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3D-2D structural phase transition is artificially induced to invoke giant electronic modulation in nonequilibrium (Pb1−xSnx)Se., Material properties depend largely on the dimensionality of the crystal structures and the associated electronic structures. If the crystal-structure dimensionality can be switched reversibly in the same material, then a drastic property change may be controllable. Here, we propose a design route for a direct three-dimensional (3D) to 2D structural phase transition, demonstrating an example in (Pb1−xSnx)Se alloy system, where Pb2+ and Sn2+ have similar ns2 pseudo-closed shell configurations, but the former stabilizes the 3D rock-salt-type structure while the latter a 2D layered structure. However, this system has no direct phase boundary between these crystal structures under thermal equilibrium. We succeeded in inducing the direct 3D-2D structural phase transition in (Pb1−xSnx)Se alloy epitaxial films by using a nonequilibrium growth technique. Reversible giant electronic property change was attained at x ~ 0.5 originating in the abrupt band structure switch from gapless Dirac-like state to semiconducting state.

High critical temperature (Tc) superconductivity is generally considered to result from a fluctuation-mediated Cooper-pairing derived from a parent phase. The question of what type of fluctuation forms in materials thus plays a key role in understanding the mechanism of superconductivity. The iron-based superconductor LaFeAsO1-xHx possesses bipartite magnetic parent phases with centrosymmetric (x ~ 0) and non-centrosymmetric (x ~ 0.5) structures. The latter is an intriguing polar-metal phase induced by temperature and carrier doping. Here, we investigate average and local structures of LaFeAsO1-xHx using X-ray diffraction and extended X-ray absorption fine-structure measurements. We found lattice C4 symmetry breaking far above the structural transition temperature, and the signature of a tiny split in As–Fe bond distances with broken spatial inversion symmetry in a wide temperature/doping range. The former reveals a nematic state, and the latter highlights a fluctuated state of polar structure which can be appropriately called polar nematic state.

We examine electronic and crystal structures of iron-based superconductors LnFeAsO1−x H x (Ln = La, Sm) under pressure by means of x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray diffraction. In LaFeAsO the pre-edge peak on high-resolution XAS at the Fe-K absorption edge gains in intensity on the application of pressure up to 5.7 GPa and it saturates in the higher pressure region. We found integrated-absolute difference values on XES for Ln = La, corresponding to a spin state, decline on the application of pressure, and then it is minimized when the T c approaches the maximum at around 5 GPa. In contrast, such the optimum value was not detected for Ln = Sm. We reveal that the superconductivity is closely related to the lower spin state for Ln = La unlike Sm case. We observed that As height from the Fe basal plane and As–Fe–As angle on the FeAs4 tetrahedron for Ln = La deviate from the optimum values of the regular tetrahedron in superconducting (SC) phase, which has been widely accepted structural guide to SC thus far. In contrast, the structural parameters were held near the optimum values up to ∼15 GPa for Ln = Sm.

A carrier doping by a hydrogen substitution in LaFeAsO1−x H x is known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO1−x H x has a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitride LnFeAsO1−x H x (Ln = La, Sm) with the range of x = 0–0.5 by using x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction. For both compounds, we observed that the pre-edge peaks of x-ray absorption spectra near the Fe-K edge were reduced in intensity on doping. The character arises from the weaker As–Fe hybridization with the longer As–Fe distance in the higher doped region. We can reproduce the spectra near the Fe-K edge according to the Anderson impurity model with realistic valence structures using the local-density approximation (LDA) plus dynamical mean-field theory (DMFT). For Ln = Sm, the integrated-absolute difference (IAD) analysis from x-ray Fe-Kβ emission spectra increases significantly. This is attributed to the enhancement of magnetic moment of Fe 3d electrons stemming from the localized picture in the higher doped region. A theoretical simulation implementing the self-consistent vertex-correction method reveals that the single dome superconducting phase for Ln = Sm arises from a better nesting condition in comparison with Ln = La.

We report the origins of the coloration of Bi2O3–SiO2, Bi2O3–B2O3, and Bi2O3–P2O5 glasses. The glasses were quantitatively analyzed to reveal the Bi valence state and network connectivity of silicate, borate, and phosphate. The electronic structure was correlated with Bi valence states, giving two types of signals: those related to optical bandgaps with Bi3+ s–p intra-transition states at 3.6–4.7 eV and visible absorption bands at 2.5–2.7 eV originating from the charge transfer between Bi3+ and Bi5+ and/or plasmonic Bi0 clusters in the colored glasses. These Bi oxide glasses with tvisible transparent and high refractive index are attractive to replace toxic Pb-containing optical glasses.

