The world of materials science is constantly evolving, with researchers striving to uncover new states of matter and understand their unique properties. One such state is liquid crystal, which exhibits properties of both liquid and solid. However, there is another intriguing state of matter that has remained elusive for over half a century – the spin-nematic phase. This phase, analogous to liquid crystal but with spin moments instead of molecules, has recently been directly observed for the first time by a team of researchers led by Professor Kim Bumjoon at the IBS Center for Artificial Low-Dimensional Electronic Systems in South Korea.
The main challenge in observing the spin-nematic phase lies in the fact that most conventional experimental techniques are not sensitive to spin quadrupoles, which are the defining features of this phase. However, through the remarkable advancements in synchrotron facility development, the research team in South Korea was able to overcome this challenge and directly observe spin quadrupoles.
The Role of Square-Lattice Iridium Oxide
The researchers focused their study on a material called square-lattice iridium oxide Sr2IrO4. This material was already known for its antiferromagnetic dipolar order at low temperatures. However, the study revealed the coexistence of a spin quadrupolar order in addition to the magnetic order. This coexistence became observable through interference between the two orders.
To directly observe the spin quadrupolar order, the researchers employed an advanced X-ray technique called ‘circular-dichroic resonant X-ray diffraction.’ This technique utilized a circularly polarized X-ray beam to detect the interference signal between the spin quadrupolar and magnetic orders.
Further verification of this discovery was obtained through ‘polarization-resolved resonant inelastic X-ray scattering.’ This technique allowed the researchers to observe the magnetic excitations and confirm that they significantly deviated from the expected behaviors of conventional magnets.
To conduct these experiments, the research team collaborated with Argonne National Laboratory in the US to construct a resonant inelastic X-ray scattering beamline at the Pohang Accelerator Laboratory over the course of four years.
In addition to X-ray techniques, the researchers utilized a series of optical techniques, including Raman spectroscopy and magneto-optical Kerr effect measurement. These techniques provided further evidence that the formation of spin quadrupole moments occurs at higher temperatures than the magnetic order, indicating the existence of a spin-nematic phase.
The discovery of the spin-nematic phase holds significant implications for the fields of quantum computing and information technologies. The highly entangled spins in this phase, as suggested by physicist P. W. Anderson, may be a critical ingredient for high-temperature superconductivity. This finding opens up new possibilities for the development of superconducting materials that operate at higher temperatures.
Moreover, the extensive study of the iridium oxide Sr2IrO4, which shares striking similarities with the copper-oxide high-temperature superconducting system, has sparked growing interest in this material as a potential candidate for a new high-temperature superconducting system. The relation between this material and the spin-nematic phase further adds to its significance in the field of superconductivity research.
The direct observation of spin quadrupole moments in the spin-nematic phase is a groundbreaking achievement in materials science. The utilization of advanced experimental techniques, combined with the collaborative efforts of researchers in South Korea and the US, has paved the way for further exploration of this unique state of matter. The implications of this discovery extend beyond fundamental research, holding promise for advancements in quantum computing, information technologies, and the search for high-temperature superconducting materials. As the infrastructure and capabilities of X-ray experiments continue to evolve, we can expect even more exciting discoveries in the field of materials science in the years to come.
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