Motoharu Imai

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.

Investigation of the energy band structure of orthorhombic BaSi2 by optical and electrical measurements and theoretical calculations

Optical and electrical properties of polycrystalline orthorhombic BaSi2 prepared by arc melting in Ar atmosphere were investigated. The optical absorption spectra measured at room temperature showed that indirect and direct absorption edges were 1.15 and 1.25 eV, respectively. The activation energy estimated from temperature dependence of the resistivity was 1.10 eV. These results agreed well with a calculated band structure of the orthorhombic BaSi2 by first principles calculation using density functional theory.

Superconductivity of Ca(Al0.5,Si0.5)2, a ternary silicide with the AlB2-type structure

A ternary silicide Ca(Al0.5,Si0.5)2 was prepared by Ar arc melting. Powder x-ray diffraction measurement indicates that it has the AlB2-type structure with lattice constants of a=4.1905(5) and c=4.3992(8) A, where Si and Al atoms are arranged in the disordered honeycomb lattice and Ca atoms are intercalated between them. The silicide is isostructural with a superconductor MgB2. Electrical resistivity and dc magnetization measurements revealed that it is a superconductor with a TC of 7.7 K.

Electrical resistivity of metastable phases of BaSi2 synthesized under high pressure and high temperature

Abstract Two metastable phases of BaSi2, cubic BaSi2 and trigonal BaSi2, were synthesized under high-pressure and high-temperature conditions. The electrical resistivity and the dominant carrier of these two phases and the normal phase, orthorhombic BaSi2, were measured at atmospheric pressure and temperature from 80 to 290 K. The electrical properties depend on the crystal structure. The orthorhombic BaSi2 is an n-type semiconductor as previously reported. It is found that the cubic BaSi2 is an n-type semiconductor and that the trigonal BaSi2 is a hole metal. The change in the electrical properties with the structural change is discussed in terms of the change in the interatomic distances.

Fermi Surface and Mass Enhancement in KFe2As2 from de Haas-van Alphen Effect Measurements

We report on a band structure calculation and de Haas–van Alphen measurements of KFe 2 As 2 . Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18 m e , m e being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement (\({\gtrsim}3\)) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors.

Superconductivity of MI(MII0.5, Si0.5)2 (MI = Sr and Ba, MII = Al and Ga), ternary silicides with the AlB2-type structure

Abstract Ternary silicides M I (M II0.5 ,Si 0.5 ) 2 (M I =Sr and Ba, M II =Al and Ga) were prepared by Ar arc melting. Powder X-ray diffraction indicates that they have the AlB 2 -type structure, in which Si and M II atoms are arranged in honeycomb layers and M I atoms are intercalated between them. Electrical resistivity and dc magnetization measurements revealed that Sr(Al 0.5 ,Si 0.5 ) 2 is superconductive, with a critical temperature for superconductivity ( T C ) of 4.2 K, while Ba(Al 0.5 ,Si 0.5 ) 2 is not at temperatures ranging above 2.0 K. Sr(Ga 0.5 ,Si 0.5 ) 2 and Ba(Ga 0.5 ,Si 0.5 ) 2 are also superconductors, with T C s of 5.1 and 3.3 K, respectively.

(Sr,Ba)(Si,Ge)2 for thin-film solar-cell applications: First-principles study

In order to meet the increasing demand for electric power generation from solar energy conversion, the development of efficient light absorber materials has been awaited. To this end, the electronic and optical properties of advanced alkaline-earth-metals disilicides and digermanides (SrSi2, BaSi2, SrGe2, and BaGe2) are studied by means of the density functional theory using HSE06 exchange-correlation energy functional. Our calculations show that all these orthorhombic structured compounds have fundamental indirect band gaps in the range Eg ≈ 0.89–1.25 eV, which is suitable for solar cell applications. The estimated lattice parameters and band gaps are in good agreement with experiments. Our calculations show that the electronic band structures of all four compounds are very similar except in the vicinity of the Γ-point. The valence band of these compounds is made up by Si(Ge)-p states, whereas the conduction band is composed of Sr(Ba)-d states. Their band alignments are carefully determined by estimating...

BaSi2 as a promising low-cost, earth-abundant material with large optical activity for thin-film solar cells: A hybrid density functional study

Employing a hybrid density functional theory, we reveal the origin of the large absorption coefficient in BaSi2, which is roughly two to eighty times larger at ħω − Eg = 0.5 eV than other conventional absorbers such as Si, GaAs, CdTe, CuInSe2, and Cu2ZnSnS4. This is explained on the basis of the electronic band structure of BaSi2, whereby the lowest conduction band (CB) has a small dispersion, owing to the localized Ba-d states, resulting in flat bands. Consequently, these flat bands contribute to a wide range of optical absorption in the low-energy region and lead to high optical activity in BaSi2.

Electrical properties of polycrystalline SrSi2

The electrical properties of polycrystalline SrSi2 were examined by electrical resistivity measurements at temperatures ranging from 2 to 760 K and Hall coefficient measurements at temperatures ranging from 10 to 300 K. These measurements revealed that SrSi2 is a narrow-gap semiconductor with an energy gap of 0.035 eV whose dominant carriers are holes.

Superconductivity of ternary silicides A(Gax,Si1−x)2 (A=Ca, Sr, and Ba)

Abstract Ternary silicides A(Ga x ,Si 1− x ) 2 (A=Ca, Sr, and Ba) were prepared by Ar arc melting. Powder X-ray diffraction indicates that they have the AlB 2 -type structure in which Si and Ga atoms are arranged in chemically disordered honeycomb layers and atoms A are intercalated between them. Resistivity and DC magnetization measurements revealed that they are superconductors with a critical temperature for superconductivity ranging from 3.3 to 3.9 K.