Chikako Moriyoshi

Giant strain in lead-free (Bi0.5Na0.5)TiO3-based single crystals

A giant electric-field-induced strain of 0.87% is reported for tetragonal (Bi0.5Na0.5)TiO3-(Bi0.5K0.5)TiO3-BaTiO3 single crystals along the [100]cubic direction, which is six times as large as that of Pb(Zr0.52Ti0.48)O3 ceramics. In situ x-ray diffraction measurements and transmission electron microscope observations show that the giant strain mainly originates from switching of nanosized 90° domains. Strain measurements indicate that the strain caused by the 90° domain switching is reversible for both unipolar and bipolar electric-field applications. The reversibility of the 90° domain switching can be explained by the interaction between the spontaneous polarization and the defect dipole composed of A-site vacancy and oxygen vacancy.A giant electric-field-induced strain of 0.87% is reported for tetragonal (Bi0.5Na0.5)TiO3-(Bi0.5K0.5)TiO3-BaTiO3 single crystals along the [100]cubic direction, which is six times as large as that of Pb(Zr0.52Ti0.48)O3 ceramics. In situ x-ray diffraction measurements and transmission electron microscope observations show that the giant strain mainly originates from switching of nanosized 90° domains. Strain measurements indicate that the strain caused by the 90° domain switching is reversible for both unipolar and bipolar electric-field applications. The reversibility of the 90° domain switching can be explained by the interaction between the spontaneous polarization and the defect dipole composed of A-site vacancy and oxygen vacancy.

Piezoelectric properties of high Curie temperature barium titanate-bismuth perovskite-type oxide system ceramics

Barium titanate (BaTiO3, BT)—bismuth magnesium titanium oxide [Bi(Mg0.5Ti0.5)O3, BMT] system ceramics were prepared in an ambient atmosphere in order to increase the Curie temperature (Tc) of BT above 132 °C. A single perovskite phase was observed for BT–BMT ceramics with BMT compositions less than 50 mol %, and their relative densities were greater than 94%. Synchrotron measured x-ray diffraction patterns revealed that all the cations in the ceramics were homogeneously distributed. The temperature dependence of the dielectric properties revealed that the BT–BMT system ceramics exhibited relaxorlike characteristics with a dielectric maximum temperature as high as 360 °C for the 0.5BT–0.5BMT ceramic. The apparent piezoelectric constant (d∗) was 60 pC/N for the 0.4BT–0.6BMT ceramic. Based upon these results, the BT–BMT system shows potential as a new type of lead-free material for high Tc piezoelectric applications.

Possible Long-Range Order with Singlet Ground State in CeRu2Al10

We investigate the thermal and transport properties of Ce x La 1- x Ru 2 Al 10 to clarify the origin of the recently discovered mysterious phase below T 0 =27 K in CeRu 2 Al 10 where a large magnetic entropy is released, however, the existence of an internal magnetic field is ruled out by 27 Al-NQR measurement. We find that T 0 decreases with decreasing x and disappears at x ∼0.45. T 0 of CeRu 2 Al 10 is suppressed down to 26 K under H =14.5 T along the a -axis. These results clearly indicate that the transition has a magnetic origin and is ascribed to the interaction between Ce ions. Considering the results of specific heat, magnetic susceptibility, thermal expansion, and electrical resistivity and also 27 Al-NQR, we propose that the transition originates from the singlet pair formation between Ce ions. Although its properties in a Ce dilute region is basically understood by the impurity Kondo effect, CeRu 2 Al 10 shows a Kondo-semiconductor-like behavior. The phase transition at T 0 may be characterized...

Experimental Verification of PbBi2Te4 as a 3D Topological Insulator

The experimental evidence is presented of the topological insulator state in PbBi2Te4. A single surface Dirac cone is observed by angle-resolved photoemission spectroscopy with synchrotron radiation. Topological invariants Z2 are calculated from the ab initio band structure to be 1;(111). The observed two-dimensional isoenergy contours in the bulk energy gap are found to be the largest among the known three-dimensional topological insulators. This opens a pathway to achieving a sufficiently large spin current density in future spintronic devices.

