Akiyuki Matsushita

A novel hydrogen-evolving photocatalyst InVO4 active under visible light irradiation

Abstract InVO4, with band gap of about 2.0 eV, was found to be a new visible light responding photocatalyst for water decomposition. The photocatalyst showed activity to visible light in a wide wavelength range up to 600 nm. Although the native photocatalyst could evolve H2 from pure water under visible light irradiation (λ>420 nm ) , the photocatalytic activity increases significantly by loading NiO as a co-catalyst. Correlation of the photocatalytic properties with crystal and electronic structure of the compound is discussed in connection with the recently reported 4d and 5d transition metal photocatalysts InNbO4 and InTaO4.

Anisotropy of superconductivity from MgB2 single crystals

Magnesium diboride (MgB2) single crystals, with a maximum size of 0.5×0.5×0.02 mm3, were grown by the vapor transport method in a sealed molybdenum crucible. A superconducting transition with the onset temperature of 38.6 K was confirmed by both transport and magnetization measurements. The upper critical field anisotropy ratio, Hc2∥ab(0)/Hc2∥c(0), was estimated to be 2.6 from the magnetic field-temperature phase diagram for MgB2 single crystals.The discovery of superconductor in magnesium diboride MgB2 with high Tc (39 K) has raised some challenging issues; whether this new superconductor resembles a high temperature cuprate superconductor(HTS) or a low temperature metallic superconductor; which superconducting mechanism, a phonon- mediated BCS or a hole superconducting mechanism or other new exotic mechanism may account for this superconductivity; and how about its future for applications. In order to clarify the above questions, experiments using the single crystal sample are urgently required. Here we have first succeeded in obtaining the single crystal of this new MgB2 superconductivity, and performed its electrical resistance and magnetization measurements. Their experiments show that the electronic and magnetic properties depend on the crystallographic direction. Our results indicate that the single crystal MgB2 superconductor shows anisotropic superconducting properties and thus can provide scientific basis for the research of its superconducting mechanism and its applications.

Definitive experimental evidence for two-band superconductivity in MgB2.

The superconducting-gap of MgB2 has been studied by high-resolution angle-resolved photoemission spectroscopy. The results show that superconducting gaps with values of 5.5 and 2.2 meV open on the sigma band and the pi band, respectively, but both the gaps close at the bulk transition temperature, providing a definitive experimental evidence for the two-band superconductivity with strong interband pairing interaction in MgB2. The experiments validate the role of k-dependent electron-phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature of MgB2.

A novel series of water splitting photocatalysts NiM2O6 (M=Nb,Ta) active under visible light

Abstract A novel series of water splitting solid photocatalysts NiM 2 O 6 ( M = Nb , Ta ) were synthesized by the solid-state reaction method. The NiNb2O6 photocatalyst crystallizes in the Columbite-type structure, orthorhombic with space group pbcn, while NiTa2O6 belongs to the tri-Rutil-type structure, tetragonal system with space group P42/mnm. Both photocatalysts showed high activity to evolve H2 from an aqueous methanol solution under ultra-violet light irradiation. The new photocatalysts can also split water to generate H2 from pure water under visible light irradiation (λ>420 nm ) without any co-catalyst. The band gaps of NiNb2O6 and NiTa2O6 were estimated to be 2.2 and 2.3 eV , respectively. The difference in the band gaps of the photocatalysts is supposed to come from their different conduction band levels formed by Nb 4d in NbO6 and Ta 5d in TaO6.

Ordered magnetic and quadrupolar states under hydrostatic pressure in orthorhombic PrCu2

We report magnetic susceptibility and electrical resistivity measurements on single-crystalline PrCu2 under hydrostatic pressure, up to 2 GPa, which pressure range covers the pressure-induced Van Vleck paramagnet-to-antiferromagnet transition at 1.2 GPa. The measured anisotropy in the susceptibility shows that in the pressure-induced magnetic state the ordered 4f-moments lie in the ac-plane. We propose that remarkable pressure effects on the susceptibility and resistivity are due to changes in the quadrupolar state of O22 and/or O20 under pressure. We present a simple analysis in terms of the singlet-singlet model.

