Hidehito Asaoka

Growth of Large Isometric YBa2Cu3Ox Single Crystals from Coexisting Region of Solid with Melt in Y2O3 Crucibles

Large isometric single crystals of oxide superconductor YBa2Cu3Ox (YBCO) have been grown from a mixed region of solid Y2BaCuO5 and melt. The crystals possess sharp, shiny habits with a maximum thickness of about 7 mm along the c-axis. The best growth condition is that wherein the solvent composition is 7BaCuO211CuO which is close to the binary eutectic of BaCuO2-CuO, and the optimum concentration of the growth system is that wherein the ratio of YBCO to 7BaCuO211CuO is 64:36. The use of a Y2O3-crucible prevents contamination from containers during growth, and facilitates the synthesis of high-purity crystals. Temperature gradient and seed crystals are both effective for controlling nucleation. The annealed crystals show a sharp superconductive transition at 91 K. The temperature dependence of the resistivities is found to be metallic in the ab- as well as the c-direction. Sufficient diamagnetism is manifested as a sharp transition at Tc and a complete shielding effect below Tc.

Growth of Polyhedral YBa2Cu3Ox Single Crystals and Their Anisotropic Properties

Polyhedral single crystals of oxide superconductor YBa2Cu3Ox have been grown under the condition where the solid phase Y2BaCuO5 and liquid phase coexist. The crystals obtained possess well-developed habits and have a thickness of up to 3 mm along the c-axis, when the solid-melt mixture is kept at 1050-1010°C and then cooled to 990-970°C. The annealed crystals show a sharp superconductive transition at 91 K. The temperature dependence of resistivity is found to be metallic both in the ab- and c-directions. A marked anisotropy in the broadening of the resistive transition is observed under magnetic fields parallel or perpendicular to the c-axis.

Observation of Initial Oxidation on Si(110)-16×2 surface by Scanning Tunneling Microscopy

The initial oxidation of a Si(110)-16×2 clean surface, both at room temperature (RT) and elevated temperatures (635, 660 °C), was investigated by scanning tunneling microscopy (STM). The effects of annealing (300 °C, 15 min) on an RT-oxidized surface were also investigated. On the RT-oxidized surface, a BN site, detected as a bright (B) spot in the filled-state image but as a normal (N) 16×2 adatom in the empty-state image, was observed. After annealing, DD, BD, and BB sites were found to exist in addition to the BN site. Here, DD (BD, BB) is a site that appears dark (bright, bright) in its filled-state image and appears dark (dark, bright) in its empty-state image. The relative population is DD>BD≈BN>BB. For oxidation at 635 °C, DD, BD, and BN sites were observed. For oxidation at 660 °C, only the BD site was observed. On the basis of these results, an atomistic process during the initial oxidation of the Si(110)-16×2 surface is discussed.

Crystal Growth Mechanism of YBa2Cu3Ox from Coexisting Region of Solid with Melt

The behavior of textured growth of YBa2Cu3Ox (123) from Y2BaCuO5 (211) and melt has been investigated using polarized optical microscopy, electron microprobe analysis and the X-ray powder diffraction method. The observations of growing interfaces of the solidified 123 phase in three-phase region, 123, 211 and the melt revealed the occurrence of diffusion of the 123 phase to the surrounding melt. This fact suggests the solubility of the 123 phase to the melt, and induction of the Ostwald ripening, which acts on the solid-melt interfaces to lower the interface free energy of the growing crystals. These situations lead to the growth of 123 crystals which progresses due to the aid of the peritectic reaction between 211 and the surrounding melt phases.

Homogeneity of Superconductivity in YBa2Cu3Ox Single Crystals.

We investigated the homogeneity of high-purity YBa2Cu3Ox (123) single crystals at different oxygen concentrations by means of the higher-harmonic susceptibility. The superconductivity of the onset temperature at 93.8 K was observed in the samples of the hypostoichiometric composition (x=6.87). Also, we succeeded in obtaining the 123 single crystals of the optimum stoichiometry (x=7.00) which showed a sharp single transition at 92.5 K corresponding to occurrence of the homogeneous superconductivity.

Measurement of Transient Photo-Induced Changes in Thin Films at J-PARC — Time-Resolved Neutron Reflectivity Measurements of Silver Photo-Diffusion into Ge-Chalcogenide Films

Study on photo-induced changes for specific light-sensitive materials is fascinating from both academic and application points of view because it is one of the frontiers in materials science including concepts of electron-excitation and non-equilibrium state, and the materials can be controlled spatially by illuminated area from distant place [1]. With the aid of significant development of light source equipment such as femtosecond ultra-short pulsed laser systems, attractive photo-induced phenomena have been found in many materials in these days. In the leading-edge field, it is quite natural to be interested in finding the structural origin of the photo-induced changes. Although neutron scattering technique has always a difficulty in beam intensity, it is highly expected to be utilized in structural determination for magnetic materials, hydrogenated materials, and X-ray sensitive materials. At the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC), the proton beam intensity is now 300kW, and it is planned to go to 1MW in

Ordering of C60 on One-Dimensional Template of Single-Domain Ge(110)-16×2 and Si(110)-16×2 Surfaces

We demonstrated the ordering of C60 in one dimension by using Ge(110)-16×2 and Si(110)-16×2 single-domain surfaces as one-dimensional templates. We achieved the formation of one-dimensional order of C60 molecules along the atomic rows of the Ge(110)-16×2 surface where surface diffusion of C60 is activated at room temperature. In contrast, the substantial reactivity of the Si(110)-16×2 surface limited the diffusion of adsorbed C60, resulting in random adsorption even at elevated temperatures.

Single-domained Si(110)-'16×2' surface

Micrometer-wide, single-domain of 16×2 reconstruction, the array of straight monoatomic steps, has been fabricated on Si(110) surface by means of controlled electromigration of surface atoms. The electromigration effect during DC current heating is found to line-up the reconstruction rows when the current direction matches to the direction of the rows. This provides not only a well-controlled surface preparation method for Si(110), but also a new template for low-dimensional nanostructures.

Initial growth stage of a highly mismatched strontium film on a hydrogen-terminated silicon (111) surface

We report the formation of an atomically abrupt interface without strain in a strontium film using a hydrogen buffer layer on silicon, in spite of large lattice mismatch such as 12%. The onset of the initial growth stage of strontium film with its bulk lattice constant occurs with one atomic layer deposition. The interfacial monoatomic layer of hydrogen together with the first one atomic layer of strontium acts as an effective buffer layer. Our results provide microscopic evidence of heteroepitaxial growth of the strain-free film with the atomically abrupt interface in a highly mismatched system.

Intrinsic Pinning does not Influence the Vortex Melting Transition of YBa2Cu3O6.94 in H ⊥c Configuration

We examine the effective regime for intrinsic pinning of an YBa 2 Cu 3 O 6.94 single crystal in the H – T plane. The irreversibility line for intrinsic pinning can be tracked by the hysteresis of the magnetic torque when the field is almost in parallel to the a b plane. The melting transition in the H ⊥ c configuration appears at higher temperatures where the intrinsic-pinning peak is absent. We argue that the intrinsic pinning may not affect the nature of the vortex melting transition in the H ⊥ c configuration.