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We investigated the multiscale characters of the crystal structure of the oxynitride perovskite LaTiO2N. While X-ray diffraction results identified the average structure as being centrosymmetric, we detected a signature of unknown structural deformation. By viewing the local structure, we unveiled the formation of a polar structure at the nanoscale.

Nanostructure-induced L10-ordering of twinned single-crystals in CoPt ferromagnetic nanowires on Si/SiO2 substrates is demonstrated, where nanostructure-induced L10-ordering is driven by ultrasmall 10 nm-scale curvature radii of the nanowires.

L10-ordered CoPt with a large coercivity (H c) of 13 kOe was demonstrated on Si/SiO2 substrates by hydrogen annealing. Equiatomic 11.2 nm thick (Co/Pt)4 multilayer thin films were fabricated by electron-beam evaporation and were annealed at 500 °C–900 °C for 10–90 min under an Ar/H2 mixed gas atmosphere. The annealing temperature and time dependences of the crystal structures, magnetic properties, and surface morphologies of the films were systematically analyzed based on the experimental results obtained from grazing incidence X-ray diffraction (GI-XRD), vibrating sample magnetometer, and scanning electron microscope, respectively. Hydrogen annealing effectively promoted the out-of-plane c-axis orientation of L10-ordered CoPt compared to the vacuum annealing according to the GI-XRD patterns. A maximum H c of 13.3 kOe was obtained in L10-ordered CoPt with angular-outlined isolated grains by hydrogen annealing at 800 °C for 60 min, where the c-axis of L10-ordered CoPt was randomly distributed.

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.

We examine electronic and crystal structures of iron-based superconductors

By tackling the challenge of extending transparent oxide semiconductors to Ge based oxides, we have found a not-yet-reported crystal structure, named $\beta$-SrGe$_2$O$_5$, which is composed of edge-sharing GeO$_6$ octahedra interconnected by GeO$_5$ bipyramids. Single crystals were successfully grown by the high-pressure flux method. $\beta$-SrGe$_2$O$_5$ has a band gap of 5.2 eV and a dispersive conduction band with an effective mass as small as 0.34 times the electron rest mass, which originates from the edge-sharing GeO$_6$ octahedra network. Although known compounds with octahedral GeO$_6$ coordination are commonly unstable at atmospheric pressure and elevated temperatures, $\beta$-SrGe$_2$O$_5$ exhibits thermal stability up to 700 $^\circ$C., Comment: 23 pages, 8 figures

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 investigated the multiscale characters of the crystal structure of the oxynitride perovskite LaTiO2N. While X-ray diffraction results identified the average structure as being centrosymmetric, we detected a signature of unknown structural deformation. By viewing the local structure, we unveiled the formation of a polar structure at the nanoscale.

A carrier doping by a hydrogen substitution in LaFeAsO1−xHx is known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO1−xHx has a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitride LnFeAsO1−xHx (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 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.

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.

Formamidinium [FA, HC(NH2)2+] lead iodide and its cation mixture have attracted interest as potentials in applications for efficient solar cells superior to well-known methylammonium lead iodide. W...

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.

We synthesized the 5d1-transition metal oxides AOsO4 (A = K, Rb, Cs) by solid-state reaction, and performed structure determination and magnetic and heat capacity measurements. It was found that they crystallize in a scheelite (A = K and Rb) or a quasi-scheelite structure (A = Cs) comprising of distorted diamond lattices of septivalent Os (d1) ions tetrahedrally coordinated by four oxide ions without local inversion symmetry; hence an antisymmetric spin-orbit coupling is expected in the crystals. The K and Rb compounds have Weiss temperatures of theta = -66 and -18 K, effective magnetic moments of mu_eff = 1.44 and 1.45 mu_B/Os, and antiferromagnetic transition temperatures of T_N = 36.9 and 21.0 K, respectively. In contrast, the Cs compound has theta = 12 K and mu_eff = 0.8 mu_B/Os without magnetic transition above 2 K, instead exhibiting a first-order structural transition at T_s = 152.5 K. The decline of the Os moment from 1.73 mu_B/Os for the simple d1 spin, particularly for Cs, is likely to originate from the antiparallel orbital moment, although the spin-orbit coupling is generally quenched in the low-lying e orbitals.

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.

To investigate chiral properties, it is important to prepare the enantiopure crystals. We grow a large enantiopure crystal of inorganic chiral magnet YbNi3Al9 via the flux method with a temperature...

