Scientists observe evidence of a new magnetic (dis)order in the square-lattice iridate Sr2IrO4

High temperature superconductivity remains one of the great mysteries in solid state physics and material science. The understanding of elementary interactions and entanglements between charge, orbital, and spin degrees of freedom may provide for keys to solve this longstanding puzzle. This motivation drives researchers to explore exotic quantum phases and phase transitions in strongly correlated electron materials.

The quantum spin liquid phase is one example for such exotic quantum phases, characterized by a large number of quantum entangled fluctuating spins that exhibit no long range order. In contrast to the quantum spin liquid phase, a nematic spin phase is also described by a macroscopic wave function; however, in this case, the spins exhibit orientational order while escaping conventional magnetic order. This is analogous to the well known nematic liquid crystal phase, where molecules tend to align in a particular direction without long-range order and, therefore, can move like in a liquid.

A team of researchers led by the Institute for Basic Sciences and the Pohang University of Science and Technology in Korea has now observed evidence for spin nematicity in the square-lattice iridate Sr₂IrO₄ using ESRF synchrotron in Grenoble, France, and the Advanced Photon Source in the US.

Alessandro Longo (left) and Christoph Sahle on ID20. Credits: S. Candé.

At the ESRF, the team used the high-resolution IXS spectrometer at ID20 in order to measure Resonant Inelastic X-ray Scattering (RIXS) spectra at the L₃-edge. “The ESRF beamline ID20 is currently the only place in Europe where we could carry out these experiments”, explains B. J. Kim, professor at Pohang University of Science and Technology and corresponding author of the paper. Christoph Sahle, scientist in charge of the beamline and co-author of the study adds: “The capabilities of our beamline proved to be exactly what the team needed to confirm the group’s results from resonant X-ray diffraction and optical Raman spectroscopy.”

Specifically, resonant elastic scattering experiments at APS and optical Raman data show the existence of spin quadrupolar order that persists even below the onset of anti-ferromagnetic order at T_N=230 K. Concomitantly, resonant inelastic X-ray scattering spectra taken with the high energy resolution spectrometer at ID20 of the ESRF show significant deviation from classical spin wave theory at the wave vector ( π, 0), constituting evidence of many-body quantum entanglement in the anti-ferromagnetic state. These results open a new avenue to investigate the entanglement of quantum spins via resonant X-ray scattering.

Reference: 

Kim, H., et al. Quantum spin nematic phase in a square-lattice iridate. Nature (2023). https://doi.org/10.1038/s41586-023-06829-4

Top image: A schematic of the RIXS geometry. X-rays with π polarization are incident to the sample with the angle α ≈ 75°, and scattered X-rays are collected without polarization analysis. A single crystal of Sr2IrO4 is placed between two permanent magnets that apply a magnetic field of 0.3 T along the b axis (a axis) at the sample position aligning the magnetic domains (that is, M∥ (M∥ )). The direction of the applied field can be changed by rotating the outer disk on which the magnets are mounted while the sample stays fixed. The angle between the incident and outgoing X-rays is fixed close to 90° to suppress elastic Thomson scattering. Credits: Kim, H., et al. Quantum spin nematic phase in a square-lattice iridate. Nature (2023).