A global inversion-symmetry-broken phase inside the pseudogap region of YBa_2Cu_3O_y
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.
© 2016 Macmillan Publishers Limited. Received 30 June 2016; accepted 13 October 2016; published online 21 November 2016. We thank D. N. Basov, P. Bourges, B. Keimer, S. A. Kivelson, P. A. Lee, J. W. Lynn, J. Orenstein, S. Raghu, B. Ramshaw, C. Varma and N.-C. Yeh for valuable discussions. This work was supported by ARO Grant W911NF-13-1-0059. Instrumentation for the RA measurements was partially supported by ARO DURIP Award W911NF-13-1-0293. D.H. acknowledges funding provided by the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center (PHY-1125565) with support of the Gordon and Betty Moore Foundation through Grant GBMF1250. N.P.A. acknowledges support from ARO Grant W911NF-15-1-0560. Work at the University of British Columbia was supported by the Canadian Institute for Advanced Research and the Natural Science and Engineering Research Council. Data availability. The data that support the plots within this paper and other findings of this study are available from the corresponding author on request. Author Contributions: L.Z., D.H. and N.P.A. planned the experiment. L.Z. and C.A.B. performed the RA measurements and N.P.A. determined the crystal alignment. L.Z., D.H. and N.P.A. analysed the data. R.L., D.A.B. and W.N.H. prepared and characterized the samples. L.Z. and D.H. wrote the manuscript. The authors declare no competing financial interests.
Submitted - 1611.08603.pdf
Supplemental Material - nphys3962-s1.pdf