Takafumi Ogawa

The electric field induced ferroelectric phase transition of AgNbO3

Coexistence of two phases of AgNbO3 is shown to explain the experimentally observed polarization–electric field hysteresis loop better than either phase in isolation, based on detailed first-principles calculations of the structural changes and stabilities of different phases of this compound. Calculations confirm a ferroelectric phase transition, whereby the symmetry of the AgNbO3 crystal switches from antiferroelectric Pbcm to ferroelectric Pmc21, under an electric field of 9 MV/cm. The calculated spontaneous polarization (0.61 C/m2) under this field compares well with the experimental value of 0.52 C/m2. After transforming, the structure remains in the ferroelectric state even after the electric field is removed, despite the structure being energetically metastable. As the energy difference between the antiferroelectric and ferroelectric phases is only +0.5 meV/f.u. and the potential energy barrier between them (∼40 meV/f.u.) is comparable to thermal fluctuation energies, it is possible for these two p...

Ferroelectricity in wurtzite structure simple chalcogenide

The possibility of the new class ferroelectric materials of wurtzite structure simple chalcogenide was discussed using modern first-principles calculation technique. Ferroelectricity in the wurtzite structure (P63mc) can be understood by structure distortion from centrosymmetric P63/mmc by relative displacement of cation against anion along c-axis. Calculated potential surface of these compounds shows typical double well between two polar variants. The potential barriers for the ferroelectric polarization switching were estimated to be 0.25 eV/f.u. for ZnO. It is slightly higher energy to the common perovskite ferroelectric compound PbTiO3. Epitaxial tensile strain on the ab-plane (0001) is effective to lower the potential barrier. The potential barrier decreased from 0.25 to 0.15 eV/f.u. by 5% ab-plane expansion in wurtzite structure ZnO. Epitaxial ZnO thin film with donor type defect reduction should be a possible candidate to confirm this ferroelectricity in wurtzite structure simple chalcogenide.

Quadrupole Effects of Vacancy Orbital in Boron-Doped Silicon

We have investigated low-temperature properties at a longitudinal elastic constant C L[111] of a boron-doped silicon crystal grown by a floating zone (FZ) method. The softening of C L[111] depending on magnetic fields indicates that a magnetic charge state V + of a vacancy orbital accommodating three electrons is stable. Appreciable anisotropy in the softening of C L[111] at magnetic fields along the [111] and [1\bar10] axes up to 10 T is described in terms of a quadrupole susceptibility for the vacancy orbital consisting of a Γ 8 quartet ground state and Γ 6 doublet excited state due to the spin–orbit interaction.

Density-Functional Analysis on Vacancy Orbital and its Elastic Response of Silicon

Recent experiments on ultrasonic measurements of non-doped and boron-doped silicon indicate that vacancies in crystalline silicon can be detected through the elastic softening at low temperature. This is attributed to enhanced response of electronic quadrupole localized at the vacancies to the elastic strain. In the present work, the electronic quadrupole moment of the vacancy orbital in silicon and their strain susceptibility are evaluated quantitatively by using the density-functional method. We show the orbital of gap state is localized around vacancy but extended over several neighbors. The effect of applied magnetic field on the vacancy orbital and its multipole structures are also investigated. We find that the results obtained from these calculations are consistent with the ultrasonic experiments.

Control of HAp Formation and Osteoconductivity on Nitrogen-Doped TiO 2 Scale Formed by Oxynitridation of Ti

The surface potential of the TiO2 scale formed on Ti was controlled by varying the Ti heat treatment conditions in a N2 atmosphere containing a trace amount of O2. The surface potential was attributed to the effective charge of nitrogen-related defects in the TiO2, where the positive and negative surface charges were associated with (N2)0+2 and (NO)0-1, respectively. The latter defects were formed only during the early stages of the heat treatment, and with increasing treatment time, this was followed by the formation of voids containing N2 in the scale rather than the disappearance of the defects in the TiO2 crystal lattice, resulting in zero surface charge. Hydroxyapatite (HAp) formation and osteoconductivity were enhanced on nitrogen-doped TiO2 scale with either a positive or negative surface potential. In contrast, for the unchanged TiO2 scale, no HAp formation was observed and the osteoconductivity was low.

