A global inversion-symmetry-broken phase inside the pseudogap region of YBa_2Cu_3O_y

Abstract

The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low-energy electronic excitations. Polarized neutron diffraction Nernst effect, terahertz polarimetry and ultrasound measurements on YBa_2Cu_3O_y suggest that the pseudogap onset below a temperature T^∗ coincides with a bona fide thermodynamic phase transition that breaks time-reversal, four-fold rotation and mirror symmetries respectively. However, the full point group above and below T^∗ has not been resolved and the fate of this transition as T^∗ approaches the superconducting critical temperature T_c is poorly understood. Here we reveal the point group of YBa_2Cu_3O_y inside its pseudogap and neighbouring regions using high-sensitivity linear and second-harmonic optical anisotropy measurements. We show that spatial inversion and two-fold rotational symmetries are broken below T^∗ while mirror symmetries perpendicular to the Cu–O plane are absent at all temperatures. This transition occurs over a wide doping range and persists inside the superconducting dome, with no detectable coupling to either charge ordering or superconductivity. These results suggest that the pseudogap region coincides with an odd-parity order that does not arise from a competing Fermi surface instability and exhibits a quantum phase transition inside the superconducting dome.