A global inversion-symmetry-broken phase inside the pseudogap region of YBa$_2$Cu$_3$O$_y$

TL;DR

This study identifies a new inversion-symmetry-broken phase within the pseudogap region of YBa₂Cu₃Oₓ, revealing a quantum phase transition that persists into the superconducting state and is distinct from charge order or superconductivity.

Contribution

The paper uncovers the specific point group symmetry breaking inside the pseudogap phase using optical anisotropy, showing a novel odd-parity order that persists within the superconducting dome.

Findings

Inversion and two-fold rotational symmetries are broken below T*.

Mirror symmetries perpendicular to the Cu-O plane are absent at all temperatures.

The symmetry-breaking transition persists inside the superconducting dome, independent of charge order or superconductivity.

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, THz polarimetery and ultrasound measurements on YBa$_2$Cu$_3$O$_y$ 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$_2$Cu$_3$O$_y$ inside its pseudogap and neighboring 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.