Grain Boundary Engineering of Alumina Ceramics

Oxygen permeability through alumina wafers was evaluated at high temperatures up to 1923 K to elucidate the mass-transfer mechanisms of polycrystalline alumina and serve as a model for protective alumina film formed on heat-resistant alloys. Oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen from the higher oxygen partial pressure (PO2) surface side to the lower PO2 surface side, along with the simultaneous GB diffusion of aluminum in the opposite direction to maintain the Gibbs–Duhem relationship. Oxygen GB diffusion coefficients in the vicinity of the PO2(hi) surface were lower than those of oxygen GB self-diffusion without an oxygen potential gradient (dµO). When dµO was applied to the wafer, the oxygen and aluminum fluxes at the outflow side of the wafer were significantly larger than those at the inflow side. Ln (Y and Lu) and Hf segregation at the GBs selectively reduced the diffusivity of oxygen and aluminum, respectively. Thus, the mesoscopic arrangements of segregating dopants, which were selected by taking into consideration the behavior of the diffusion species and the role of dopants, enabled the alumina film to have enhanced oxygen shielding capability and structural stability at high temperatures. Furthermore, the GB diffusion data derived from the oxygen permeation experiments were compared to those for alumina scale formed by the so-called two-stage oxidation of alumina-forming alloys.

MC3T3-E1 Cellular Response and Protein Detection on Surface Potential-Controlled TiO2 Scale in Serum-Containing Medium

MC3T3-E1 cell differentiation and related surface potentials of rutile-type TiO2 scales formed on Ti are controlled by varying the Ti heat treatment conditions in a N2 atmosphere containing a trace amount of O2. The zeta potentials of the samples heated at 873 and 973 K for 1 h show large negative and positive values, respectively, while cell differentiation on the surface is enhanced in both cases (14 days incubation). In the case of untreated Ti, the cell differentiation diminishes and the zeta potential becomes more neutral. Protein detection by an immunogold-labeling technique and Ca and P detection by time-of-flight secondary ion mass spectrometry reveal that Ca and P, rather than an adhesive protein such as fibronectin, predominantly adsorbed on the scales formed in 1 h at 873 and 973 K, respectively. In the case of untreated Ti, both fibronectin and a non-adhesive protein such as albumin adsorbed, but no Ca and P were detected. The present findings illuminate the relationship between charged surfaces and MC3T3-E1 cellular response.

Mechanism of polarization switching in wurtzite-structured zinc oxide thin films

The properties of a potentially new class of ferroelectric materials based on wurtzite-structured ZnO thin films are examined using the first-principles calculations. Theoretical P-E hysteresis loops were calculated using the fixed-D method for both unstrained and (biaxially) strained single crystals. Ferroelectric polarization switching in ZnO (S.G. P63mc) is shown to occur via an intermediate non-polar structure with centrosymmetric P63/mmc symmetry by displacement of cations relative to anions in the long-axis direction. The calculated coercive electric field (Ec) for polarization switching was estimated to be 7.2 MV/cm for defect-free monocrystalline ZnO. During switching, the short- and long-axis lattice parameters expand and contract, respectively. The large structural distortion required for switching may explain why ferroelectricity in this compound has not been reported experimentally for pure ZnO. Applying an epitaxial tensile strain parallel to the basal plane is shown to be effective in loweri...

Structure and thermodynamics of spherical Yukawa and Coulomb clusters

Structures in the ground state and thermodynamics related to melting of spherical Yukawa and Coulomb clusters confined in external fields of less than 103 particles are discussed on the basis of molecular dynamics and Monte Carlo simulations and theoretical methods. At low temperatures, particles form concentric shell structures which are expressed by simple formulas and insensitive to the strength of screening. Structures are reproduced by the shell model when the intrashell cohesive energy is properly taken into account. The specific heat of clusters of relatively small sizes 100 ⩽ Nd ⩽ 400 has two peaks as a function of temperature. By analyses of diffusions, it is shown that the peaks at lower and higher temparatures corresponds to the loss of ordering in the azimuthal and radial (and therefore total) ordering, respectively. Magnitudes of fluctuations in the outermost shell are consistent with the conventional Lindemann criterion.

Structure of spherical Yukawa clusters

Structure of clusters of Yukawa particles is analysed by simulations and theoretical approaches in an isotropic environment which can be realized under microgravity or by active cancellation of the effect of gravity. Yukawa particles model dust particles in dusty plasmas and such an environment is suitable to observe their inherent properties. At low temperatures, clusters are composed of spherical shells and, when scaled by the mean distance, the structure seems to depend almost only on the system size or the number of particles. The positions and populations of shells are given by simple interpolation formulae. It is shown that shells have an approximately equal spacing close to that of triangular lattice planes in the bulk close-packed structures. When the cohesive energy in each shell is properly taken into account, the shell model reproduces these structures of spherical Yukawa clusters to a good accuracy.