Takanori Fujiyoshi

Amorphous gallium phosphide (a-GaP) pn-diode by ion beam assisted deposition

Abstract Amorphous gallium phosphide (a-Ga x P) has been formed as pn-diode by using electron beam deposition under hydrogen and/or nitrogen ion beam assistance. The rectification performance limited due to localized states, is improved to an optical device application grade through a higher current source of hydrogen and/or nitrogen ions. It is suggested, that a selective usage of the two ions can optimize the carrier type and junction states. In the case of an optimized ion-assisted amorphous film on crystal GaP, blue light luminescence has been observed.

Influence of discontinuous columnar defects on flux pinning properties in GdBCO coated conductors

To investigate the effect of discontinuity of 1-D pinning centers on the flux pinning in a wide range of magnetic field directions, discontinuous columnar defects (CDs) and continuous CDs were formed in GdBCO coated conductors using Xe-ion irradiations with 80 and 270 MeV, respectively. An overall shift upward in Jc, for the 80-MeV-irradiated sample compared to the 270-MeV-irradiated one, can be observed in every direction of magnetic field, which is more remarkable as temperature decreases. This implies a synergetic effect of the pinning interaction between the linearity and the discontinuity for the discontinuous CDs. When the density of CDs is lower, however, the difference in Jc between the discontinuous and the continuous CDs becomes less at lower temperature, even in the magnetic field parallel to the c-axis. At lower density of CDs, the average spacing between CDs is wider, which would hardly lead to automatic entanglement of flux lines in the discontinuous CDs, i.e., to the synergetic effect.

Inductive measurement of in-field critical current density of YBCO thin film on a SrTiO3 bicrystal substrate using the third harmonic voltage method

The spatial distribution and the magnetic field dependence of the critical current density Jc for a YBa2Cu3Oy (YBCO) thin film deposited on a SrTiO3 (STO) bicrystal substrate have been investigated using the third harmonic voltage method. When a pick-up coil is mounted on the area without a crystal grain-boundary, the third harmonic voltage V3 increases monotonically with increasing coil current I0 over a certain threshold value. On the other hand, when the pick-up coil is mounted on the area with a crystal grain-boundary, the V3?I0 curve shows a strange behavior. However, the V3?I0 curves show a monotonic increase when a magnetic field over 0.07?T is applied to the YBCO thin film, because the shielding current hardly flows across the crystal grain-boundary in the large magnetic field. The magnetic field dependence of Jc is also measured using the four-probe method to clarify the strange behavior of the V3?I0 curve in the third harmonic voltage method.

High flux pinning efficiency by columnar defects dispersed in three directions in YBCO thin films

A systematic investigation of flux pinning by widely direction-dispersed columnar defects (CDs) in YBa2Cu3O y thin films was carried out by using heavy-ion irradiation: a parallel configuration of CDs aligned along the c-axis, and two trimodal splay configurations composed of CDs crossing at 0° and ± 45° relative to the c-axis, where the splay plane defined by the three irradiation angles is perpendicular (trimodal-A) or parallel (trimodal-B) to the transport current direction. The trimodal configurations show high pinning efficiency over a wide range of magnetic field orientations compared to the parallel one at low magnetic field. In particular, trimodal-B shows the higher critical current density of the two trimodal configurations. The crossed CDs at ± 45° in the trimodal configurations provide uncorrelated flux pinning at B || c due to the large tilting angle off the c-axis, which effectively reinforce the flux pinning of CDs parallel to the c-axis. This assist effect is more remarkable for trimodal-B: a kink sliding motion of flux lines along the CDs is more effectively reduced by the splay plane, not only at B || c but also at inclined magnetic fields off the c-axis.

Influences of microstructure on critical current properties in MgB 2/Al Film

The metal Al is lighter in weight than other substrate materials for MgB₂ filmssuch as Si and Ni. This property inspires MgB₂ fabrication on a large-scale Al substrate as a new route to MgB₂ tapes. Here we report microstructural factors influencing critical current density, <i>J</i>_c, in MgB₂/Al films. MgB₂/Al films were prepared by the following steps: deposit a boron layer of 3 nm in thickness on an Al substrate heated at 280 ^°C ; deposit Mg and boron on the boron layer (sample A). For comparison, Mg and boron were deposited directly on an Al substrate heated at 265 ^°C (sample B). The microstructure was observed by transmission electron microscopy (TEM) and scanning TEM. <i>J</i>_c values at 20 K in the self-field were 4.9 × 10⁶ A cm^-2 for sample A and 2.7 ×10⁶ A cm^-2 for sample B. Both the samples form an oxygen-rich layer of 10 nm in thickness at the substrate surface. This oxygen-rich layer may suppress Al diffusion into MgB₂ lattices. The [001] texture of columnar MgB₂ crystals grown on the substrate is stronger in sample A than in sample B. This indicates that the boron layer deposition on the Al substrate is effective for fabricating well-textured MgB₂ polycrystals, resulting in the higher <i>J</i>_c enhancement for sample A.

