Giyuu Kido

Electronic structures of promising photocatalysts InMO4 (M=V, Nb, Ta) and BiVO4 for water decomposition in the visible wavelength region

The compounds InMO4 (M=V, Nb, Ta) and BiVO4 are promising photocatalysts which are able to induce hydrolysis of water molecules under visible light irradiation. By first principles calculations, supported by experiments, we inspect their peculiar electronic structure in an attempt to rationalize the link between the bulk crystal architecture of the materials and the related electronic properties. We find that the bottom of the conduction band of InMO4 systems consists of a large contribution (about 20%) due to 5s orbitals of In atoms. Another dominant component comes from d orbitals of V, Nb, and Ta. On the other hand, the top of the valence band of the BiVO4 shows a contribution from 6s orbitals of Bi of about 18% as well as a dominant component due to 2p states of O. We can infer that the photocatalytic activity could be improved by the large mobility coming from the s orbital component as well as by tuning the electron affinity (position of the bottom of the conduction band) and ionization potential (t...

Optical transitions in quantum ring complexes

High Magnetic Field Center, National Institute for Materials Science, Sakura 3-13, Tsukuba 305-0003, Japan(Dated: February 2, 2008)Making use of a droplet-epitaxial technique, we realize nanometer-sized quantum ring complexes, consistingof a well-defined inner ring and an outer ring. Electronic structure inherent in the unique quantum system isanalyzed using a micro-photoluminescence technique. One advantage of our growth method is that it presentsthe possibility of varying the ring geometry. Two samples are prepared and studied: a single-wall ring and aconcentric double-ring. For both samples, highly efficient photoluminescence emitted from a single quantumstructure is detected. The spectra show discrete resonance lines, which reflect the quantized nature of the ring-type electronic states. In the concentric double–ring, the carrier confinement in the inner ring and that in theouter ring are identified distinctly as split lines. The obse rved spectra are interpreted on the basis of singleelectron effective mass calculations.

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.

Self-assembly of laterally aligned GaAs quantum dot pairs

The authors report the fabrication of self-assembled, strain-free GaAs∕Al0.27Ga0.73As quantum dot pairs which are laterally aligned in the growth plane, utilizing the droplet epitaxy technique and the anisotropic surface potentials of the GaAs (100) surface for the migration of Ga adatoms. Photoluminescence spectra from a single quantum dot pair, consisting of a doublet, have been observed. Finite element energy level calculations of a model quantum dot pair are also presented.

Field-induced magnetic ordering in the quantum spin system KCuCl3

KCuCl 3 is a three-dimensional coupled spin-dimer system and has a singlet ground state with an excitation gap Δ/k B =31 K. High-field magnetization measurements for KCuCl 3 have been performed in static magnetic fields ofup to 30 T and in pulsed magnetic fields of up to 60 T. The entire magnetization curve including the saturation region was obtained at T= 1.3 K. From the analysis of the magnetization curve, it was found that the exchange parameters determined from the dispersion relations of the magnetic excitations should be reduced, which suggests the importance of the renormalization effect in the magnetic excitations. The field-induced magnetic ordering accompanied by the cusplike minimum of the magnetization was observed, as in the isomorphous compound TlCuCl 3 . The phase boundary was almost independent of the field direction, and is represented by the power law. These results are consistent with the magnon Bose-Einstein condensation picture for field-induced magnetic ordering.

Conduction band caused by oxygen vacancies in aluminum oxide for resistance random access memory

As a next-generation memory, we have developed a rare-metal-free memory using Al oxide with a high-density of oxygen vacancies (Vos). The electronic structure has been simulated using first-principles calculations. In this paper, we report the electronic structure of the band gap, analyzed using thermally stimulated current measurements, to evaluate the simulated results. We observed electronic states corresponding to resistance changes for the first time. These results show that Vo+2 (electron empty Vo) changes to Vo+1 by electron injection; the overlapped Vo+1 electron changes into a “Vo conduction band” (VoCB), and the changed structure is stabilized by structural relaxation of Al ions around Vo. VoCB is considered as a kind of mid-gap impurity band. The origin of the on/off switching is considered to be generation/degeneration of the VoCB caused by increasing/decreasing numbers of Vo electrons. Based on knowledge of the electronic mechanism, we have changed metal/insulator/metal structure to a metal/i...

Lasing in GaAs∕AlGaAs self-assembled quantum dots

The authors have demonstrated photopumped laser action of self-assembled ring-shaped GaAs quantum dots (QDs) grown by droplet epitaxy. Morphological control of the QD shape from conelike dots to ringlike ones results in a narrow spectral band of photoluminescence from the QD ensemble, reflecting their small size distribution. Using ring-shaped QDs as an active laser medium, they observed multimodal stimulated emissions from the ground state at temperatures of up to 300K.

Confirmation of a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor and antiferromagnetic second-nearest-neighbor interactions in Rb

We have investigated magnetic properties of Rb

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