Publisher：Physical Review B  

We describe the existence of local distortion discovered in the synchrotron x-ray single-crystal structure analysis of layered ditelluride 1T-MTe2 (M=V, Nb, Ta). In 1T-TaTe2, the double zigzag chain structure of Ta is deformed at about 170 K, and heptamer molecules are formed periodically at low temperatures. We found that some of the Ta atoms that compose the double zigzag chain structure appearing at high temperatures are locally displaced, resulting in local dimerization. This tendency weakens when Ta is replaced by V or Nb. Our results indicate that the local distortion persistently survives in these ditellurides, where the electronic degrees of freedom, including orbitals, are weakened. We further discuss the origin of local distortion in these ditellurides, which is different from many usual material systems where molecular formation occurs at low temperatures.

DOI： 10.1103/PhysRevB.107.245113

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Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/JPSJ.92.064706

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.actamat.2023.118738

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Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/JPSJ.92.023703

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevB.107.045141

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.107.045141/fulltext

Publisher：Physical Review B  

Vanadium atoms in layered LiVO2 form in-plane periodic vanadium trimers at low temperatures, but the trimers appear randomly in the stacking direction because there are many trimer configurations with comparable lattice energy. We detailed an original modeling scheme to represent glassy states with a completely disordered trimer configuration in the stacking structure. Through pair distribution function analysis using this model, we show that the synthesis method can yield two types of low-temperature stacking structures: a completely disordered stacking structure and a short-range order in the stacking structure. The phase-transition temperature of the former sample is about 15 K lower than that of the latter. We discuss that this is due to the strong trimer frustration that appears in the sample without short-range order, which suppresses the phase-transition temperature, similar to the frustration effect in conventional spin systems.

DOI： 10.1103/PhysRevB.107.L020101

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Publisher：Nature Physics  

Crystal structures with degenerate electronic orbitals are unstable towards lattice distortions that lift the degeneracy. Although these Jahn–Teller distortions have profound effects on magnetism, they are typically unaffected by the onset of magnetic ordering because of a separation in energy scales. Here we show the contrary case in Pr2Zr2O7, where orbital degeneracy remains down to the millikelvin range due to an interplay between spins and orbitals. Pr2Zr2O7 is a multipolar spin ice with strongly localized 4f electrons in an even-number configuration, giving rise to a non-Kramers doublet that carries transverse quadrupolar and longitudinal dipolar moments. Our study of ultrapure single crystals of Pr2Zr2O7 finds comprehensive evidence for enhanced spin–orbital quantum dynamics of the non-Kramers doublet. This dynamical Jahn–Teller effect is encapsulated by the liquid–gas metamagnetic transition that is characteristic of spin ice being accompanied by strong lattice softening. This behaviour suggests that a spin–orbital liquid state forms on the pyrochlore lattice at low temperatures and low magnetic fields.

DOI： 10.1038/s41567-022-01816-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

Orbital degrees of freedom appear in partially filled degenerate orbitals and are responsible for a variety of physical properties. Although many methods for directly observing orbital states in ordered states have been proposed, it is difficult to determine degenerate orbital states directly. We found that high-precision diffraction data obtained by synchrotron radiation X-rays can be used to observe only the valence electron density distribution with minimal analytical bias. The results obtained using this method clearly showed the anisotropy of the partially filled degenerate 3d orbital states in spinel vanadium oxide FeV2O4. The 3d orbital quantum state was successfully determined from this anisotropy.

DOI： 10.1039/d1ma01113h

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Language：Japanese   Publisher：Journal of the Physical Society of Japan  

An excitonic insulator, in which an optical gap is formed by exciton condensation, is expected to exhibit large photoresponses. Here, we report the results of femtosecond reflection spectroscopy covering a wide infrared region on a typical excitonic insulator, Ta2NiSe5, which gives direct information not only about a change of excitonic gap but also a metallization. With a weak excitation, the excitonic gap decreases due to the weakening of excitonic effect by photocarriers. With a strong excitation, a Drude-like high-reflection band appears, showing an excitonic-insulator-to-metal transition. Analyses of spectral and temporal changes of reflectivity revealed that the metallization is accompanied by large structural changes. This demonstrates an important role of electron–lattice interaction on the stabilization of excitonic phase.