High-oxygen-pressure crystal growth of ferroelectric Bi4Ti3O12 single crystals

We have investigated the effects of high-oxygen-pressure crystal growth of ferroelectric Bi4Ti3O12 on the polarization properties along the a(b) axis. Domain observations by piezoresponse force microscope demonstrate that a small remanent polarization (Pr) for the crystals grown at 0.02MPa is attributed to the clamping of 90° domain walls by oxygen vacancies. The vacancy formation of Bi and O during crystal growth at high temperatures is suppressed at a higher oxygen pressure, leading to a larger Pr of 47μC∕cm2 for the crystals grown at 1MPa oxygen. High-oxygen-pressure sintering is proposed to be effective for obtaining Bi4Ti3O12-based devices with enhanced polarization properties.

Extremely High Resolution Single Crystal Diffractometory for Orbital Resolution using High Energy Synchrotron Radiation at SPring‐8

The investigation of accurate structure at charge density level will not only understand the function mechanism of physical property but also lead to design new functional materials. We have successfully installed large cylindrical camera at the BL02B1/SPring‐8. In conceptual design, the image plate (IP) was selected as the detector, because such IP will not only detect wide range in one shot but also yield reliable data. In commissioning, the performance of this camera demonstrated to be suitable for the direct observation of d‐electron system.

Existence of Fine Structure inside Spin Gap in CeRu2Al10

We investigate the magnetic field effect on the spin gap state in CeRu 2 Al 10 by measuring the magnetization and electrical resistivity. We found that the magnetization curve for the magnetic field H ∥ c shows a metamagnetic-like anomaly at H * ∼4 T below T 0 =27 K, but no anomaly for H ∥ a or H ∥ b . A shoulder of the electrical resistivity at T s ∼5 K for I ∥ c is suppressed by applying a longitudinal magnetic field above 5 T. Many anomalies are also found in the magnetoresistance for H ∥ c below ∼5 K. The obtained magnetic phase diagram consists of at least two or three phases below T 0 . These results strongly indicate the existence of a fine structure on a low-energy side in a spin gap state with an excitation energy of 8 meV, which has recently been observed in inelastic neutron scattering experiments.

In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure.

BiCh2-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated. In this study, we investigate the relationship between crystal structure and superconducting properties of the BiCh2-based superconductor family, specifically, optimally doped Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)2. We use powder synchrotron X-ray diffraction to determine the crystal structures. We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle. Furthermore, we show that the superconducting transition temperature for all REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

Direct observation of oxygen stabilization in layered ferroelectric Bi3.25La0.75Ti3O12

Electron charge density distributions in layered ferroelectrics Bi4Ti3O12 (BiT) and Bi3.25La0.75Ti3O12 (BLT) are investigated by analyzing high-energy synchrotron-radiation powder diffraction data using the maximum entropy method/Rietveld method. BiT shows that chemical bonding resulting from orbital hybridization is established between Bi–O in the perovskite layer only along the a axis, whereas BLT exhibits isotropic chemical bonding of Bi∕La–O with a high electron density both along the a and b axes. High endurance to polarization fatigue reported for BLT films is suggested to originate from the stabilization of oxygen in the perovskite layer due to the isotropic chemical bonding of Bi∕La–O.

Enhanced piezoelectric response of BaTiO3–KNbO3 composites

The piezoelectric response of solvothermally synthesized BaTiO3 (BT)–KNbO3 (KN) composites (the nominal BT/KN ratio was 1) with distinct interfaces was investigated. The x-ray diffraction pattern showed two distinct peaks began to merge into a singular broad peak at a two-theta position between (200) and (002) tetragonal-related peaks of BT. The transmission electron microscopy observation showed a heteroepitaxial interface region between BT single-crystal particles and deposited KN crystals. The large-field piezoelectric constant was 136 pC/N, which was three times larger than that of a sintered 0.5BT–0.5KN composite. The enhanced piezoelectric response was attributed to the strained epitaxial interface region.