The First Direct Observation of Spin Quadrupoles in a Spin-Nematic Phase: A Breakthrough in Materials Science

Liquid crystal has revolutionized the technology industry with its unique characteristics, combining the properties of both liquid and solid states. However, its magnetic counterpart, the spin-nematic phase, has remained elusive for nearly half a century. Recently, a breakthrough study conducted by researchers at the IBS Center for Artificial Low-Dimensional Electronic Systems in South Korea has successfully observed spin quadrupoles for the first time in the world. This groundbreaking achievement opens new doors in the field of materials science and has profound implications for various applications, including quantum computing, information technologies, and high-temperature superconductivity.

The main challenge in directly observing the spin-nematic phase has been the insensitivity of conventional experimental techniques to spin quadrupoles, the defining features of this phase. However, the team led by Professor Kim Bumjoon utilized the advancements in synchrotron facility development to overcome this obstacle. Their study focused on square-lattice iridium oxide Sr2IrO4, a material known for its antiferromagnetic dipolar order at low temperatures.

The researchers successfully discovered the coexistence of a spin quadrupolar order by employing the interference with the magnetic order. The interference signal was detected through the groundbreaking technique of “circular-dichroic resonant X-ray diffraction,” which utilizes a circularly polarized X-ray beam. This method allowed the direct observation of spin quadrupoles and provided valuable insights into their behavior.

To ensure the validity of their discovery, the team collaborated with the Argonne National Laboratory in the United States. Over the course of four years, a resonant inelastic X-ray scattering beamline was constructed at the Pohang Accelerator Laboratory. The researchers utilized this facility to perform “polarization-resolved resonant inelastic X-ray scattering,” which revealed significant deviations in the magnetic excitations from those expected in conventional magnets.

In addition to X-ray techniques, the researchers employed a series of optical techniques to further validate their findings. Raman spectroscopy and magneto-optical Kerr effect measurement were used to demonstrate that the formation of spin quadrupole moments occurs at higher temperatures than the magnetic order. This unique phase, known as the spin-nematic phase, exclusively possesses spin quadrupole moments without magnetic order.

The discovery of the spin-nematic phase holds significant implications for quantum computing and information technologies. The highly entangled spins within the spin-nematic phase, as suggested by physicist P. W. Anderson, serve as a critical ingredient for high-temperature superconductivity. Consequently, the spin-nematic phase offers potential applications in the development of high-temperature superconducting systems.

The iridium oxide Sr2IrO4, the focus of this study, has garnered significant attention due to its similarities with the copper-oxide high-temperature superconducting system. The researchers’ findings not only shed light on the spin-nematic phase but also contribute to the growing interest in exploring this material as a potential high-temperature superconducting system. The intricate relationship between the spin-nematic phase and high-temperature superconductivity holds promise for future advancements in this field.

The direct observation of spin quadrupoles in a spin-nematic phase marks a groundbreaking achievement in materials science. The successful application of advanced X-ray techniques and collaborative efforts with international institutions have paved the way for new discoveries and further understanding of complex material phenomena. The implications of this research extend beyond the realms of fundamental science, encompassing quantum computing, high-temperature superconductivity, and information technologies. As the field of materials science continues to progress, this groundbreaking research sets the stage for future advancements and opens new avenues for innovation.