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arXiv · 2310.00886

Quantum spin nematic phase in a square-lattice iridate

Hoon Kim·Jin-Kwang Kim·Jimin Kim·Hyun-Woo J. Kim·Seunghyeok Ha·Kwangrae Kim·Wonjun Lee·Jonghwan Kim·Gil Young Cho·Hyeokjun Heo·Joonho Jang·J. Strempfer·G. Fabbris·Y. Choi·D. Haskel·Jungho Kim·J. -W. Kim·B. J. Kim
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Abstract

Spin nematic (SN) is a magnetic analog of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid. Particularly intriguing is a valence-bond SN, in which spins are quantum entangled to form a multi-polar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here, we establish a SN phase in the square-lattice iridate Sr$_2$IrO$_4$, which approximately realizes a pseudospin one-half Heisenberg antiferromagnet (AF) in the strong spin-orbit coupling limit. Upon cooling, the transition into the SN phase at T$_C$ $\approx$ 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 (AF) phase below T$_N$ $\approx$ 230 K, and becomes directly observable through its interference with the AF 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 resonating-valence-bond-like quantum entanglement in the AF state. Taken together, our results reveal a quantum order underlying the N\'eel AF that is widely believed to be intimately connected to the mechanism of high temperature superconductivity (HTSC).

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Hoon Kim, Jin-Kwang Kim, Jimin Kim, Hyun-Woo J. Kim, Seunghyeok Ha, Kwangrae Kim, Wonjun Lee, Jonghwan Kim, Gil Young Cho, Hyeokjun Heo, Joonho Jang, J. Strempfer, G. Fabbris, Y. Choi, D. Haskel, Jungho Kim, J. -W. Kim, B. J. Kim. 2023-10-02. Quantum spin nematic phase in a square-lattice iridate. https://doi.org/10.1038/s41586-023-06829-4

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