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

Nature Physics volume 12, pages 32–36 (2016)Cite this article

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Abstract

A rare combination of strong spin–orbit coupling and electron–electron correlations makes the iridate Mott insulator Sr2IrO4 a promising host for novel electronic phases of matter1,2. The resemblance of its crystallographic, magnetic and electronic structures1,2,3,4,5,6 to La2CuO4, as well as the emergence on doping of a pseudogap region7,8,9 and a low-temperature d-wave gap10,11, has particularly strengthened analogies to cuprate high-Tc superconductors12. However, unlike the cuprate phase diagram, which features a plethora of broken symmetry phases13 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 Sr2IrO4 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 Sr2IrO4 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 order14,15,16,17,18. 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 Sr2IrO4.

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Figure 1: Symmetry of the hidden order in Sr2IrO4.
Figure 2: Spatial mapping of hidden order domains.
Figure 3: Degenerate ground-state configurations of the hidden order.
Figure 4: Temperature and hole-doping dependence of the hidden order.

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Acknowledgements

We thank S. Lovesey and D. Khalyavin for providing information about the magnetic point group of the Néel order in Sr2IrO4. 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.

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Authors and Affiliations

  1. Department of Physics, California Institute of Technology, Pasadena, California 91125, USA

    L. Zhao, D. H. Torchinsky, V. Ivanov, R. Lifshitz & D. Hsieh

  2. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA

    L. Zhao, D. H. Torchinsky, H. Chu & D. Hsieh

  3. Department of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA

    H. Chu

  4. Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel

    R. Lifshitz

  5. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA

    R. Flint

  6. Department of Physics and Astronomy, Center for Advanced Materials, University of Kentucky, Lexington, Kentucky 40506, USA

    T. Qi & G. Cao

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  1. L. Zhao
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  9. D. Hsieh
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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.

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Correspondence to D. Hsieh.

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Zhao, L., Torchinsky, D., Chu, H. et al. Evidence of an odd-parity hidden order in a spin–orbit coupled correlated iridate. Nature Phys 12, 32–36 (2016). https://doi.org/10.1038/nphys3517

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