5d pyrochlore oxides with all-in-all-out magnetic order are prime candidates for realizing strongly correlated, topological phases of matter. Despite significant effort, a full understanding of all-in-all-out magnetism remains elusive as the associated magnetic excitations have proven difficult to access with conventional techniques. Here we report a Raman spectroscopy study of spin dynamics in the all-in-all-out magnetic state of the 5d pyrochlore Cd2Os2O7. Through a comparison between the two-magnon scattering and spin-wave theory, we confirm the large single ion anisotropy in this material and show that the Dzyaloshinskii–Moriya and exchange interactions play a significant role in the spin-wave dispersions. The Raman data also reveal complex spin–charge–lattice coupling and indicate that the metal–insulator transition in Cd2Os2O7 is Lifshitz-type. Our work establishes Raman scattering as a simple and powerful method for exploring the spin dynamics in 5d pyrochlore magnets. Pyrochlore 5d transition metal oxides are expected to have interesting forms of magnetic order but are hard to study with conventional probes. Here the authors show that Raman scattering can be used to measure magnetic excitations in Cd2Os2O7 and that it exhibits complex spin-charge-lattice coupling.

We investigate the ultrafast dynamics of ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$, a prototype material showing a Lifshitz-type transition as a function of temperature. In the paramagnetic metallic state, the photoreflectivity shows a subpicosecond relaxation, followed by a featureless small offset. In the antiferromagnetic state slightly below ${T}_{N}$, however, the photoreflectivity resurges over hundreds of picoseconds, which goes beyond the usual realm of the effective-temperature model. Our observations are consistent with the Lifshitz phase transition of ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ driven by the evolution of the local magnetic moment.

Volborthite Cu3V2O7(OH)2·2H2O is a copper mineral that materializes a two-dimensional quantum magnet comprising a kagome net of spin-1/2 Cu2+ ions. We prepared single crystals of volborthite using hydrothermal conditions and investigated their crystal structures and magnetic properties. Unusual orbital "switching" and "flipping" transitions were observed: in the former type of transition (switching), the Cu 3d orbital occupied by an unpaired electron changes between the d(3z2-r2) and d(x2-y2) types, and in the latter type of transition (flipping), the d(x2-y2)-type orbitals change their directions. Their origin is ascribed to variations in the orientation of water molecules in the gap between the kagome layers and the accompanying changes of hydrogen bonding. These orbital transitions dramatically modify the magnetic interactions between Cu2+ spins, from the anisotropic kagome type to the formation of spin trimers over the kagome net. The effective spin 1/2 generated on the trimers exhibits a frustrated magnetism, resulting in a rich phase diagram in the magnetic fields. Volborthite is a unique compound showing an exceptional interplay between the orbital and spin degrees of freedom.

Inelastic neutron scattering was performed for an iron-based superconductor ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{D}}_{0.1}$, where most of D (deuterium) replaces oxygen, while a tiny amount goes into the interstitial sites. By first-principles calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature ${T}_{\mathrm{c}}=26\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, excitations emerge in the range 5--15 meV, while they are absent in the reference system ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$. The strong excitations at 14.5 and 11.1 meV broaden rapidly around 15 and 20 K, respectively, where each energy becomes comparable to twice the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two.

The spin-driven component of electric polarization in a single crystal of multiferroic ${\mathrm{BiFeO}}_{3}$ was experimentally investigated in pulsed high magnetic fields up to 41 T. Sequential measurements of electric polarization for various magnetic field directions provide clear evidence of electric polarization normal to the hexagonal $c$ axis $({\mathbit{P}}_{\mathrm{t}})$ in not only the cycloidal phase, but also the field-induced canted antiferromagnetic phase. The direction of ${\mathbit{P}}_{\mathrm{t}}$ is directly coupled with the ferromagnetic moment in the canted antiferromagnetic phase, and thus controlled by changing the direction of the applied magnetic field. This magnetoelectric coupling is reasonably reproduced by the metal-ligand hybridization model.

Magnetic excitations in a heavily electron-doped antiferromagnet, ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$, have been investigated using powder inelastic neutron scattering. Unlike other parent compounds of the iron-based superconductors, the magnetic excitation gap in ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$ was not detected down to the lowest measured temperature of 4 K. This result can be understood as a result of quasi-isotropy within the $ab$ plane, which is consistent with the band calculation result that the ${d}_{xy}$ orbital plays the dominant role in the magnetism of ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{H}}_{0.5}$. In addition, the intensities of the magnetic excitations in this phase are much stronger than those in nondoped LaFeAsO. Even in the paramagnetic phase, the magnetic excitation in ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$ persists. These results corroborate recent studies showing that the electron doping enhances the localized nature in this system.

The effects of aluminum substitution on the structure and electric properties of langasite (La3Ga5SiO14) family crystals are reported. The maximum Al content x of La3Ga5-xAlxSiO14 (LGAS), La3Nb0.5Ga5.5-xAlxO14 (LNGA), and La3Ta0.5Ga5.5-xAlxO14 (LTGA) crystals grown by the conventional Czochralski technique are x = 0.9, 0.2, and 0.5, respectively. Single-crystal X-ray structural analysis reveals that Al atoms are distributed in all cation sites except for the decahedral one occupied by La, rather than favoring the smallest tetrahedral sites. The electrical properties of the Al-substituted crystals were also compared with those of pure ones. Al substitution was found to affect the piezoelectric constant d14, but not d11. The LGAS and LTGA crystals exhibit higher electric resistivity ρ as a result of Al substitution. Among the Al-substituted and pure crystals, the LTGA crystal featured the lowest temperature dependence of d11 and highest ρ, making it the most suitable of the group for high-temperature piezoelectric applications.

