy technique. I will show evidence of a global inversion symmetry broken magnetic phase that onsets at the pseudogap transition temperature and persists inside the superconducting dome in the overdoped regime, but shows no coupling to either charge density wave or superconducting order parameters. I will then discuss how these results may reconcile previous polarized neutron diffraction, Nernst effect and THz polarimetry data on YBa2Cu3Oy. Finally, I will show evidence of a remarkably similar phenomenon occurring in doped Sr2IrO4, a strongly spin-orbit coupled analog of La2CuO4, hinting at a robust connection between this hidden order and pseudogap physics that extends beyond the cuprates.