Abstract:
Spin nematic (SN) is a magnetic analogue of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid [1,2]. This elusive phase of matter is often manifested by a spontaneous symmetry breaking with spin-quadrupole order; however, its experimental observation has been limited to indirect approaches [3,4], given that the spin-quadrupole moments preserve the time-reversal symmetry and thus are insensitive to most experimental probes.
In this talk, we propose a SN phase in the square-lattice iridate Sr2IrO4, which approximately realizes a pseudospin one-half Heisenberg antiferromagnet (AF) in the strong spin-orbit coupling limit. Much research on this compound has focused on its analogy with high-Tc cuprates [5]. Here, a new possibility is explored by three experimental probes, which, when taken together, suggest the formation of a SN phase at around 263 K: (i) polarization-resolved resonant inelastic x-ray scattering, (ii) circular-dichroic resonant x-ray diffraction, and (iii) Raman spectroscopy. Our results reveal a quantum order underlying the Néel AF that is widely believed to be intimately connected to the mechanism of high-Tc superconductivity [6].

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[3] Y. Kohama et al., Proc. Natl. Acad. Sci. USA 116, 10686 (2019)
[4] M. Mourigal et al., PRL 109, 027203 (2012).
[5] J. Bertinshaw, Y. K. Kim, G. Khaliullin, and B. J. Kim, Annu. Rev. Condens. Matter Phys. 10, 315 (2019)
[6] H. Kim et al., Nature 625, 264 (2024)