The superconducting pyrochlore oxide Cd2Re2O7 ( T c = 1 K ), which is now considered as a candidate of the spin-orbit-coupled metal, shows an inversion-symmetry-breaking structural transition at T s 1 = 200 K . T s 1 decreases with increasing pressure and disappears at around P c = 4.2 GPa , where at least four high-pressure phases with tiny structural distortions are suggested by means of powder X-ray diffraction [Yamaura PRB 2017]. We have carried out Raman scattering experiments to investigate changes in the crystal symmetry under high pressures up to 4.8 GPa. A structural transition at 1.9–3.0 GPa and the recovery of inversion symmetry above P c are observed at 12 K.

5d pyrochlore oxides with all-in-all-out magnetic order are prime candidates for realizing strongly correlated, topological phases of matter. Despite significant effort, a full understanding of all-in-all-out magnetism remains elusive as the associated magnetic excitations have proven difficult to access with conventional techniques. Here we report a Raman spectroscopy study of spin dynamics in the all-in-all-out magnetic state of the 5d pyrochlore Cd2Os2O7. Through a comparison between the two-magnon scattering and spin-wave theory, we confirm the large single ion anisotropy in this material and show that the Dzyaloshinskii–Moriya and exchange interactions play a significant role in the spin-wave dispersions. The Raman data also reveal complex spin–charge–lattice coupling and indicate that the metal–insulator transition in Cd2Os2O7 is Lifshitz-type. Our work establishes Raman scattering as a simple and powerful method for exploring the spin dynamics in 5d pyrochlore magnets., Pyrochlore 5d transition metal oxides are expected to have interesting forms of magnetic order but are hard to study with conventional probes. Here the authors show that Raman scattering can be used to measure magnetic excitations in Cd2Os2O7 and that it exhibits complex spin-charge-lattice coupling.

$^{51}\mathrm{V}$ NMR measurements have been conducted on a single crystal of the quasi-one-dimensional conductor $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{0.33}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ which undergoes a metal-insulator (MI) transition at ${T}_{\mathrm{MI}}\ensuremath{\sim}170$ K. In the metallic phase, we obtain $^{51}\mathrm{V}$ Knight shift and electric field gradient tensors. From the analysis of the $^{51}\mathrm{V}$ Knight shifts, we find that the charge disproportionation appears even in the metallic state and the electronic structure is represented within a model of weakly coupled ladders containing two types of ladders with distinct carrier densities. Based on the $^{51}\mathrm{V}$ nuclear spin-lattice relaxation rate, we discuss the spin dynamics within the one-dimensional electron gas model. From the analysis of several nonmagnetic ${\mathrm{V}}^{5+}$ spectra observed in the insulating phase, we propose a possible charge-order pattern which has a superlattice modulation larger than those in other family members of $\ensuremath{\beta}\text{\ensuremath{-}}{A}_{0.33}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ $(A=\mathrm{Na}$ and Ag). Finally, we discuss the $A$-ion dependence of the electronic structure, the charge disproportionation, and the charge order in $\ensuremath{\beta}\text{\ensuremath{-}}{A}_{0.33}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$.

We report single-crystal 51V NMR studies on volborthite Cu3V2O7(OH)2 2H2O, which is regarded as a quasi-two-dimensional frustrated magnet with competing ferromagnetic and antiferromagnetic interactions. In the 1/3 magnetization plateau above 28 T, the nuclear spin-lattice relaxation rate 1/T1 indicates an excitation gap with a large effective g factor in the range of 4.6-5.9, pointing to magnon bound states. Below 26 T where the gap has closed, the NMR spectra indicate small internal fields with a Gaussian-like distribution, whereas 1/T1 shows a power-law-like temperature dependence in the paramagnetic state, which resembles a slowing down of spin fluctuations associated with magnetic order. We discuss the possibility of an exotic spin state caused by the condensation of magnon bound states below the magnetization plateau., 10 pages, 13 figures

Two-dimensional (2D) electrides are a new concept material in which anionic electrons are confined in the interlayer space between positively charged layers. We have carried out angle-resolved photoemission spectroscopy measurements on ${\mathrm{Y}}_{2}\mathrm{C}$, which is a possible 2D electride, in order to verify the formation of 2D electride states in ${\mathrm{Y}}_{2}\mathrm{C}$. We clearly observe the existence of semimetallic ``electride bands'' near the Fermi level, as predicted by ab initio calculations, which conclusively demonstrates that ${\mathrm{Y}}_{2}\mathrm{C}$ is a quasi-2D electride with electride bands derived from interlayer anionic electrons.