Revealing properties of single-walled carbon nanotubes under high pressure

It was found by the x-ray diffraction experiment under hydrostatic pressure that the carbon nanotubes are compressed easily with a high volume compressibility of 0.024 GPa−1. The single-walled carbon nanotubes are polygonized when they form bundles of hexagonal close-packed structure and the inter-tubular gap is smaller than the equilibrium spacing of graphite. Under high pressure, further polygonization occurs to accommodate the extra amount of volume reduction. The ratio of the short and the long diagonals in the hexagonalized cross section is found to have changed from 0.991 at zero pressure to 0.982 at 1.5 GPa pressure, when the Bragg reflection from the nanotube lattice diminished. Accompanying polygonization, a discontinuous change in electrical resistivity was observed at 1.5 GPa pressure, suggesting a phase transition had occurred.

Superconducting and Transport Properties of B-Y-Cu-O Compounds –Orthorhombic and Tetragonal Phases

The superconducting properties and crystal structures of the Ba-Y-Cu-O compounds have been studied. Annealing and quenching experiments have been carried out to study the phase transformation between the high temperature tetragonal phase (oxygen deficient) and low temperature orthorhombic phase (oxygen rich). The former phase is semiconductor-like down to 4.2 K, whereas the latter is metallic and superconducting with Tc as high as 94 K. The conspicuous difference in the transport and superconducting properties between the two phases has been attributed to the loss or formation of oxygen and Cu atom chain in the crystal structure.

Synthesis and superconducting properties of (Y1−xPrx)Ba2Cu4O8 and (Y1−xPrx)2Ba4Cu7O15−y compounds

Abstract Both (Y1−xPrx)Ba2Cu4O8 (hereafter abbreviated as Y/Pr124) and (Y1−xPrx)2Ba4Cu7O15−y (Y/Pr247) compounds were successfully synthesized over the whole concentration range 0≤x≤1 by means of either hot isostatic pressing (HIP) technique or high pressure oxygen gas technique. We revealed that the superconducting transition temperature disappears at xc=0.8 in both systems in contrast to xc=0.5 for the (Y1−xPrx)Ba2Cu3O7 (Y/Pr123) system reported in the literature and concluded from this that the suppression of superconductivity due to the Pr-doping is weaker in both 124 and 247 systems than that in the 123 system. Furthermore, a combination of the magnetic susceptibility data in the normal state with the Pr–O distance derived from the neutron diffraction experiments led us to conclude that the Pr ion is in the trivalent state for all samples in both 124 and 247 systems. An increase in the critical concentration xc in the Y/Pr124 and Y/Pr247 systems relative to that for the Y/Pr123 system is attributed to an increase in the hybridization of the Pr-4f and O-2p orbitals.

Low Temperature Specific Heat and Electrical Resistivity in Orthorhombic YBa2Cu3O6.8 and Tetragonal YBa2Cu3O6.0

The low temperature specific heat and electrical resistivity of YBa2Cu3O6.8 and YBa2Cu3O6.0 have been measured. The specific heat data were taken at a temperature range from 2 K to 40 K. Abnormal upturns at the lower temperature end and a large electron contribution were observed in the C/T vs T2 curve. The resistivity ρ was measured from 20 K to 270 K for YBa2Cu3O6.0. A strong current density dependence was observed. Log ρ measured at low current density was found to show T-1/3 dependence below 170 K.

Structure of High-Tc Superconductor YBa2Cu3O6.6 at Low Temperatures

A single-phase YBa2Cu3O6.6 shows a sharp superconductive transition at 94 K. The crystal structure of this compound has been determined by the Rietveld profile analysis of neutron powder diffraction data measured at 10 K and 120 K. The space group Pmmm is confirmed, and the positions and occupations of oxygen atoms are established. No phase transitions or other structural anomalies were observed in relation to the superconductive transition.