DOI： 10.7566/JPSJ.91.023701

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevB.105.L041103

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.105.L041103/fulltext

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/JPSJ.90.074706

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.103.184112

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.inorgchem.1c00208

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Photoinduced phase transitions have been intensively studied owing to the potential capability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it is currently a central issue as to how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photoinduced phase transitions. Here, we use a recently developed analysis method, which we call frequency-domain angle-resolved photoemission spectroscopy (FDARPES), and reveal mode- and band-selective electron-phonon couplings during the photoinduced insulator-to-metal transition for Ta2NiSe5. We find that the lattice modulation corresponding to the 2 THz phonon mode, where the Ta lattice is sheared along the a axis, is the most relevant for the emergence of photoinduced semimetallic state. Furthermore, we find that the semimetallic and semiconducting bands coexist in the transient state, and demonstrate that FDARPES spectra can selectively detect the phonon-specific couplings to the two coexistent band structures during the photoinduced phase transition by resolving them in the frequency domain.

DOI： 10.1103/PhysRevB.103.L121105

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>Electronic instabilities in transition metal compounds often spontaneously form orbital molecules, which consist of orbital-coupled metal ions at low temperature. Recent local structural studies utilizing the pair distribution function revealed that preformed orbital molecules appear disordered even in the high-temperature paramagnetic phase. However, it is unclear whether preformed orbital molecules are dynamic or static. Here, we provide clear experimental evidence of the slow dynamics of disordered orbital molecules realized in the high-temperature paramagnetic phase of LiVS₂, which exhibits vanadium trimerization upon cooling below 314 K. Unexpectedly, the preformed orbital molecules appear as a disordered zigzag chain that fluctuate in both time and space under electron irradiation. Our findings should advance studies on soft matter physics realized in an inorganic material due to disordered orbital molecules.

DOI： 10.1038/s41535-021-00313-w

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Other Link： http://www.nature.com/articles/s41535-021-00313-w

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the precise crystal structures and electronic states in a
quasi-two-dimensional molecular conductor ${\alpha}$-(BETS)$_2$I$_3$ at ambient
pressure. The electronic resistivity of this molecular solid shows a
metal-to-insulator (MI) crossover at $T_{MI}$=50 K. Our x-ray diffraction and
$^{13}$C nuclear magnetic resonance experiments revealed that
${\alpha}$-(BETS)$_2$I$_3$ maintains the inversion symmetry below $T_{MI}$. The
first-principles calculations found a pair of anisotropic Dirac cones at a
general k-point, where the degenerated contact points are located at the Fermi
level. Furthermore, the origin of the insulating state in this system is
explained by a small energy gap of ~2 meV opened by a spin-orbit interaction,
in which the Z$_2$ topological invariants indicate a weak topological
insulator. Our results suggest that ${\alpha}$-(BETS)$_2$I$_3$ is a promising
material for studying the bulk Dirac electron system in two-dimension.

DOI： 10.1103/PhysRevB.103.035135

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arXiv

Other Link： http://arxiv.org/pdf/2006.08978v1

Publishing type：Research paper (scientific journal)   Publisher：The Crystallographic Society of Japan  

DOI： 10.5940/jcrsj.62.221

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/JPSJ.89.124703

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

The physical properties of molecular crystals are governed by the frontier orbitals of molecules. A molecular orbital, which is formed by superposing the atomic orbitals of constituent elements, has complicated degrees of freedom in the crystal because of the influence of electron correlation and crystal field. Therefore, in general, it is difficult to experimentally observe the whole picture of a frontier orbital. Here, we introduce a new method called “core differential Fourier synthesis” (CDFS) using synchrotron X-ray diffraction to observe the valence electron density in materials. By observing the valence electrons occupied in molecular orbitals, the orbital state can be directly determined in a real space. In this study, we applied the CDFS method to molecular materials such as diamond, C60 fullerene, (MV)I2, and (TMTTF)2X. Our results not only demonstrate the typical orbital states in some materials, but also provide a new method for studying intramolecular degrees of freedom.