Ferroic materials, such as ferromagnetic or ferroelectric materials, have been utilized as recording media for memory devices. A recent trend for downsizing, however, requires an alternative, because ferroic orders tend to become unstable for miniaturization. The domain wall nanoelectronics is a new developing direction for next-generation devices, in which atomic domain walls, rather than conventional, large domains themselves, are the active elements. Here we show that atomically thin magnetic domain walls generated in the antiferromagnetic insulator Cd2Os2O7 carry unusual ferromagnetic moments perpendicular to the wall as well as electron conductivity: the ferromagnetic moments are easily polarized even by a tiny field of 1 mT at high temperature, while, once cooled down, they are surprisingly robust even in an inverse magnetic field of 7 T. Thus, the magnetic domain walls could serve as a new-type of microscopic, switchable and electrically readable magnetic medium which is potentially important for future applications in the domain wall nanoelectronics.

The $5d$-transition-metal pyrochlore oxide $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$, which was recently suggested to be a prototype of the spin-orbit-coupled metal [Phys. Rev. Lett. 115, 026401 (2015)], exhibits an inversion-symmetry-breaking (ISB) transition at 200 K and a subsequent superconductivity below 1 K at ambient pressure. We study the crystal structure at high pressures up to 5 GPa by means of synchrotron x-ray powder diffraction. A rich structural phase diagram is obtained, which contains at least seven phases, and is almost consistent with the electronic phase diagram determined by previous resistivity measurements. Interestingly, the ISB transition vanishes at $\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ where the enhancement of the upper critical field was observed in resistivity. Moreover, it is shown that the point groups at 8 K, probably kept in the superconducting phases, sequentially transform into piezoelectric, ferroelectric, and centrosymmetric structures on the application of pressure.

A systematic study of the crystal structure of a layered iron oxypnictide LaFeAsO1−xHx as a function of pressure was performed using synchrotron X-ray diffraction. This compound exhibits a unique phase diagram of two superconducting phases and two parent phases. We established that the As–Fe–As angle of the FeAs4 tetrahedron widens on the application of pressure due to the interspace between the layers being nearly infilled by the large La and As atoms. Such rarely observed behaviour in iron pnictides implies that the FeAs4 coordination deviates from the regular tetrahedron in the present systems. This breaks a widely accepted structural guide that the superconductivity favours the regular tetrahedron, albeit the superconducting transition temperature (Tc) increases from 18 K at ambient pressure to 52 K at 6 GPa for x = 0.2. In the phase diagram, the second parent phase at x ~ 0.5 is suppressed by pressure as low as ~1.5 GPa in contrast to the first parent phase at x ~ 0, which is robust against pressure. We suggest that certain spin-fluctuation from the second parent phase is strongly related to high-Tc under pressure.

In quasi-one-dimensional (quasi-1D) systems, because of the finite interchain coupling and fluctuation, the charge density wave (CDW) transition temperature ${T}_{\mathrm{CDW}}$ is much lower than the mean-field-theory predicted ${T}_{\mathrm{MF}}$ and the CDW fluctuation region exists between ${T}_{\mathrm{CDW}}$ and ${T}_{\mathrm{MF}}$. Usually, CDW fluctuation has little effect on the electronic transport properties. Moreover, the CDW state is usually insensitive to magnetic field. The work done here on a PdTeI single crystal with quasi-1D PdTe chains shows that the above statements need to be revisited. It is found that the CDW fluctuation leads to a gradual decrease of carrier concentration before the static long-range CDW order occurs at ${T}_{1}$~110 K. On the other hand, the pinning and sliding of CDW state are observed below ${T}_{2}$~6 K. When such a low ${T}_{2}$ is combined with the existence of multiple quasi-1D bands, PdTeI exhibits exotic crossover behavior from negative to huge positive magnetoresistance under magnetic field and field-induced localization. It indicates that the CDW fluctuation could have a remarkable influence on transport properties of CDW materials. Moreover, there is an interaction between the CDW state and magnetic field.

We investigated the metal-insulator transition (MIT) driven by all-in-all-out (AIAO) antiferromagnetic ordering in the 5d pyrochlore Cd(2)Os(2)O(7) using optical spectroscopy and first-principles calculations. We showed that the temperature evolution in the band-gap edge and free carrier density were consistent with rigid upward (downward) shifts of electron (hole) bands, similar to the case of Lifshitz transitions. The delicate relationship between the band gap and free carrier density provides experimental evidence for the presence of an AIAO metallic phase, a natural consequence of such MITs. The associated spectral weight change at high energy and first-principles calculations further support the origin of the MIT from the band shift near the Fermi level. Our data consistently support that the MIT induced by AIAO ordering in Cd(2)Os(2)O(7) is not close to a Slater type but instead to a Lifshitz type.

The effects of off-stoichiometry and elemental substitution on electronic properties of iron-based ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ are investigated. Resistivity and magnetization are revealed to be quite sensitive to the stoichiometry of Fe atoms, and 10% deficiency at Fe sites reduces the antiferromagnetic transition temperature by 40 K. The antiferromagnetic transition temperature decreases even faster and collapses to zero with hole doping through 10% K substitution at the Ba site, while the antiferromagnetic ordering phase remains with electron doping through 20% Co substitution at the Fe site. Such electron-hole asymmetry is opposite to two-dimensional iron-based superconductors, and can be explained on the basis of both itinerant and localized electronic pictures.