Influences of Microstructure on Critical Current Properties in

The metal Al is lighter in weight than other substrate materials for MgB₂ filmssuch as Si and Ni. This property inspires MgB₂ fabrication on a large-scale Al substrate as a new route to MgB₂ tapes. Here we report microstructural factors influencing critical current density, <i>J</i>_c, in MgB₂/Al films. MgB₂/Al films were prepared by the following steps: deposit a boron layer of 3 nm in thickness on an Al substrate heated at 280 ^°C ; deposit Mg and boron on the boron layer (sample A). For comparison, Mg and boron were deposited directly on an Al substrate heated at 265 ^°C (sample B). The microstructure was observed by transmission electron microscopy (TEM) and scanning TEM. <i>J</i>_c values at 20 K in the self-field were 4.9 × 10⁶ A cm^-2 for sample A and 2.7 ×10⁶ A cm^-2 for sample B. Both the samples form an oxygen-rich layer of 10 nm in thickness at the substrate surface. This oxygen-rich layer may suppress Al diffusion into MgB₂ lattices. The [001] texture of columnar MgB₂ crystals grown on the substrate is stronger in sample A than in sample B. This indicates that the boron layer deposition on the Al substrate is effective for fabricating well-textured MgB₂ polycrystals, resulting in the higher <i>J</i>_c enhancement for sample A.

\hbox{MgB}_{2}/\hbox{Al}

Abstract The thermal diffusivity of YBa 2 Cu 3 O z , typical one of high T c superconductors, has been measured by a transient method, in order to make clear the electron-phonon interaction around T c in the material. The diffusivity shows an abrupt increase with decreasing temperature below T c . The BCS energy gap in the material has been estimated from the results, by using the Bardeen-Rickayzen-Tewordt (BRT) theory on the phonon transport.

Film

Techniques of doping with zinc and sulfur to a-GaP:H films during hydrogen-reactive evaporation (RE) and sputtering (SP), and properties of p-n junction diodes made of different a-GaP:H films are described. The conductivity of a-GaP:H films, made by RE and SP method, increases drastically by doping. Junction diodes composed of p-type RE /or SP a-GaP:H films doped with Zn on n-type crystalline GaP substrates, show a comparatively good rectification property, while that of a diode of (p-type SP film doped with Zn) /(n-type RE film doped with S) is in bad shape at present. On the other hand, the latter diode shows the photo-voltaic effect, the spectrum of which is compared with the photoconductivity one of RE a-GaP:H films reported by us previously.

Electron-phonon interaction and thermal diffusivity of high Tc superconductors

The optical response of high-quality epitaxial YBa2Cu3O7-δ thin films was measured for temperatures around the superconducting transition region under the magnetic field parallel to the c-axis. The nonbolometric component of the optical response is dominant at the temperature below the transition region and increases with the magnetic field. Moreover, we measured the transport properties under light or heavy ion irradiation. In the scaling analysis of transport properties, it is found that the dynamic critical exponent, z changes drastically due to light or heavy ion irradiation.

Doping for making n- and p-type a-GaP:H films during reactive evaporation and sputtering

The critical scaling analysis of current-voltage characteristics was performed on a YBa2Cu3O7-δ thin film to study the influence of pinning strength on the flux line dynamics in the vicinity of the glass-liquid transition temperature. To change the pinning strength, the thin film was irradiated with 200MeV Au ions at two angles, ±15° with respect to c-axis, which produced planar splayed columnar defects. As one of the important results, it was found that installed defects led to the increase of the dynamic critical exponent z. The origin of the interesting change in z can be explained by the depinning model considering the pinning strength distribution, as well as the glass-liquid transition model which is popular for interpreting the flux line dynamics. This result indicates that the pinning interaction is a main factor determining the flux line dynamics in the vicinity of the glass-liquid transition temperature.