DOI： 10.3390/cryst10110998

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The orbital degree of freedom of electrons greatly influences the physical properties of materials such as magnetic order and unconventional superconductivity. An orbital is a minimal unit of "shape," and the orbital state can be unraveled by observing the spatial anisotropic distribution of electrons. However, it is difficult to experimentally extract the orbital information in a crystal because of various technical problems. Here, the Ti-3d orbital state in perovskite-type oxides RTiO3 (R = Y, Sm, and La) is directly determined by a core differential Fourier synthesis (CDFS) method using synchrotron x-ray diffraction. The valence electron-density distribution, including information on the anisotropy and the hybridization between atomic orbitals, can be extracted from the CDFS analysis. Our study not only demonstrates the relationship between the magnetic- and orbital-ordered states called the Kugel-Khomskii model, but also provides a nontrivial picture of the orbital state reconstructed by the orbital hybridization.

DOI： 10.1103/PhysRevResearch.2.033503

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevB.102.081106

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.102.081106/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Since the discovery of the Verwey transition in magnetite, transition metal compounds with pyrochlore structures have been intensively studied as a platform for realizing remarkable electronic phase transitions. We report on a phase transition that preserves the cubic symmetry of the beta -pyrochlore oxide CsW2O6, where each of W 5d electrons are confined in regular-triangle W-3 trimers. This trimer formation represents the self-organization of 5d electrons, which can be resolved into a charge order satisfying the Anderson condition in a nontrivial way, orbital order caused by the distortion of WO6 octahedra, and the formation of a spin-singlet pair in a regular-triangle trimer. An electronic instability due to the unusual three-dimensional nesting of Fermi surfaces and the strong correlations of the 5d electrons characteristic of the pyrochlore oxides are both likely to play important roles in this charge-orbital-spin coupled phenomenon. Many rules have been suggested to rationalize the behaviour of transition metal oxides but the complex underlying physics often leads to deviations from the expected behaviour. Here the authors show that the metal-insulator transition in CsW2O6 satisfies the Anderson condition in an unusual way.

DOI： 10.1038/s41467-020-16873-7

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Cu+ ion behavior is one of the heavily discussed topics in physical and material fields because of its unclear behavior and high potential in materials application. To provide a breakthrough in this field, comprehensive research that connects the real space and the reciprocal lattice (momentum) space is effective. Here, we investigated the Cu+ ion behavior in two-dimensional layered system Cu0.33TiSe2, which shows the Cu+ disorder-order phase transition at 200 K. Inelastic x-ray scattering revealed a large change of the Cu+ ion dynamics at the phase transition. It is expected that the complex interactions are involved in the behavior of Cu+ ions intercalated in the negatively charged TiSe2 layers by charge transfer. However, Monte Carlo simulations using a simple combination of repulsive forces between Cu+ ions well reproduce the characteristic temperature dependence of the anisotropic diffuse scattering. These insights provide a unified understanding in controlling the intercalated system.

DOI： 10.1103/PhysRevB.101.094108

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DOI： 10.1088/1742-6596/1609/1/012001

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Language：Japanese   Publishing type：Research paper (scientific journal)  

© 2019 American Physical Society. Ta2NiS5 is supposed to be a simple semiconductor in which excitonic instability of Ta2NiSe5 is suppressed due to its large band gap. However, the Ni 2p core-level photoemission of Ta2NiS5 exhibits a satellite indicating Ni 3d orbitals are mixed into its conduction band as expected in an excitonic insulator. The valence band does not show dispersion flattening and spectral sharpening which are fingerprints of an excitonic insulator. Instead, Ni 3p-3d resonant photoemission indicates Mottness of the Ni 3d electron in Ta2NiS5 with negative charge-transfer energy. The present results show that Ta2NiS5 can be viewed as a valence bond insulator with a band gap exceeding the exciton binding energy.