We report the results of the low-temperature specific heat measurements of the single crystalline β -pyrochlore oxide superconductors AOs2O6 (A=K, Rb, and Cs) under high pressure up to 13 GPa. We find that superconducting transition temperature (Tc) monotonically increases for CsOs2O6 and RbOs2O6, while the one for KOs2O6 decreases by applying the pressure. With further increasing the pressure, Tc is suddenly suppressed at the same lattice volume for all compounds, concomitant with the first-order structural phase transition.

Single crystals of a two-dimensional quantum spin system with geometric frustration lead to the observation of a ‘pinwheel’ valence-bond ground state. In this case, the distortion of the ideal kagome lattice structure helps to stabilize the quantum spin state. Determining ground states of correlated electron systems is fundamental to understanding unusual phenomena in condensed-matter physics. A difficulty, however, arises in a geometrically frustrated system in which the incompatibility between the global topology of an underlying lattice and local spin interactions gives rise to macroscopically degenerate ground states1, potentially prompting the emergence of quantum spin states, such as resonating valence bond2,3,4,5,6,7 and valence-bond solid8,9,10,11 (VBS). Although theoretically proposed to exist in a kagome lattice—one of the most highly frustrated lattices in two dimensions being comprised of corner-sharing triangles—such quantum-fluctuation-induced states have not been observed experimentally. Here we report the first realization of the ‘pinwheel’ VBS ground state in the S=1/2 deformed kagome lattice antiferromagnet Rb2Cu3SnF12 (refs 12, 13). In this system, a lattice distortion breaks the translational symmetry of the ideal kagome lattice and stabilizes the VBS state.

The crystal structure of BaFe2S3 is determined up to ~12 GPa by means of x-ray powder diffraction experiments using synchrotron radiation at room temperature. BaFe2S3 is the first superconductor in iron-based ladder compounds, showing the critical temperature at T c = 24 K under 11.6 GPa. The superconductivity emerges on the application of pressure after suppressing an antiferromagnetic Mott insulating state. The atomic positions in the superconducting state remain hitherto unknown experimentally. We found significant changes of structural parameters: shrinkage of the Fe-ladder unit and decreasing height of the S atom position from the Fe plane. Moreover, a reduction of the S–S distance between the ladders was observed under pressure. The pressure dependence of the structural parameters is strikingly different from the prediction of the band calculation.

We report the structure and electric properties of La 3 Ta 0.5 Ga 5.5-x Al x O 14 (LTGAx,x = 0.0 (LTG), 0.3 and 0.5) single crystals, with a Ca 3 Ga 2 Ge 4 O 14 -type structure, suitable for high temperature piezoelectric application. A single-crystal X-ray structure analysis reveals that Al atoms are distributed in all cation sites except for the decahedral site occupied by La, which favors instead the octahedral site. The piezoelectric moduli d 11 kept a constant, whereas [d 14 ] increased 14% with the increasing Al content, x, up to 0.5. The LTGA0.5 crystals showed a low temperature dependence (2% deviation from room temperature to 400°C) of d 11 and a higher electric resistivity p than those of the LTG crystals. The Al substitution is effective for improving the electrical properties of the LTG.

The superconducting pyrochlore oxide Cd2Re2O7 is revisited with a particular emphasis on the sample-quality issue. The compound has drawn attention as the only superconductor (Tc = 1.0 K) that has been found in the family of {\alpha}-pyrochlore oxides since its discovery in 2001. Moreover, it exhibits two characteristic structural transitions from the cubic pyrochlore structure, with the inversion symmetry broken at the first one at 200 K. Recently, it has attracted increasing attention as a candidate spin-orbit coupled metal (SOCM), in which specific Fermi liquid instability is expected to lead to an odd-parity order with spontaneous inversion-symmetry breaking [L. Fu, Phys. Rev. Lett. 115, 026401 (2015)] and parity-mixing superconductivity [V. Kozii and L. Fu: Phys. Rev. Lett. 115 (2015) 207002; Y. Wang et al., Phys. Rev. B 93 (2016) 134512]. We review our previous experimental results in comparison with those of other groups in the light of the theoretical prediction of the SOCM, which we consider meaningful and helpful for future progress in understanding this unique compound., Comment: 15 pages, 11 figures, J. Phys. Soc. Jpn, in press

The crystal structure of the β -pyrochlore oxide superconductor KOs2O6 is re-examined. A single-crystal X-ray diffraction (XRD) analysis at room temperature first revealed that the compound crystallizes in a cubic structure with the centrosymmetric space group F d 3 m , as in conventional pyrochlore oxides. Later, however, Schuck et al. claimed a different non-centrosymmetric F 4 3 m structure based on their single-crystal XRD analysis. To unambiguously determine the true crystal structure of KOs2O6, we carried out high-resolution synchrotron powder X-ray and convergent-beam electron diffraction measurements at room temperature. The space group was determined with high reliability to be centrosymmetric F d 3 m , not F 4 3 m . This confirms the importance of the K atom location in a high-symmetry site, which causes unusually large rattling of the K atom.

