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

By Liuyan Zhao, D. Torchinsky, Hao Chu, V. I. Ivanov, R. Lifshitz, R. Flint, T. Qi, G. Cao, D. Hsieh. Nature Physics, 2015

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The Mott insulator Sr2IrO4 is intensively studied because of its electronic similarity to the high-temperature cuprate superconductor La2CuO4. Now, spectroscopic experiments reveal evidence for a hidden order with odd-parity symmetry in this system. 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.

Subfields: Condensed Matter Physics, Electronic, Optical and Magnetic Materials

View Paper: https://www.semanticscholar.org/paper/e893571da22ef04febf89b34fc79acfa32828ee0