DOI： 10.1103/PhysRevB.100.245129

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.100.235120

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Language：Japanese   Publisher：Physical Review X  

The thermal conductivity κ of the quasi-2D organic spin-liquid candidate EtMe3Sb[Pd(dmit)2]2 (dmit-131) was measured at low temperatures, down to 0.07 K. We observe a vanishingly small residual linear term κ0/T, in κ/T vs T as T→0. This shows that the low-energy excitations responsible for the sizable residual linear term γ in the specific heat C, seen in C/T vs T as T→0, are localized. We conclude that there are no mobile gapless excitations in this spin-liquid candidate, in contrast with a prior study of dmit-131 that reported a large κ0/T value [Yamashita, Science 328, 1246 (2010)SCIEAS0036-807510.1126/science.1188200]. Our study shows that dmit-131 is in fact similar to κ-(BEDT-TTF)2Cu2(CN)3, another quasi-2D organic spin-liquid candidate where a vanishingly small κ0/T and a sizable γ are seen. We attribute heat conduction in these organic insulators without magnetic order to phonons undergoing strong spin-phonon scattering, as observed in several other spin-liquid materials.

DOI： 10.1103/PhysRevX.9.041051

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Language：Japanese   Publishing type：Research paper (scientific journal)  

©2019 The Physical Society of Japan We have measured the dielectric constant and thermopower of single crystal samples of the excitonic insulator candidate Ta2NiSe5. The dielectric constant below 50 K shows relaxor-like relaxation, implying the existence of randomly distributed electric dipoles. We have evaluated the concentration of the dipoles to be 1019 cm−3 at 30 K, which gives a rough estimate of the exciton concentration in this compound. A large thermopower of 600 µV=K at 100 K suddenly drops toward zero down to 50 K, and this zero thermopower evidences the neutral particles flow against the applied temperature gradient. We have semi-quantitatively explained the above two anomalies in terms of spontaneously generated excitons at low temperatures where thermally excited holes are quenched.

DOI： 10.7566/JPSJ.88.113706

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.inorgchem.9b00186

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PubMed

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.100.174401

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

<p>Gold(<sc>iii</sc>) atoms reversibly deviate from the molecular plane on receiving thermal and photon energy.</p>

DOI： 10.1039/c9dt02377a

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PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)  

© 2019 American Chemical Society. Li2SrNb2O7 (LSNO) crystallizes in a structure closely related to n = 2 Ruddlesden-Popper-type compounds, which are generally formed by intergrowth of two-dimensional perovskite-type blocks and rocksalt-type layers. The present study demonstrates the coexistence of spontaneous polarization and an anti-ferroelectric-like nonlinear response in LSNO at 80 K, suggesting weak ferroelectricity below the phase transition temperature of 217 K. A combination of first-principles calculations and single-crystal X-ray diffractions clarifies a polar P21cn structure for the ground state of LSNO, where an in-plane antiferroelectric displacement and an out-of-plane polar shift simultaneously take place. The present study offers a new perspective to design ferroelectric and antiferroelectric materials with Ruddlesden-Popper-type compounds.

DOI： 10.1021/acs.chemmater.9b02373

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.99.104109

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Publisher：American Journal of Pathology  

Localization of the abnormal and normal isoforms of prion proteins to detergent-resistant membrane microdomains, lipid rafts, is important for the conformational conversion. Lipid rafts are enriched in sialic acid–containing glycosphingolipids (namely, gangliosides). Alteration in the ganglioside composition of lipid rafts can affect the localization of lipid raft–associated proteins. To investigate the role of gangliosides in the pathogenesis of prion diseases, we performed intracerebral transmission study of a scrapie prion strain Chandler and a Gerstmann-Sträussler-Scheinker syndrome prion strain Fukuoka-1 using various knockout mouse strains ablated with ganglioside synthase gene (ie, GD2/GM2 synthase, GD3 synthase, or GM3 synthase). After challenge with the Chandler strain, GD2/GM2 synthase knockout mice showed 20% reduction of incubation time, reduced prion protein deposition in the brain with attenuated glial reactions, and reduced localization of prion proteins to lipid rafts. These results raise the possibility that the gangliosides may have an important role in prion disease pathogenesis by affecting the localization of prion proteins to lipid rafts.