New copper complexes with Dmt-Pyra-STF (=dimethylthio-pyrazino-selenathiafulvalene) as the ligand—trans-[CuIICl2(Dmt-Pyra-STF)2] (1) and [CuI2Br2.5(Dmt-Pyra-STF)] (2)—have been synthesized by a vertical diffusion method. In complex 1, the donors form uniform stacking columns along the c axis to have a θ-type donor arrangement, but 1 is an insulator due to the absence of carriers. The magnetic susceptibility of 1 can be fitted to the Curie–Weiss rule (θ = −9.02 K) and suggests that the oxidation state of Cu ions in 1 is 2+. In contrast, complex 2 has supramolecular chains along the c axis, which are formed by CuI and Br−. In addition, this complex is a semiconductor with σrt = 0.16 S cm−1 and Ea = 0.10 eV. The magnetic susceptibility of 2 suggests that the oxidation state of Cu ions in 2 is 1+, namely [CuI2Br2.5(Dmt-Pyra-STF)0.5+]. Both complexes 1 and 2 demonstrate that pyrazino-fused STF derivatives can be coordinated to a copper even though the atomic orbital of Se is so large. Furthermore, we can firstly synthesize complex 2, in which both charge transfer and coordination occur simultaneously.

To elucidate the superconducting gap structure and the influence of rattling motion on quasiparticle dynamics in the superconducting state of KOs 2 O 6 , thermal conductivity and microwave surface impedance were measured at low temperatures. The magnetic field dependence of thermal conductivity at the lowest temperature and the temperature dependence of penetration depth strongly indicate a fully gapped superconductivity in KOs 2 O 6 . The quasiparticle scattering time is strongly enhanced in the superconducting state, indicating a strong electron–electron inelastic scattering in the normal state. These results highlight that KOs 2 O 6 is unique among superconductors, which is characterized by the coexistence of fully gapped superconductivity and strong electron correlations.

FeVO4 (I) single crystals are grown by the flux method using V2O5 as the self-flux. The grown crystals exhibit a characteristic morphology with natural facets. The quality of the crystals is confirmed by X-ray diffraction and EPMA techniques. Magnetic properties are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. Two magnetic phase transitions are observed at ∼13 and ∼20 K. Such unusual magnetic behaviors are suggested to originate from two different Fe ligand environments of octahedral FeO6 and trigonal bipyramidal FeO5 in a six-column doubly bent chain.

Last five years we have mainly investigated phonons in the following cage compounds by Raman scattering: RT 4 X 12 , A 8 Ga 16 Ge 30 , La 3 Pd 20 Ge 6 , RB 6 , KOs 2 O 6 . From this systematic investigation we have clarified the following dynamical properties of the guest ion trapped in the large cages. The vibration of the guest ion is highly anharmonic due to weak atomic interaction between the guest ion and cage atom. This anharmonicity universally decreases the vibration energy of the guest ion with decreasing temperature. The existence of carriers decreases the interaction between the guest atoms and changes the dispersion curve of the guest ion mode to flat, which is the experimental evidence of the weak interaction between the guest atoms. For thermoelectricity, it is found in A 8 Ga 16 Ge 30 that the off-center rattling decreases lattice thermal conductivity.

We report 51V NMR experiments in the metallic phase of the quasi-one-dimensional (1D) conductor β - Sr 0.33 V 2 O 5 . The Knight shift and the quadrupole splitting of all six vanadium sites were determined as a function of the direction of magnetic field perpendicular to the conducting b -axis. Magnetic properties of the three 1D structural units are remarkably heterogeneous. In particular, the V2 ladder unit shows pronounced charge disproportionation among the two (V2a and V2b) sites. At one of these sites, the absolute value of the Knight shift and 1 / ( T 1 T ) increase steeply with decreasing temperature, suggesting development of ferromagnetic correlation.

The effect of magnetic field on S = 3 2 Heisenberg antiferromagnets on pyrochlore lattice was investigated. Chromium spinels CdCr 2 O 4 and HgCr 2 O 4 exhibit first order field-induced transitions to magnetization plateau states with one-half moment of Cr 3 + . One-half moment is reasonably understood as formation of collinear three-up and one-down spin configuration for each tetrahedron, which is a building block of pyrochlore lattice. The effect of lattice distortions to stabilize this spin ordering state makes these magnetization plateau phases appear within wide range of magnetic field. It is quite likely that field-induced collinear three-up and one-down state is universal for antiferromagnetic Heisenberg spin systems on pyrochlore lattice, however, such pure spin system seems to be limited to chromium spinel oxides at present.