L12-ordered CoPt3 and L10-ordered CoPt are formed in electron-beam-deposited (Co/Pt)4 multilayer thin films with and without a Ti underlayer, respectively, on Si/SiO2 substrates by rapid thermal annealing. The crystal structures, magnetic properties, and surface morphologies of the films are investigated by grazing incidence X-ray diffraction (GI-XRD), vibrating sample magnetometer (VSM), and scanning electron microscope (SEM), respectively. In the film without a Ti underlayer, L10-ordered CoPt with an isolated round grain structure is confirmed, showing an in-plane coercivity of 2.7 kOe. In contrast, in the film with a Ti underlayer, L12-ordered CoPt3 is confirmed together with Co-rich A1-disordered CoPt, showing an in-plane coercivity of 500 Oe, which exhibits an angular-outlined continuous film structure. The three sets of experimental results from GI-XRD, VSM, and SEM coincide well with each other.

The structural properties of the iron mixed-valence complex ( n-C

The structural properties of the iron mixed-valence complex ( n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato, C2O2S2) have been investigated by single-crystal X-ray diffraction (SCXRD) at low temperatures. ( n-C3H7)4N[FeIIFeIII(dto)3] has two-dimensional (2D) honeycomb layers consisting of alternating FeII and FeIII arrays bonded by bis-bidentate dithiooxalato ligands. Upon cooling, a superlattice structure with q = (1/3, 1/3, 0) was observed below 260 K, which corresponds to an order-disorder transition of the ( n-C3H7)4N+ ions between the honeycomb layers. The charge-transfer phase transition (CTPT) occurs at TC↑1/2 ∼ 120 K and TC↓1/2 ∼ 90 K upon heating and cooling, respectively, with an electron transfer between the FeII and FeIII ions, accompanied by a spin-state change, FeII ( S = 2; HS)-O2C2S2-FeIII ( S = 1/2; LS) ↔ FeIII ( S = 5/2; HS)-O2C2S2-FeII ( S = 0; LS). During the CTPT, the intersheet [FeIIFeIII(dto)3] distance decreased monotonously upon cooling, and an abrupt structural contraction was observed in the hexagonal 2D network. The volume contraction during the CTPT was quite small (∼0.7%), and differences in the structural distortions at the FeS6 and FeO6 sites were not found in the vicinity of the CTPT. We also calculated the orbital energies and the occupied spin states for the [Fe(O2C2S2)3] and [Fe(S2C2O2)3] octahedra in the vicinity of the CTPT by density functional theory (DFT). Because the local symmetry around the two coordinating iron ions is already lowered to trigonal symmetry, the CTPT did not cause any further deformation. This symmetry invariance resulted in an absence of orbital contributions to the total entropy change (Δ S) in the CTPT, which is in agreement with the previous heat capacity measurements. [Nakamoto, T; Miyazaki, Y; Itoi, M; Ono, Y; Kojima, N; Sorai, M. Heat Capacity of the Mixed-Valence Complex {[( n-C3H7)4N][FeIIFeIII(dto)3]}∞, Phase Transition because of Electron Transfer, and a Change in Spin-State of the Whole System. Angew. Chem., Int. Ed. 2001, 40, 4716-4719.].

This paper focuses on effects of isovalent La substitution on the crystal structure and dielectric properties of ferroelectric $\mathrm{B}{\mathrm{i}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$. Polycrystalline samples of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ are synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses with a composition range of $0\ensuremath{\le}x\ensuremath{\le}0.1$. The crystal structure changes from monoclinic to tetragonal with increasing La-substitution rate $x$ at room temperature. This structural variation stems from the change in orientation of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra that form one-dimensional chains when they are in the ordered configuration, thus suggesting that lone-pair electrons play an important role in sustaining one-dimensional chains of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra. Synchronizing with the disordering of $\mathrm{Si}{\mathrm{O}}_{4}$ chains, ferroelectric phase transition temperature of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ sharply decreases as $x$ increases, and ferroelectricity finally vanishes at around $x=0.03$. The present results demonstrate that lone-pair electrons of Bi play an important role in the ferroelectricity of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ through propping the ordered structure of one-dimensional $\mathrm{Si}{\mathrm{O}}_{4}$ chains with stereochemical activity. Furthermore, an additional phase transition has been first discovered in the low-temperature region of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ with $x\ensuremath{\le}0.01$, where the ordered one-dimensional $\mathrm{Si}{\mathrm{O}}_{4}$ chains remain.

