Yusuke Wakabayashi

Layered transition-metal dichalcogenide 1T-TaS ₂ has long been considered a prime example of a Mott insulator driven by strong electron correlations. However, this view has been recently challenged by proposals which favor a band insulating state arising from interlayer dimerization. This ongoing debate opens a frontier in 1T-TaS ₂ research focusing on stacking order, and raises a fundamental question with broad implications for two-dimensional materials: can electronic phases be engineered by actively tuning interlayer coupling? Here, using micro-focused angle-resolved photoemission spectroscopy supported by first-principles calculations, we demonstrate that uniaxial strain drives 1T-TaS ₂ from its insulating charge-density-wave phase into a stable metallic phase. This metallic phase, distinct from hydrostatic-pressure-induced or non-equilibrium hidden phases, emerges via a strain-induced rearrangement in the interlayer stacking of the Star-of-David charge order, effectively disrupting interlayer dimerization. Furthermore, the emergent metallic phase exhibits a significant quasiparticle mass renormalization, a hallmark of strong electron correlations, indicating its correlated nature and proximity to a Mott instability. This finding necessitates a re-evaluation of the interplay between structural and strong correlations in 1T-TaS ₂ . The present results establish uniaxial strain as a powerful method for tuning the interlayer coupling, opening pathways for exploring quantum phases inaccessible under ambient conditions.

CrAs undergoes a first-order helimagnetic transition accompanied by a huge lattice distortion whose microscopic mechanism remains unresolved. Here, the authors demonstrate through stress-strain measurements that the helimagnetic transition in CrAs is substantially more sensitive to uniaxial stress than to hydrostatic pressure. First-principles calculations reveal that this strong response originates from an entanglement between anisotropic strain and orbital-dependent correlations, showing that local correlations can drive nonlinear elastic responses in strongly correlated systems.

Epitaxial strain in perovskite oxide thin films is conventionally understood through lattice parameter modulation, characterized by the c / a ratio of the pseudocubic cell. However, internal atomic arrangements such as octahedral tilting can relax independently, a phenomenon that has remained largely unexplored. Here, we demonstrate that electrochemical operation induces internal structural relaxation in epitaxial La 0.7 Sr 0.3 CoO 3 films while in-plane lattice parameters remain pinned to the substrate. Synchrotron x-ray diffraction reveals that prolonged electrochemical treatment drives the octahedral tilt mode from the as-grown strained configuration toward the bulk structure without conventional lattice parameter changes. This relaxation is enabled by ionic oxygen migration during electrochemical reactions, which reduces energy barriers between structural configurations. Our findings reveal a relaxation pathway in epitaxial oxides that operates independently of conventional lattice parameter relaxation, and demonstrate the importance of monitoring internal atomic arrangements under operating conditions. This establishes electrochemically driven ionic migration as a versatile tool for controlling internal structure in epitaxial oxides, opening opportunities for designing strain-engineered functional materials.

Hydrogen doping offers a versatile method to dynamically control the doping level of metal oxides over a wide range, yet it introduces strong lattice modulation whose structural details have been poorly investigated. Here, we elucidate the hydrogen-induced structural modulation in NdNiO 3 thin films using synchrotron x-ray diffraction. While the strain relaxed pristine film exhibits a bulklike 2 × 2 × 2 structure, the hydrogen-induced insulating phase adopts a 1 × 1 × 2 structure with some incommensurate modulation. Our results provide the first direct experimental evidence for hydrogen-induced formation of a superstructure in rare-earth nickelates.

X-ray scattering has long been a cornerstone for elucidating structures in condensed matter. Sharp Bragg reflections at reciprocal lattice points provide detailed structural information with lattice translational symmetry, while the diffuse X-ray scattering captures deviations from the lattice-averaged structure. Recent advances in synchrotron light sources, detectors, and data-analysis methods have greatly expanded the potential of diffuse scattering based X-ray techniques in condensed matter science. In this paper, we exhibit that potential by presenting representative studies of lattice dynamics using inelastic X-ray scattering, low-dimensional order characterized through pair distribution function analysis, short range correlations of point defects in strongly correlated oxides, and nanoscale morphology revealed by coherent X-ray diffraction imaging. These case studies demonstrate how diffuse scattering bridges length scales from atomic vibrations to mesoscopic order, unlocking information inaccessible to Bragg-only approaches.

The surface structure of the La_0.6Sr_0.4CoO₃ film, a typical model water-splitting catalyst, is examined under vacuum and electrochemical conditions using surface X-ray diffraction. The pristine sample has a two-unit-cell-thick strongly polarized SrCoO₃ layer at the surface, and the surface termination is predominantly a CoO₂ layer with apical oxygen atoms. After electrochemical treatment, the surface was covered with an additional edge-shared CoO₆ double layer. The polarization in the SrCoO₃ region was greatly reduced. These structural changes were accompanied by an increase in the working electrode current, suggesting a strong relationship between surface structure modulation and catalytic activity. Reversible structural modulation induced by the film's electric potential was observed and was qualitatively understood as atomic displacements caused by the local electric field and change in the Co ionic radii.

A furan derivative, 2-[1,1'-biphenyl]-4-yl-5-(5'-[1,1'-biphenyl]-4-yl-[2,2'-bithiophen]-5-yl)furan (BPFTT), was designed and synthesized and shown to have excellent optical and electronic characteristics. The synthesized compound exhibits a photoluminescence quantum yield (PLQY) of 28%. In addition, a field-effect transistor fabricated using this material in the active layer exhibits ambipolar mobilities of 0.54 (hole) and 0.03 (electron) cm² V^-1 s^-1. Single crystals with their natural crystal edges acting as Fabry-Pérot resonators exhibit dual gain narrowing phenomena with optical pumping thresholds as low as 15 μJ/cm². Introducing planar distributed feedback resonators reduces the threshold to 2.41 μJ/cm², with the Q-factor reaching 1.5 × 10⁴. The full width at half-maximum is measured to be 0.074 nm, which is an excellent value reported to date for any organic single-crystal laser.

Local polarization and charge transfer at the interface between 7-unit cell-thick LaMnO3 (LMO) ultrathin films and substrates composed of Nb-doped (0.05 wt. %) and undoped SrTiO3 (STO) are examined by resonant surface x-ray diffraction. Notably, this photon-in photon-out technique allows us to simultaneously examine the valence distribution and local polarization with minimal radiation damage. Consistent with previous reports, Mn2+ is observed at the interface. Furthermore, the degree of charge transfer is nearly unchanged by the 0.05 wt. % of Nb doping. In the middle of the LMO films, the valence of Mn is 3+. Local polarization estimated from the cation/anion displacements shows that the electric field points outward in the LMO films, and polarization is suppressed in the STO substrate region.