Magnetic properties of SrCo 2 V 2 O 8 single crystal are investigated by means of ac magnetic susceptibility, dc magnetization and heat capacity measurements. The results show that SrCo 2 V 2 O 8 possesses two canted antiferromagnetic transitions at ∼5 and ∼3 K, which is different from isostructural BaCo 2 V 2 O 8 with only one antiferromagnetic transition at ∼5 K. We suggest that such different magnetic properties are mainly due to their different structural symmetry.

Substitution of an oxygen anion with a hydrogen anion induced the low dimensionalization of magnetic ordering in a transition metal oxide $\mathrm{S}{\mathrm{r}}_{2}\mathrm{V}{\mathrm{O}}_{4}$. Upon increasing $x$ up to $\ensuremath{\sim}1$ in $\mathrm{S}{\mathrm{r}}_{2}\mathrm{V}{\mathrm{O}}_{4\ensuremath{-}x}{\mathrm{H}}_{x}$, the hydride ions were ordered linearly, and the magnetic susceptibility was simultaneously suppressed. It was found that this suppression was attributed to the formation of a quasi-one-dimensional antiferromagnetic spin chain, maintaining that each of the vanadium cations is two-dimensionally bridged by hydride and oxide ions. Density functional theory calculations demonstrate that the quasi-one-dimensional property is caused by much enhanced anisotropic exchange couplings $({J}_{1}/{J}_{2}\ensuremath{\sim}6)$ originating from the absence of $\ensuremath{\pi}$ bonding between $\mathrm{H}\phantom{\rule{0.28em}{0ex}}1s$ and $\mathrm{V}\phantom{\rule{0.28em}{0ex}}3d$ orbitals. Utilizing a hydride ion that has an ionic radius similar to an oxygen anion and only one energetically available orbital of $1s$ is a different approach to realization of magnetic low dimensionalization in $3d$ early transition metal oxides.

We have synthesized high-quality single crystals of volborthite, a seemingly distorted kagome antiferromagnet, and carried out high-field magnetization measurements up to 74 T and 51V NMR measurements up to 30 T. An extremely wide 1/3 magnetization plateau appears above 28 T and continues over 74 T at 1.4 K, which has not been observed in previous study using polycrystalline samples. NMR spectra reveal an incommensurate order (most likely a spin-density wave order) below 22 T and a simple spin structure in the plateau phase. Moreover, a novel intermediate phase is found between 23 and 26 T, where the magnetization varies linearly with magnetic field and the NMR spectra indicate an inhomogeneous distribution of the internal magnetic field. This sequence of phases in volborthite bear a striking similarity to those of frustrated spin chains with a ferromagnetic nearest-neighbor coupling J1 competing with an antiferromagnetic next-nearest-neighbor coupling J2., 12 pages, 5 figures

We investigate the metal-insulator transition (MIT) of the osmium pyrochlore oxide Cd2Os2O7 through transport and magnetization measurements. The MIT and a magnetic transition to the all-in/all-out (AIAO) order occur simultaneously at 227 K. We propose a mechanism based on a Lifshitz transition induced by the AIAO magnetic order probably via strong spin-orbit couplings in the specific semimetallic band structure. It is suggested, moreover, that two observed puzzles, a finite conductivity near T = 0 and an emergence of weak ferromagnetic moments, are not bulk properties but originate at magnetic domain walls between two kinds of AIAO domains., 11 pages, 8 figures, to be published in APL Materials

The crystal structure and electromagnetic properties of the A-site ordered perovskite manganite, TbBaMn 2 O 6 , have been studied. TbBaMn 2 O 6 has successive phase transitions; the structural transition at T t =450 K, the charge/orbital order transition at T CO =430 K and the antiferromagnetic transitions on Mn ions at T N =210 K and on Tb ions at T N =12 K. At T t , both a- and b -axis expand by 0.7%, and the c -axis shrinks by 1.3% in the low-temperature phase. The magnetic interaction changes from ferromagnetic (the high-temperature triclinic phase) to antiferromagnetic (the low-temperature monoclinic phase) across T t . The electrical resistivity clearly shows anisotropic behaviors at T t ; an increase of resistivity along the c -axis and no change within the ab -plane. The obtained results strongly suggest the dx 2 − y 2 -type orbital order at T t .

Pressure effects on β′-type Pd(dmit)2 anion radical salts, which belong to a strongly correlated electron system with a quasi-triangular lattice of dimerized Pd(dmit)2 units, were studied with X-ray crystal structure analysis and resistivity measurements. The electronic state of this system is governed by various parameters including the effective on-site Coulomb energy on the dimer (Ueff.), the band width (W), the degree of frustration, and the degree of band-filling. X-ray crystal structure analysis under hydrostatic pressure at low temperatures was performed for the Et2Me2P salt The tight-binding band calculations based on structural data indicate that the application of hydrostatic pressure reduces the electron correlation effect and leads the system from the Mott insulating state to the metallic state accompanied by the superconductivity. Further application of hydrostatic pressure induces another non-metallic behavior. This would originate from a partial nesting of the anisotropic Fermi surface associated with a structural transition which removes the glide plane and the two-fold axis. Based on cation dependence in the pressure effect, we propose a possible interplay of the correlation and the frustration, and discuss the uniaxial strain effect for this unique strongly correlated electron system.

