K. Kitazawa

Study on chemical diffusion of oxygen in Ba2YCu3O7−δ

Abstract Chemical diffusion in the high- T c oxide superconductor Ba 2 YCu 3 O 7−δ system was thermogravimetrically studied in a temperature range from 550 to 850°C under an oxygen pressure of 1 to 10 −2 atm, using highly densified polycrystalline specimens. Isothermal weight relaxation after stepwise changes of the oxygen pressure were measured and analyzed in terms of oxygen diffusion-controlled kinetics. The chemical diffusion coefficient was found to be strongly sensitive to the oxygen composition. Its absolute value sharply increased with decreasing oxygen deficiency, δ, although it did not change appreciably at the structural transition between orthorhombic and tetragonal phases. Such a compositional dependence of the chemical diffusion is not expected from a simple vacancy diffusion mechanism. It is interpreted in terms of composition-sensitive activation energy for the two-dimensional motion of oxygen within the unique crystal structure of this oxide system.

Superconductivity and metal-semiconductor transition in BaPb1-xBixO3

Abstract The perovskite solid solutions BaPb1-xBixO3 exhibit unique properties in both metallic and semiconducting phases. In the metallic phase (0 ≤ x ≤ 0.35) they are superconducting with high transition temperatures. It turns out that the high T c originates from extremely strong electron-phonon interaction. At the metal-semiconductor transition the superconductivity disappears and the semiconducting properties persist over a wide range of Bi composition (0.35 ≤ x ≤ 1). Recent studies of the optical spectrum, as well as Raman scattering experiments, have given direct evidence for charge-density-wave formation in BaBiO3. The CDW is found to be local in the present system and be stable throughout the semiconducting phase. The local CDW instability appears to manifest itself even in the metallic phase, giving rise to various precursory effects.

A new mercury-based superconductor: (Hg, Cr)Sr2CuOy

Abstract A new mercury-based cuprate superconductor, (Hg 1- x Cr x )Sr 2 CuO y (0⩽ x ⩽0.8), having T c of 58 K ( x =0.3 has been synthesized. While a chromium-free sample ( x =0) was multi-phased and the superconducting volume fraction was less than 1%, samples for x =0.3 and 0.4 were of nearly single phase and their superconducting shielding volume fraction reached 100% of perfect diamagnetism. X-ray diffraction analysis revealed that its crystal symmetry was tetragonal with lattice parameters a 0 =3.840−3.854 A and c 0 =8.639−8.705 A which slightly varied with x . The observed c 0 value, corresponding to the interlayer distance of superconducting CuO 2 planes, was found to be the shortest among all the known mercury-based “12( n -1) n ” superconductors. Compared to the barium-containing compounds, the present barium-free materials have been found to be insensitive to the ambient air atmosphere during the synthesis and also to be quite stable after storage in air.

Strong flux pinning up to liquid nitrogen temperature discovered in heavily Pb-doped and oxygen controlled Bi2212 single crystals

Abstract Heavily Pb-doped Bi2212 single crystals with carefully controlled oxygen compositions were prepared and their magnetization curves were studied as a function of carrier doping level from heavily overdoped (Tc = 67 K) to near optimally-doped (Tc = 96 K) states. All the Pb-doped samples showed much improved Jc-H properties compared to the conventional Pb-free Bi2212 crystals. In particular, the moderately overdoped crystals maintained high Jc and Hirr up to high temperatures and these values surpassed those for the Pb-ion irradiated Bi2212 with an optimized fluence. Dramatically improved flux pinning properties in the present material are possibly due to generation of temperature and field-induced new pinning centers, as suggested by anomalous dependence of Jc both on temperature and magnetic field.

Charge transfer associated with iodine intercalation in Bi2Sr2Can−1CunOy, n=1−3

Abstract Intercalation of iodine into the Bi 2 Sr 2 Ca n -1 Cu n O y ( n =1,2,3) superconductors has been studied. A variety of measurements indicate charge transfer is associated with the intercalation process, both directly to the BiO layers and also to the superconducting CuO 2 sheets. Changes in T c on intercalation can be understood in terms of hole doping to the CuO 2 planes. Hall coefficient measurements on a single crystal n =2 sample supported an increase in hole concentration, and an XPS study clearly showed the ionic character for the intercalated iodine, accompanied by changes in the Fermi level. Investigation of the c -axis resistivity, also on n =2 single crystals, indicated a change from the semiconductive behaviour of the parent crystal to an apparently metallic character, though the c -axis resistivity remained high.

Fine crystal lattice fringes observed using a transmission electron microscope with 1 MeV coherent electron waves

A transmission electron microscope with a 1 MeV cold field-emission electron source has been developed for coherent and penetrating electron waves. We confirmed the coherence and overall stability of the microscope by observing Au(337) lattice fringes. These fringes have a 0.498 A spacing.

Bulk superconductivity in both tetragonal and orthorhombic solid solutions of (La1−xSrx)2CuO4−δ

Comparative measurements of low-field Meissner and shielding signals of both powder and sintered pellet samples of (La[sub 1[minus][ital x]]Sr[sub [ital x]])[sub 2]CuO[sub 4[minus][delta]] have revealed that bulk superconductivity is present over a wide range of Sr composition, 0.0425[le][ital x][le]0.125, extending over the orthorhombic-to-tetragonal phase boundary at [ital x][congruent]0.105, except for a narrow anomaly region [ital x]=0.0625. Samples were prepared by three different methods through a combination of spray-drying or powder-mixing preparation with either slow cooling or quenching from the sintering temperature of 1100 [degree]C down to 500 [degree]C, at which oxygenation annealing was performed. Homogeneous solid solutions were obtained only for specimens prepared by spray drying and slow cooling. Dependences of the Meissner and shielding signals on the Sr content, on the intensity of magnetic field, and on the sample size were examined in terms of pinning effect and penetration depth.

Growth and properties of pure C60 single crystals from vapor

Abstract C60 single crystals free from solvent contamination were grown from its vapor up to a size of 0.5×1.2×1.6-2mm3 by a method of periodic oscillation of the crystal temperature. The heat capacity of the crystal exhibited a sharp peak at 260 K with hysteresis of less than 1.2 K. The XRD patterns from room-temperature to 50 K show a transition of structure from face-centered cubic to simple cubic at 260 K. Values of 14.5 ± 0.6 and 17.5 ± 0.5 for the Vickers hardness of C60 crystals on the (100) and (111) faces, respectively, indicated that C60 is a very soft material.

Anisotropy of Bi2Sr2CaCu2O8+y revisited

Abstract The anisotropic superconductivity in Bi 2 Sr 2 CaCu 2 O 8+ y is investigated by precision measurements of resistivity and torque as a function of the magnetic field angle in the reversible regime. Scaling of the angular dependences of these quantities with an effective field, H (sin 2 θ+γ −2 cos 2 θ) 1 2 , θ being the field angle measured from the basal plane, suggests values of the anisotropy parameter γ≈200 or larger, which substantially exceeds previous estimates. In the small θ region, the effect of a layer gains importance.

DC susceptibility of type-II superconductors in field-cooled processes

Abstract The DC susceptibility of oxide superconducting specimens in the field-cooled process has been experimentally found to depend not only on the applied DC magnetic field but also on the size of the specimens. The DC susceptibility is calculated using the critical state model in which the diamagnetism and the flux-pinning effect of superconductors are taken into account. It is shown that the saturated value of the DC susceptibility at sufficiently low temperatures, i.e., the so-called Meissner fraction, decreases with increasing DC field and/or increasing specimen size.