DOI： 10.1016/j.ajpath.2018.11.009

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

The phase transition phenomenon in a semimetallic 1T-TiSe2 has attracted attention as an excitonic insulator. However, as the phase transition accompanying superlattice peaks has the q vector connecting the Fermi surfaces of the three-dimensional shape of hole and electron pockets, it also assumes the charge density wave (CDW) state owing to the electron-phonon interaction. To understand the electronic state at the low temperature, control of the chemical potential was attempted by electronic doping through Cu+ intercalation. Physical properties measurements and synchrotron x-ray diffraction experiments in CuxTiSe2 (x=0-0.35) were performed. The phase transition was determined to occur as a cooperative phenomenon between the honeycomb lattice type CDW corresponding to the nesting vector and the ordered state of the Cu+ ions between TiSe2 layers at a specific doping amount (x=1/3). The behavior of Cu+ ions in highly doped regions suggests the occurrence of a two-dimensional liquid-solid state transition based on the temperature dependence of the x-ray diffuse scattering.

DOI： 10.1103/PhysRevB.99.081111

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DOI： 10.1088/1361-648X/aae106

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PubMed

Language：English   Publishing type：Research paper (scientific journal)  

Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realisation of photo-induced insulator-to-metal transitions in Ta2Ni(Se1-xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron-hole coupled systems.

DOI： 10.1038/s41467-018-06801-1

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PubMed

DOI： 10.1364/OE.26.024784

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

© 2018 American Physical Society. We discuss herein the emergence of a large entropy change in metallic Li0.33VS2 derived from orbitally assisted loose σ bond formation. Comprehensive structural studies based on synchrotron x-ray and neutron diffraction analyses clarify the fabrication of ribbon chains at 375 K, consisting of multiple three-centered two-electron σ bonds based on the viewpoint of local chemical bonding. Although the metallic conductivity persists down to the lowest temperature measured, an exceptionally large entropy change as a metal, as much as ΔS=6.6Jmol-1K-1, appears at the transition. The emergence of a large entropy change in a metallic state may possibly lead to different functional materials, such as a heat-storage material with a rapid thermal response.

DOI： 10.1103/PhysRevB.98.081104

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Language：English   Publisher：AMER PHYSICAL SOC  

© 2018 American Physical Society. Ta2NiSe5 is considered a promising excitonic insulator (EI) candidate, in which the EI state is due to Coulomb interaction between electrons and holes. In Ta2NiSe5, the role of phonons is significant since the EI transition is accompanied by an orthorhombic-to-monoclinic structural transition at Ts=328K. Our inelastic x-ray scattering measurements reveal that the transverse acoustic mode exhibits strong softening just above Ts. Furthermore, the transverse optical mode exhibits linewidth broadening caused by strong electron-optical-phonon coupling above Ts. Density functional theory calculations indicate that the coupled optical modes arise due to the vibration of Ta ions. Consequently, in the monoclinic phase, the coupled optical modes are &quot;frozen&quot; such that Ta and Se approach each other, forming atomic-displacement-type electric dipoles that are obviously different from a Peierls transition or Jahn-Teller distortion. The characteristic of electronic toroidal moment formation by the antiferroelectric arrangements of the electric dipoles forms the common feature between Ta2NiSe5 and 1T-TiSe2.

DOI： 10.1103/PhysRevB.98.045139

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We have investigated the structural and magnetic properties of S=3/2Cr3+ Heisenberg-like kagome lattice antiferromagnets Cs2KCr3F12 and Cs2NaCr3F12 using single crystals. Each compound has a slightly distorted kagome lattice, although the space groups differ for the two compounds. The magnetic data suggest that each compound exhibits the formation of antiferromagnetic ordering with in-plane-spin canting approximately below 6 K. The in-plane coplanar structures are clearly different from disordered ground states of the isostructural Ti3+ compounds and out-of-plane coplanar structures of the V3+ compounds, which is mainly caused by the differences in the strength of quantum fluctuation and the strength of magnetic anisotropy for these three spin systems. In contrast, as well as the Ti3+ and V3+ compounds, the magnetization curves show 1/3-magnetization-plateaulike anomalies.

