Seiji Yaegashi

Epitaxial Growth of CeO2 Films on Si(111) by Sputtering

CeO2 films were epitaxially grown on Si(111) substrates by reactive sputtering following the single-crystal CeO2 seed layer formation by oxygen-reactive solid-phase epitaxy. Formation of metallic Ce layers and oxidation of the layers prior to reactive sputtering was found to be the key process for epitaxial growth of CeO2 on Si substrates. An Auger electron spectroscopy depth profiling analysis showed that the Ce–Si interlayer was formed after the Ce metal deposition. Reflection high-energy electron diffraction and transmission electron microscopy proved that epitaxial CeO2 films were grown by reactive sputtering on the seed layers formed by oxidation of Ce metal layers with thickness of 5 to 10 nm. Crystalline quality of the films depended on the sputtering conditions, especially on total sputtering pressure and oxygen concentration. The optimization of the conditions for seed layer formation by oxygen-reactive solid-phase epitaxy and reactive sputtering was an important factor for improving the crystalline quality of epitaxially grown CeO2 film.

Synthesis of YBa2Cu4O8 Powders by Sol-Gel Method under Ambient Pressure

The 124 superconductor YBa2Cu4O8 was synthesized using a sol-gel method with starting solutions contained stoichiometric compositions of Y/Ba/Cu=1/2/3 and 1/2/4. Heattreatment was done below 800°C in 1 atm oxygen atmosphere; neither special equipment nor additives were required. Our result suggests that the 123 phase is thermodynamically metastable in the temperature region that had been thought to be the equilibrium region of 123-orthorhombic phase.

Low-Temperature Synthesis of YBa2Cu3O7-x and YBa2Cu4O8 by Sol-Gel Method

The 123 and 124 superconductors were synthesized by sol-gel methods at low temperature. Heat treatment was done below 800°C; neither special equipment nor additives were required. The 123 pellets, which were prepared at 650°C under vacuum condition (0.1 to 1.0 Pa) followed by annealing in flowing oxygen at 750° and 800°C, exhibited zero resistance at 70 and 80 K, respectively. Both the 124 pellets, which were prepared at 780°C in 1 atm oxygen atmosphere followed by either furnace cooling to room temperature or quenching from 780°C to liquid nitrogen temperature, had almost the same transition temperature (Tc(onset)=83-80 K, and Tc(zero)=76–69 K).

Electron Tunneling Measurements of High-Tc Superconductor Bi-Sr-Ca-Cu-O by STM

The tunneling spectra of Tc=80 K single crystal and 110 K polycrystalline specimens have been measured at 4 K using a scanning tunneling microscope with a tungsten counter electrode. For the 80 K phase, we estimated the energy gaps 2Δ/kBTc to be 4.9–5.3 and 4.2–5.0 in the directions perpendicular and parallel to the a-b plane, respectively, while 2Δ/kBTc for the 110 K phase was in the range of 3.4–4.2. The energy gap data obtained in the present study will be discussed in terms of the anisotropy and coupling strength of the superconductivity.

Resistivity Anomaly Near Room Temperature of Y-Ba-Cu-O and Related Oxides as Created by the Surface Effect of Water

A number of papers have recently reported the possible presence of superconductivity near room temperature in Y-Ba-Cu-O oxide and related materials, based mainly on resistivity measurements. Close examinations of the electrode contacts during resistivity measurements in the present study showed that similar resistivity anomalies could be reproducibly observed when the specimen surface was wet. The anomaly turned out to be related with artifacts of the electrode contacts which resulted from the reaction of the specimen surface with water, creating electrochemical current paths between the electrodes.

Preparation and soft magnetic properties of epitaxial Fe–Si(111) monolayer films and Fe–Si(111)/Cr(111) multilayer films

Epitaxial Fe–Si(111) monolayer films and [Fe–Si(111)/Cr(111)]4 multilayer films were grown on Si(111) substrates by dc facing-targets sputtering. The thickness dependence of the Fe–Si layer and the Cr layer on the crystallinity and the magnetic properties was studied. In the case of Fe-7.2 wt% Si(111) monolayer films, excellent soft magnetic properties were observed in films thinner than 200 nm. The sudden deterioration of soft magnetic properties was observed at the thickness of 200 nm. Torque measurements of the films revealed that the increase of effective anisotropy energy caused the deterioration of soft magnetic properties. In the [Fe-7.2 wt% Si(111)/Cr(111)]4 multilayer films, permeability increased with increase of the Cr layer thickness, and excellent soft magnetic properties of multilayer films were obtained with a Cr layer thickness of 20 nm. Soft magnetic characteristics were improved in the multilayered structure compared with the monolayer film with identical total thickness of magnetic layers.

Preparation of YBa2CuZ3O7-x and YBa2Cu4O8 Thick Films by Gas Deposition Using Fine Powders

Superconducting thick films 1 mm wide and 25 µm thick were prepared on MgO substrates by gas deposition of YBa2Cu4O8 (124) fine powders. A 124 thick film with random grain orientation had a Jc of 490 A/cm2 at 36 K. A YBa2Cu3O7-x (123) thick film produced by converting 124 into 123+CuO had a Jc of 1470 A/cm2 at 30 K.

EFFECT OF TEMPERATURE AND OXYGEN PRESSURE ON PREPARATION OF BI2SR2CACU2OX THIN FILMS BY METALORGANIC DEPOSITION

Bi2Sr2CaCu2Ox superconducting thin films have been prepared by metalorganic deposition (MOD) under various oxygen pressures. The pressure-temperature (P-T) phase diagram of Bi2Sr2CaCu2Ox was determined under oxygen pressures from 0.02 to 0.2 atm. A film which was fired at 785°C in 0.06 atm oxygen with the thickness of about 0.1 µm showed a critical current density (Jc) of 2×104 A/cm2 at 30 K. The relationship between Jc and firing conditions is discussed.

Bi-based Superconducting Films Synthesized by a Combination of Metalorganic Deposition (MOD) and a Diffusion Process

Bi-based superconducting thin films have been prepared by a combination of metalorganic deposition(MOD) and a diffusion process. The films derived from a layer of Bi/Sr/Ca/Cu=2/2/1/3 and a layer of Ca/Pb=2/1, after firing for only 4 h, yielded a nearly single phase layer of the 2223(high Tc) phase. It was found that Ca2PbO4 played a very important role in formation of the 2223 phase in short firing times, acting as a souce of Ca ions to react with the 2212 phase and CuO.

Preparation of Superconducting Thin Films of Bi2Sr2CaCu2Oy by Metallo-Organic Decomposition Method

Metallo-organic decomposition is a nonvacuum technique for thin film fabrication which allows easy alteration of chemical components and no requirement of expensive systems. We report the preparation of ultra-thin film superconductors, which are almost transparent with homogeneous surface by this method using calboxylic acid salts solutions. By optimum coating, pyrolizing, and firing conditions, single phase of 80K superconductor with strongly c-axis aligned parpendicular to the substrate surface was obtained.