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

The inelastic response of AV2Al20 (with A = Al, Ga and Y) was probed by high-resolution inelastic neutron scattering experiments and density functional theory (DFT) based lattice dynamics calculations (LDC). Features characteristic of the dynamics of Al, Ga and Y are established experimentally in the low-energy range of the compounds. In the stereotype 'Einstein-solid' compound AlV2Al20 we identify a unique spectral density extending up to 10 meV at 1.6 K. Its dominating feature is a peak centred at 2 meV at the base temperature. A very similar spectral distribution is established in GaV2Al20 albeit the strong peak is located at 1 meV at 1.6 K. In YV2Al20 signals characteristic of Y dynamics are located above 8 meV. The spectral distributions are reproduced by the DFT-based LDC and identified as a set of phonons. The response to temperature changes between 1.6 and ∼300 K is studied experimentally and the exceptionally vivid renormalization of the A characteristic modes in AlV2Al20 and GaV2Al20 is quantified by following the energy of the strong peak. At about 300 K it is shifted to higher energies by 300% for A = Al and 450% for A = Ga. The dynamics of A = Y in YV2Al20 show a minor temperature effect. This holds in general for modes located above 10 meV in any of the compounds. They are associated with vibrations of the V2Al20 matrix. Atomic potentials derived through DFT calculations indicate the propensity of A = Al and Ga to a strong positive energy shift upon temperature increase by a high quartic component. The effect of the strong phonon renormalization on thermodynamic observables is computed on grounds of the LDC results. It is shown that through the hybridization of A = Al and Ga with the V2Al20 dynamics the matrix vibrations in the low-energy range follow this renormalization.

A novel charge-disproportionation state with δ = 0.75 was observed in an electron-donor (D)/-acceptor (A) D(δ+)2A(2δ-) layered framework by chemically tuning the electron-donating affinity of D at the boundary between D(0.5+)2A(-) and D(+)2A(2-) phases, which was pressure-sensitive due to the formation of the D(+)2A(2-) oxidation state.

A thin film superconductor $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ is the first electron-doped compound of $\mathrm{BaF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ obtained by the substitution of Ba with La within a blocking layer. By contrast, in conventional electron-doped $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$, the dopant (Co) is inserted into the conduction layer. The different shapes, or geometries, of $\mathrm{FeA}{\mathrm{s}}_{4}$ are expected in the two compounds above. However, the structure of $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ is investigated here because of being a thin film. To clarify the effect of the geometry, we make use of an up-to-date method to determine the atomic positions of the $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ thin film using synchrotron x-ray diffraction. We established that the $\mathrm{FeA}{\mathrm{s}}_{4}$ geometry in $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ indicates a systematic variation upon doping, which is opposite to that in $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$. However, the superconducting transition temperatures nearly coincide with each other for the same amount of dopings. The present result contrasts with a suggestion that the $\mathrm{FeA}{\mathrm{s}}_{4}$ geometry strongly influences the superconductivity in iron pnictides. Hence, we propose that the carrier density is the more important parameter than the $\mathrm{FeA}{\mathrm{s}}_{4}$ geometry and that the electronic correlation plays a significant role in the superconductivity for the electron-doped $\mathrm{BaF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$.

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.

A conducting molecular crystal TPP[MnIII(Pc)(CN)2]2 (MnIII: d4, S = 1, TPP = tetraphenylphosphonium and Pc = phthalocyanine) was fabricated. In its crystal structure, the [MnIII(Pc)(CN)2] units formed a one-dimensional regular chain along the c-axis with an overlap integral value of 8.6 × 10-3. TPP[MnIII(Pc)(CN)2]2 showed a semiconducting behaviour that also has been observed for isostructural TPP[CoIII(Pc)(CN)2]2 (CoIII: d6, S = 0) and TPP[FeIII(Pc)(CN)2]2 (FeIII: d5, S = 1/2) whose ground states are charge-ordered states. In spite of the local magnetic moment of the MnIII ion (S = 1) at the centre of the Pc ligand, TPP[MnIII(Pc)(CN)2]2 exhibited an almost isotropic and small negative magnetoresistance (MR) effect (the MR ratio was -8.7% under 8 T at 10.7 K), contrarily to the anisotropic giant negative MR effect of TPP[FeIII(Pc)(CN)2]2. The isotropy was found to be due to a (dxz)1(dyz)1 electronic configuration, and the smaller MR effect was explained by a weaker antiferromagnetic interaction between MnIII ions than that between FeIII ions, as suggested by a Weiss temperature Θ of -3.1 K (|Jdd|/kB = 1.2 K).

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.

Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without considerable anion vacancy were synthesized by high-pressure solid state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x > 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion ordering is strongly correlated with the bonding interaction between vanadium and the ligands., 19 pages, 5 figures

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.