Quantum spin nematic phase in a square-lattice iridate

Hoon Kim, Jin-Kwang Kim, Junyoung Kwon, Jimin Kim, Hyun-Woo J. Kim, Seunghyeok Ha, Kwangrae Kim, Wonjun Lee, Jonghwan Kim, Gil Young Cho, Hyeokjun Heo, Joonho Jang, C. J. Sahle, A. Longo, J. Strempfer, G. Fabbris, Yoonseok Choi, D. Haskel, Jungho Kim, J. -W. Kim and B. J. Kim ()
Additional contact information
Hoon Kim: Institute for Basic Science
Jin-Kwang Kim: Institute for Basic Science
Junyoung Kwon: Pohang University of Science and Technology
Jimin Kim: Institute for Basic Science
Hyun-Woo J. Kim: Institute for Basic Science
Seunghyeok Ha: Institute for Basic Science
Kwangrae Kim: Institute for Basic Science
Wonjun Lee: Institute for Basic Science
Jonghwan Kim: Institute for Basic Science
Gil Young Cho: Institute for Basic Science
Hyeokjun Heo: Seoul National University
Joonho Jang: Seoul National University
C. J. Sahle: The European Synchrotron
A. Longo: The European Synchrotron
J. Strempfer: Argonne National Laboratory
G. Fabbris: Argonne National Laboratory
D. Haskel: Argonne National Laboratory
Jungho Kim: Argonne National Laboratory
J. -W. Kim: Argonne National Laboratory
B. J. Kim: Institute for Basic Science

Nature, 2024, vol. 625, issue 7994, 264-269

Abstract: Abstract Spin nematic is a magnetic analogue of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid1,2. Particularly intriguing is a valence-bond spin nematic3–5, in which spins are quantum entangled to form a multipolar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here we establish a spin nematic phase in the square-lattice iridate Sr2IrO4, which approximately realizes a pseudospin one-half Heisenberg antiferromagnet in the strong spin–orbit coupling limit6–9. Upon cooling, the transition into the spin nematic phase at TC ≈ 263 K is marked by a divergence in the static spin quadrupole susceptibility extracted from our Raman spectra and concomitant emergence of a collective mode associated with the spontaneous breaking of rotational symmetries. The quadrupolar order persists in the antiferromagnetic phase below TN ≈ 230 K and becomes directly observable through its interference with the antiferromagnetic order in resonant X-ray diffraction, which allows us to uniquely determine its spatial structure. Further, we find using resonant inelastic X-ray scattering a complete breakdown of coherent magnon excitations at short-wavelength scales, suggesting a many-body quantum entanglement in the antiferromagnetic state10,11. Taken together, our results reveal a quantum order underlying the Néel antiferromagnet that is widely believed to be intimately connected to the mechanism of high-temperature superconductivity12,13.

Date: 2024
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DOI: 10.1038/s41586-023-06829-4

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