A global inversion-symmetry-broken phase inside the pseudogap region of YBa2Cu3Oy

By Liuyan Zhao, Carina A. Belvin, R. Liang, D. Bonn, W. Hardy, N. P. Armitage, D. Hsieh. Nature Physics, 2016

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A spectroscopic study of the canonical cuprate materials YBCO reveals the point group symmetry of this system inside its pseudogap phase. The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low-energy electronic excitations1. Polarized neutron diffraction2,3,4, Nernst effect5, terahertz polarimetry6 and ultrasound measurements7 on YBa2Cu3Oy 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 Tc is poorly understood. Here we reveal the point group of YBa2Cu3Oy 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.

Subfields: Atomic and Molecular Physics, and Optics, Condensed Matter Physics

View Paper: https://www.semanticscholar.org/paper/768fe2f864c24ea64fa3014c72f015c9ab8c8630