Photocarrier injection (PCI) is efficient method of the carrier control for the inorganic thin films, where the photo-excited holes in the wide-gap semiconductor under the UV light irradiation are injected to the film through the interface. We examine the PCI method to various organic films provided on the inorganic wide-gap semiconductor substrate. We prepared organic films deposited on the surface of a Nb-doped TiO2 (SrTiO3) single crystal substrate, possessing a band gap of 3.0 eV (3.2 eV), by using casting or vacuum evaporation. Fabricated organic films were polythiophene (1), Ni phthalocyanine (2), polyaniline (3), C60 (4), and DNA (Herring Sperm) (5) on Nb-doped TiO2 substrate. We determined the carrier injection to the organic films from the across-plane open-circuit voltage measurement under the UV light. The values of the surface density of the injected holes to organic films are estimated at 1.4 × 1013 cm−2 (1), 4.2 × 1012 cm−2 (2), 6.9 × 1012 cm−2 (3), 1.5 × 1011 cm−2 (4), and 8.1 × 1012 cm−2 (5) at the irradiance of 10 mW/cm2. Moreover, we also measure the photovoltages for polythiophene films provided on Nb-doped SrTiO3 substrates with various Nb concentrations.

We have succeeded in growing a high-quality single crystal of UCu 2 Si 2 with the tetragonal structure by the Sn-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization and specific heat. UCu 2 Si 2 is found to order antiferromagnetically below T N =106 K, and follows a successive ferromagnetic ordering at T C =100 K. The magnetic properties are highly anisotropic, reflecting the crystal structure. An easy-axis of magnetization is found to be the [001] direction ( c -axis) both in the antiferromagnetic and ferromagnetic phases, while the [100] direction ( a -axis) corresponds to the hard-axis in magnetization. The magnetization curve in the antiferromagnetic phase indicates a clear metamagnetic transition at a low field of about 1 kOe and changes into a ferromagnetic magnetization curve below T C =100 K. The saturation moment is determined as 1.75 µ B /U at 2 K. The electronic specific heat coefficient is also determined as 20 mJ/(K 2 ·mol).

The crystal structure and electronic and magnetic properties of magnetic molecular conductor (EDTDM) 2 FeBr 4 are investigated. The material undergoes a SDW transition at T MI ∼11 K and an antiferromagnetic transition of Fe 3+ d -spins at T N =3 K. In addition to the appearance of an anomaly in the resistivity around T N , a large magnetoresistance is observed below T N , which diverges at the critical pressure ( P c ∼9.2 kbar) of the MI transition. The perturbation potential from the antiferromagnetic Fe 3+ spin arrangement stabilizes the SDW state, leading to the anomaly in the resistivity. The application of magnetic field reduces the potential of the Fe 3+ spins, which leads to the large negative magnetoresistance. This indicates that the π– d interaction plays an important role in the interplay between the magnetism and the electron transport.

The magnetic structure of the antiferromagnetically ordered state of a quasi-one-dimensional conductor β-Na 0.33 V 2 O 5 below T N =24 K was examined by neutron diffraction. The presence of magnetic Bragg reflections at ( h , k ±1/6,0), h + k =odd below T N indicates that the amplitudes of the magnetic moments at the V sites in three types of V–O chains are modulated with a period of 3 b along the chain direction (∥ b ). On the other hand, it was confirmed that the lattice modulation along the b direction occurs at ∼244 K and at ∼131 K with a period of 2 b and 6 b , respectively. Although the latter structural transition had been attributed to a “charge ordering” transition, which would result in a charge disproportionation in the V 3 d orbitals with a period of 6 b , the 3 b periodicity in the amplitude of magnetic moments strongly suggests that the charge disproportionation occurs with a period of 3 b , not 6 b . The 6 b lattice modulation developing below 131 K is therefore attributed to an instability...

The magnetic and transport properties of a new cubic KSbO 3 -type ruthenate, (Ba 1− x Sr x ) 2 Ru 3 O 9 ( x ≈0.35), have been investigated. The crystal structure has a singular geometry in which ruthenium atoms form an ideal three-dimensional orthogonal dimer lattice. The magnetic susceptibility is Pauli-paramagnetic but exhibits an anomalous temperature dependence reminiscent of a gap-like behavior. The resistivity exhibits a metallic behavior, except for a rise at low temperature.

(n-C3H7)4N[FeIIFeIII(dto)3] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between FeII and FeIII occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P63, a=10.0618(5) A , c=16.0424(7) A , V=1406.54(12) A 3 , Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [FeIIFeIII(dto)3]∞, contracts by 0.1 A at the charge-transfer transition temperature (TCT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 A at TCT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around TCT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C3H7)4N[FeIIFeIII(dto)3] is regarded as spin entropy driven phase transition.