Electronic structure and covalency in superionic conductors

The electronic states of noble metal halides and alkali halides are calculated by the DVXα cluster method to get more microscopic evidence for the p — d hybridization and the covalency in noble metal halides. It is found that both components of anti-bonding and bonding exist in the diagram of overlap population (DOP) for AgX (X=halogen) and these two components are made up of the 4d band of Ag ion and the p band of halogen ion, which form the p — d hybridization. The covalency of noble metal halides is in the border between that of the fourfold coordinated compounds such as AgI and that of the sixfold coordinated compounds such as AgCl. These calculation results on the covalency are compared with the Phillips's ionicity. Paper presented at the Patras Conference on Solid State Ionics, Patras, Greece, Sept. 14 – 18, 2004.     

Single-electron devices formed by pattern-dependent oxidation: microscopic structural evaluation

The embedded Si structure formed by pattern-dependent oxidation (PADOX) in single-electron device (SED) is analyzed by novel microscopic methods using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface charge imaging SEM reveals the outline of the embedded Si nanowire of the electrically-measured SED. The size of the wire in the device is small enough to make a potential barrier caused by the quantum mechanical size effect. The result of the Si height in the oxidized structure estimated by AFM indicates that the huge stress induced by oxidation is applied to the narrow Si wire. The experimental results support the theoretical model of the SED fabricated by PADOX that the potential profile responsible for the SED operation is produced by two effects, the quantum mechanical size effect and the strain-induced bandgap reduction.     

Differential cross sections of neutral pion photoproduction from hydrogen in the energy region from 390 MeV to 975 MeV

The differential cross sections of the reaction γp→π⁰p have been measured at photon energies between 390 MeV and 975 MeV with an energy step of 20 or 25 MeV. The angular region covers 11 angles between 15° and 130° in the c.m.s. A combination of a magnetic spectrometer and photon detector was used for the measurement in the angular region from 50° to 130°. At forward angles of 15°, 35° and 50°, π⁰ mesons were detected using a π⁰ detector consisting of a pair of photon detectors. The data obtained with two different detection systems overlap at the angle of 50° and coincide with each other within the experimental error. The results are compared with those of phenomenological partial-wave analyses.     

Twin boundary structure of a superconductive YBa2Cu3O6.9 crystal

An orthorhombic YBa₂Cu₃O_6.9 crystal with Tc=91 K, in which synthetic twins are inevitably contained, is so sensitive to electron irradiation that a structural change easily occurs at the twin boundary, causing the phase transformation from orthorhombic to tetragonal system. We have succeeded in taking images, in which lattice fringes sharply bend at the boundary, using an ultra-high-resolution high-voltage electron microscopy (UHR-HVEM) and the intrinsic structure of the twin boundary could be analyzed to be of oxygen-centered type. The diffraction streak, which is excited only from the boundary region, is explained with help of optical diffraction for a model structure of tilted anti-phase boundary. Atomic knock-on displacements start to occur also in the matrix, being a little apart from the boundary, even at very early stage of electron beam irradiation.Presented at the workshop on High-Voltage and High Resolution Electron Microscopy, February 21–24, 1994, Stuttgart, Germany.     

A high-pressure and high-temperature synthesis of platinum carbide

High-pressure synthesis is a powerful method for the preparation of novel materials with high elastic moduli and hardness. Additionally, such materials may exhibit interesting thermal, optoelectronic, semiconducting, magnetic, or superconducting properties. We report on the new high-pressure, high-temperature synthesis of platinum carbide. The experiments were performed in a laser-heated diamond anvil cell and data were collected using the synchrotron X-ray diffraction method at pressures >75 GPa at high-temperatures. The new platinum carbide has a rock-salt type structure, with space group Fm3m and cubic symmetry. It was confirmed to remain stable to at least 120 GPa. This structure is the same as that of other metal carbides reported in previous studies. After decompression, the new high-pressure phase was recoverable at ambient pressure. The Birch-Murnaghan equation of state for this new phase was determined from the experimental unit cell parameters, with K₀=301 (±15) GPa, and K′₀=5.2 (±0.4).     

Fabrication of self-ordered nanohole arrays on Si by localized anodization and subsequent chemical etching

Nanohole arrays with a 60 nm hole periodicity were fabricated on a Si substrate by the anodization of an aluminum film sputtered on a Si substrate in sulfuric acid and subsequent chemical etching. The transfer of the nanoporous pattern of anodic alumina into the Si substrate was achieved by the selective removal of silicon oxide, which was produced by the anodic oxidation of the underlying Si substrate through the anodic porous alumina used as a mask.