[Seminar] Prof. Bumjoon Kim "Intertwinned Néel and spin nematic orders in a square-lattice antiferromagnet"
Prof. B. J. Kim / Pohang University of Science and Technology, Korea
Intertwinned Néel and spin nematic orders in a square-lattice antiferromagnet
Spin nematic is an elusive phase of matter predicted half a century ago, a quantum spin analogue of classical nematic liquid crystals. It is characterized by spontaneously broken rotational symmetries without breaking time reversal symmetry, and hence is invisible to most conventional probes. Particularly intriguing is when the nematic order is bond centered, entangling spins at different sites. In this talk, I will discuss our recent observation of a spin nematic order intertwinned with a conventional Néel antiferromagnetic order in a quasi-two- dimensional square lattice. Using resonant x-ray diffraction on a 5d transition metal oxide Sr2IrO4, we observe an interference between scattering amplitudes from dipolar and quadrupolar sectors, giving rise to a circular dichroism in the x-ray scattering intensities. The interactions between the two orders is encoded in the way in which the Néel order is distorted to realize an unprecedented magnetic structure forbidden for the underlying lattice symmetry. Through a rigorous symmetry analysis, we are able to uniquely specify the spatial structure of the nematic order and its XY-like anisotropy. Furthermore, using Raman spectroscopy we observe novel emergent low-energy excitations showing distinct responses from those of magnons upon applying external magnetic fields, which is naturally interpreted as quadrupole waves entangled with spin waves.
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