DOI： 10.1103/PhysRevB.97.224421

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevMaterials.2.054402

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：International Union of Crystallography  

The crystal structure of the excitonic insulator Ta2NiSe5 has been investigated under a range of pressures, as determined by the complementary analysis of both single-crystal and powder synchrotron X-ray diffraction measurements. The monoclinic ambient-pressure excitonic insulator phase II transforms upon warming or under a modest pressure to give the semiconducting C-centred orthorhombic phase I. At higher pressures (i.e. &gt
3GPa), transformation to the primitive orthorhombic semimetal phase III occurs. This transformation from phase I to phase III is a pressure-induced first-order phase transition, which takes place through coherent sliding between weakly coupled layers. This structural phase transition is significantly influenced by Coulombic interactions in the geometric arrangement between interlayer Se ions. Furthermore, upon cooling, phase III transforms into the monoclinic phase IV, which is analogous to the excitonic insulator phase II. Finally, the excitonic interactions appear to be retained despite the observed layer sliding transition.

DOI： 10.1107/S2052252517018334

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

The distribution of d-orbital valence electrons in volborthite [Cu3V2O7(OH)212H2O] was investigated by charge density analysis of the multipole model refinement. Diffraction data were obtained by synchrotron radiation single-crystal X-ray diffraction experiments. Data reduction by detwinning of the multiple structural domains was performed using our developed software. In this study, using high-quality data, we demonstrated that the water molecules in volborthite can be located by the hydrogen bonding in cavities that consist of Kagome lattice layers of CuO4(OH)2 and pillars of V2O7. Final multipole refinements before and after the structural phase transition directly visualized the deformation electron density of the valence electrons. We successfully directly visualized the orbital flipping of the d-orbital dx2–y2, which is the highest level of 3d orbitals occupied by d9 electrons in volborthite. The developed techniques and software can be employed for investigations of structural properties of systems with multiple structural domains.

DOI： 10.7566/JPSJ.87.024601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry  

We report a K2CaPO4F:Eu2+ phosphor with a new crystal structure. This phosphor has a large Stokes shift and converts near-ultraviolet light to red luminescence without absorption of other visible light. The mechanism was elucidated by applying a constrained density functional theory to the solved crystal structure.

DOI： 10.1039/c7cc08202a

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We present structural studies of the spin-1/2 honeycomb-lattice antiferromagnet, Cu-2(pymca)(3)(ClO4) (pymca: pyrimidine-2-carboxylate), using synchrotron X-ray diffraction experiments. Previous research reported that its crystal structure consists of a regular honeycomb lattice, which exhibits quantum spin-liquid behavior. However, due to its threefold Cu2+ site symmetry, this proposed crystal structure cannot describe Jahn-Teller distortion. We analyzed the structure of Cu-2(pymca)(3)(ClO4), including superlattice reflections that were overlooked during previous X-ray diffraction studies of the material, and clarified that its distorted honeycomb lattice consists of Cu-2(pymca)(3) sections. Further investigation of Cu-2(pymca)(3)(ClO4) crystal structure may reveal magnetic susceptibilities of the distorted honeycomb lattice.

