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Published January 2016 | Supplemental Material
Journal Article Open

Evidence of an odd-parity hidden order in a spin–orbit coupled correlated iridate


A rare combination of strong spin–orbit coupling and electron–electron correlations makes the iridate Mott insulator Sr_2IrO_4 a promising host for novel electronic phases of matter. The resemblance of its crystallographic, magnetic and electronic structures to La_2CuO_4, as well as the emergence on doping of a pseudogap region and a low-temperature d-wave gap, has particularly strengthened analogies to cuprate high-T_c superconductors. However, unlike the cuprate phase diagram, which features a plethora of broken symmetry phases in a pseudogap region that includes charge density wave, stripe, nematic and possibly intra-unit-cell loop-current orders, no broken symmetry phases proximate to the parent antiferromagnetic Mott insulating phase in Sr_2IrO_4 have been observed so far, making the comparison of iridate to cuprate phenomenology incomplete. Using optical second-harmonic generation, we report evidence of a hidden non-dipolar magnetic order in Sr_2IrO_4 that breaks both the spatial inversion and rotational symmetries of the underlying tetragonal lattice. Four distinct domain types corresponding to discrete 90°-rotated orientations of a pseudovector order parameter are identified using nonlinear optical microscopy, which is expected from an electronic phase that possesses the symmetries of a magneto-electric loop-current order. The onset temperature of this phase is monotonically suppressed with bulk hole doping, albeit much more weakly than the Néel temperature, revealing an extended region of the phase diagram with purely hidden order. Driving this hidden phase to its quantum critical point may be a path to realizing superconductivity in Sr_2IrO_4.

Additional Information

© 2015 Macmillan Publishers Limited. Received 05 August 2015. Accepted 15 September 2015. Published online 26 October 2015. We thank S. Lovesey and D. Khalyavin for providing information about the magnetic point group of the Néel order in Sr_2IrO_4. We acknowledge useful discussions with P. Armitage, L. Fu, A. Kaminski, P. A. Lee, O. Motrunich, J. Orenstein, N. Perkins, S. Todadri, C. Varma and V. Yakovenko. This work was support by ARO Grant W911NF-13-1-0059. Instrumentation for the SHG 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. R.F. acknowledges the hospitality of the Aspen Center for Physics, supported by NSF Grant PHYS-1066293, where some of this work was carried out. G.C. acknowledges NSF support via Grant DMR-1265162. R.L. acknowledges support from the Israel Science Foundation through Grant 556/10. Contributions: L.Z. and D.H. planned the experiment. L.Z., D.H.T., H.C. and V.I. performed the measurements. L.Z. and R.L. performed the magnetic point group symmetry analysis. R.F. performed the Landau free energy calculation. T.Q. and G.C. prepared and characterized the samples. L.Z., R.F. and D.H. analysed the data and wrote the manuscript. The authors declare no competing financial interests.

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