The spin dimer system is a quantum magnet in which two S = 1/2 spins form a dimer by strong antiferromagnetic interaction, and weak interdimer interactions couple the dimers. We found a quantum phase transition from a singlet to an ordered state in a spin dimer magnet TlCuCl₃ induced by hydrostatic pressure through magnetization measurements and neutron scattering experiments. We verified that the excitation gap closes with increasing pressure. Our observation encouraged the theoretical study, which led to the discovery of the first Higgs mode in condensed matter. We also clarified the structures of magnetic excitations in Ba₃CoSb₂O₉ and Cs₂Cu₃SnF₁₂, described as S = 1/2 triangular lattice and kagome lattice antiferromagnets, respectively. In both systems, we found that single magnon excitation energies are significantly renormalized downwards and that an intense excitation continuum extends to high energies. These features strongly suggest spinon excitations as an elementary excitation. Combining our results with recent theories, we can deduce that the superposition of the ordered state and the quantum spin liquid state gives the ground states of these systems.

Star polymers are unique polymers that have attracted industrial and scientific interest. I have conducted research on the structural analysis of various soft matter materials using neutrons, X-rays, and light scattering. Among them, I have been fascinated by the physics of star polymers and the corresponding gels. In my research for star polymers, I have revealed the unique osmotic properties of critical polymer clusters and excellent properties as biomaterials. Using contrast-tuned small-angle neutron scattering, I also visualized gel mesh in the partially deuterated star polymer gel network. After all these studies, I successfully developed extremely homogeneous gels based on the bond percolation of star polymers.

The advancement of LCD technology has greatly impacted our daily lives. Liquid crystal molecules exist in an intermediate state between liquid and crystal, and are stabilized by the domain structure of polar and non-polar portions, as well as the polarity of anisotropic molecules. In order to use liquid crystals for displays, they must be uniformly oriented. However, achieving this requires an alignment agent to be applied to the substrate in a clean environment, followed by heat treatment, which increases the number of processes and limits the number of substrates that can be used. Recently, there has been a lot of attention on alignment agents that can induce alignment simply by mixing with the liquid crystal and adsorbing to the substrate. In this study, neutron reflectivity measurements were conducted to investigate the liquid crystal structure and the structure formed by the alignment agent. The results showed that the model alignment agent adsorbs on the substrate to form a monolayer, and that the structure changes from horizontal to vertical with increasing mixing concentration.

Spin-contrast-variation (SCV) small-angle neutron scattering (SANS) enabled us to determine structure of nano-ice crystals that were generated in rapidly frozen sugar solution. In the frozen glucose solution, we found that the nano-ice crystals formed a planar structure with a radius larger than several tens of nanometers and a thickness of 2-3 nm, which was close to the critical nucleation size of ice crystals in supercooled water. This result suggests that the glucose molecules were preferentially bound to a specific face of nano-ice crystals, and then blocked the crystal growth perpendicular to that face.

A multiscale water visualization system for polymer electrolyte fuel cells (PEFCs) was established by using operando multiprobe radiography with pulsed spallation neutron and synchrotron X-ray sources. The multiscale three-dimensional water distribution revealed that water back-diffusion from the cathode to the anode contributes to the “drainability” of PEFCs toward fuel cell electric vehicles.