DOI： 10.7566/JPSJ.86.123302

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

High luminescence efficiency is obtained in halide- and chalcogenide-based phosphors, but they are impractical because of their poor chemical durability. Here we report a halide-based nanocomposite phosphor with excellent luminescence efficiency and sufficient durability for practical use. Our approach was to disperse luminescent single nanocrystals of caI(2):Eu2+ in a chemically stable, translucent crystalline SiO2 matrix. Using this approach, we successfully prepared a nanocomposite phosphor by means of self-organization through a simple solid-state reaction. Single nanocrystals of 6H polytype (thr notation) CaI2:Eu2+ with diameters of about SO rim could be generated not only in a SiO2 amorphous powder but also in a SiO2 glass plate. The nanocomposite phosphor formed upon solidification of molten CaI2 left behind in the crystalline SiO2 that formed from the amorphous SiO2 under the influence of a CaI2 flux effect. The resulting nanocomposite phosphor emitted brilliant blue luminescence with an internal quantum efficiency up to 98% upon 407 nm violet excitation. We used cathodoluminescence microscopy, scanning transmission electron microscopy, and Rietveld refinement of the X-ray diffraction patterns to confirm that the blue luminescence was generated only by the CaI2:Eu2+ single nanocrystals. The phosphor was chemically durable because the luminescence sites were embedded in the crystalline SiO2 matrix. The phosphor is suitable for use in near-ultraviolet light-emitting diodes. The concept for this nanocomposite phosphor can be expected to be effective for improvements in the practicality of poorly durable materials such as halides and chalcogenides.

DOI： 10.1021/acsami.7b14132

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

A quasi-one-dimensional organic charge-transfer salt (TMTTF)(2)PF6 undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a "structureless transition," and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer delta(CO) = 0.20e. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.

DOI： 10.1103/PhysRevLett.119.065701

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have investigated crystal structure and magnetic properties of three S = 1 V3+ Ising-like anisotropic kagome lattice antiferromagnets, Cs2KV3F12, Cs2NaV3F12, and Rb-2 NaV3F12, using single crystals. Each compound crystallizes in a monoclinic system and has a slightly distorted kagome lattice. Although the magnetic properties are similar, the details depend on the magnitude of magnetic anisotropy. The magnetization and magnetic susceptibility are highly anisotropic owing to anisotropic g factors and Ising-like single ion anisotropy originating from partially unquenched orbital moments of V3+, and the easy axis of Ising-like anisotropy is perpendicular to the kagome plane. In contrast to disordered ground states in the isostructural Ti3+ (S = 1/2) compounds, the V3+ (S = 1) compounds exhibit antiferromagnetic ordering with distorted 120 degrees. structure or other nearly coplanar magnetic structure whose basal spin plane includes the easy axis, which is mainly due to large magnetic anisotropy and decrease of quantum fluctuation. Moreover, when magnetic fields are applied perpendicular to the kagome plane, magnetization curves of them show distinct 1/3 and 2/3 magnetization plateaus, which are stabilized by Ising-like single ion anisotropy. In particular, the 2/3 magnetization plateaus are notable magnetic phenomena in kagome lattice antiferromagnets and are due to the combination of spin frustration, Ising-like single ion anisotropy, and the small monoclinic distortion of kagome lattices.

DOI： 10.1103/PhysRevB.95.134436

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Two novel 112-type palladium-doped iron arsenides were synthesized and identified using comprehensive studies involving synchrotron X-ray diffraction and X-ray absorption near-edge structure (XANES) experiments. Whereas in-plane arsenic zigzag chains were found in the 112-type superconducting iron arsenide Ca1-xLaxFeAs2 with maximum T-c = 34 K, deformed arsenic network structures appeared in other 112-type materials, such as longitudinal arsenic zigzag chains in CaFe1-yPdyAs2 (y similar to 0.51) and arsenic square sheets constructed via hypervalent bonding in Ca1-xLaxFe1-yPdyAs2 (x similar to 0.31, y similar to 0.30). As K-edge XANES spectra clarified the similar oxidization states around FeAs4 tetrahedrons, alluding to possible parents for high-T-c 112-type iron arsenide superconductors.

DOI： 10.1021/acs.inorgchem.6b03115

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Scopus

PubMed

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DOI： 10.1038/srep39280

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高輝度放射光による先端物性研究

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DOI： 10.1103/PhysRevLett.103.073001

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DOI： 10.11316/jpsgaiyo.72.1.0_1077

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単結晶高分解能電子密度分布解析による精密構造物性研究

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