Keikichi Nakamura

Growth of the 2223 Phase in Leaded Bi–Sr–Ca–Cu–O System

Growth of the 2223 phase has been studied in the leaded Bi-Sr-Ca-Cu-O system. Differential thermal analysis showed that the enhanced formation of the 2223 phase occurs in the temperature range between 835°C and 869°C where partial melting occurs. The density of the pellet was found to increase by 30% at one hour and then to decrease by 40% from the highest density attained. The former is attributed to the growth of the 2223 phase with the aid of the partially melted liquid phase and the latter to the flaky shape of the resultant 2223 crystals. The effect of Pb addition for enhanced 2223 phase formation will be discussed.

Growth of the 2223 Phase in Leaded Bi-Sr-Ca-Cu Oxide under Reduced Oxygen Partial Pressure

The growth process of 2223 phase in a leaded Bi-Sr-Ca-Cu oxide system has been investigated under various oxygen partial pressures using differential thermal analysis, combined with XRD of the quenched specimen. It is shown that under a slow heating rate, the DTA curves exhibit two or three endothermic peaks between 820 to 900°C, and the peak positions are lowered with reduction in the O2 partial pressure. The 2223 phase is found to grow most efficiently in the partial melting state sandwiched between the two endothermic peaks, and the temperature range between the two peaks becomes maximum around 0.1-atm O2 partial pressure.

Uniformly layered mixtures of the Bi2Sr2Can−1CunOn+4 phases formed in artificially layered films—structural analysis and superconducting properties

Uniformly layered mixtures of the succeeding members of Bi2Sr2Can−1 CunO2n+4 series were found in the films synthesized by alternating sputter deposition of BiO, SrCu0.5O1.5, and CaCuO2 layers. The uniformly layered intergrowth was caused by the deviation of the CaCuO2 supply amount in each sequence, which is constant throughout the synthesis, from the stoichiometric amount. It is presented that the x‐ray diffraction peaks from the films with uniform intergrowth can be indexed by treating them as the satellites of superlattices. In this treatment, the number of CuO2 layers, which varies randomly but uniformly along the c axis, is expressed on average by noninteger n. The superconducting transition temperature (Tc) and concentration of holes in the CuO2 layers in those films decreased with increasing n (2.88≤n≤6) monotonically. From the continuity of the decrease in Tc with n, we conclude that the hole concentration is averaged between the two phases in the intergrowth by the diffusion of the holes.

Synthesis of YNi2B2C thin films by magnetron sputtering

Thin films of YNi2B2C superconductor by the magnetron sputtering technique have been synthesized. The YNi2B2C phase was found to crystallize very uniformly after annealing at 1050 °C in vacuum. The structure was investigated and the electric and magnetic properties were characterized. The films were c‐axis oriented and showed a Tc onset of ∼15 K by both magnetization and resistivity measurements. The results of the critical current measurement suggested the existence of anisotropies of pinning force and an upper critical field Bc2.

Superconducting and Transport Properties of B-Y-Cu-O Compounds –Orthorhombic and Tetragonal Phases

The superconducting properties and crystal structures of the Ba-Y-Cu-O compounds have been studied. Annealing and quenching experiments have been carried out to study the phase transformation between the high temperature tetragonal phase (oxygen deficient) and low temperature orthorhombic phase (oxygen rich). The former phase is semiconductor-like down to 4.2 K, whereas the latter is metallic and superconducting with Tc as high as 94 K. The conspicuous difference in the transport and superconducting properties between the two phases has been attributed to the loss or formation of oxygen and Cu atom chain in the crystal structure.

Synthesis of Artificially Layered Bi–Sr–Ca–Cu Oxide Films and Their Thermal Stability

Bi-O/Sr-Ca-Cu-O multilayered films have been synthesized using two target reactive magnetron sputtering. It is shown that the modulation wavelength \varLambda (c/2 for the crystals) can be simply controlled by varying the deposition time for a one layer unit of Sr-Ca-Cu-O while keeping that for Bi-O constant. The films thus obtained have smooth surfaces and have \varLambdas ranging from 12 to 26 A, i.e., almost equal to those of the crystalline or intergrowth phases o bserved in the Bi-Sr-Ca-Cu-O system. The thermal stability of these artificially layered films is also reported.

Oxygen Ordering in YBa2Cu3O6+x

The orthorhombic and tetragonal phase relation, i.e. the order-disorder phase transformation in YBa2Cu3O6+x, has been studied by employing statistical thermodynamics, and the equilibrium value of order parameter -1s1 has been determined as a function of the temperature and oxygen concentration. In the above-mentioned treatment, plus and minus signs of s in the ordered state (|s|≠0) are found to describe the degree of order in the twin related crystals. The calculated properties are compared with the experimental data, i.e. the temperature dependence of oxygen occupancy at a constant oxygen pressure and the concentration dependence of axial lengths, a and b, at room temperature. The agreement is excellent for both cases when the axial lengths at room temperature are measured for the samples with oxygen removed or doped at low temperatures. The statistical thermodynamical expression for further ordering has also been derived.

Structure of High-Tc Superconductor YBa2Cu3O6.6 at Low Temperatures

A single-phase YBa2Cu3O6.6 shows a sharp superconductive transition at 94 K. The crystal structure of this compound has been determined by the Rietveld profile analysis of neutron powder diffraction data measured at 10 K and 120 K. The space group Pmmm is confirmed, and the positions and occupations of oxygen atoms are established. No phase transitions or other structural anomalies were observed in relation to the superconductive transition.

The grain size dependence of the resistance behaviors in doped lanthanum manganite polycrystalline films

Using the sol–gel spin-coating method, La0.67Sr0.33MnO3 and La0.7Ca0.3MnO3 polycrystalline films have been prepared. The polycrystalline films show much lower resistivity peak temperature Tp than that of the corresponding epitaxial film, and Tp shifts to higher temperature when the annealing temperature increases. The magnetization measurement reveals that all of the polycrystalline films have the same magnetic transition temperature as the corresponding epitaxial films. These results are different from the previous works. We ascribe the difference to the grain size of our polycrystalline samples being much smaller, so the conduction through the grain boundaries predominates over the intragrain conduction. In this case, spin disorder and magnetocrystalline anisotropy should be taken into account in explaining the magnetotransport behavior.

Synthesis and characterization of Bi2Sr2Can−1CunOy(n=1–7) thin films grown by off‐axis, three target magnetron sputtering

An alternate deposition method using three target magnetron sputtering and off‐axis substrate geometry has been applied to control the number of CuO2 planes n in Bi2Sr2Can−1CunOy thin films. During alternate deposition, the substrate stayed sequentially at each target as Bi→Sr5Cu2O7→CaCuO2→Sr5Cu2O7→Bi. When the staying times at the Bi and Sr5Cu2O7 targets were fixed constant but varied for the CaCuO2 target, single phase films with n=1–7 were obtained. The x‐ray diffraction patterns showed a systematic change in the relative line intensity with n in accordance with the calculated patterns, and all possible (00l) lines, including the (004_0_) line of the n=7 film, can be clearly observed below 2θ<60° region. The c dimension increases stepwise by 0.63 nm and approaches 6.27 nm for n=7, indicating a perovskite CaCuO2 plane is regularly inserted with n. The resistivity zero temperature, Tc zero, decreases systematically with n. For n=7, Tc decreased to onset 55 K and end 4 K. The systematic decrease in Tc zer...