Yoshio Muto

Critical currents at 77.3 K under magnetic fields up to 27 T for an Y‐Ba‐Cu‐O film prepared by chemical vapor deposition

Critical currents of an Y1Ba2Cu3O7–δ film prepared by a chemical vapor deposition technique have been measured resistively. Excellent critical current properties in high fields up to 27 T are obtained at 77.3 K. The value of Jc is 4.1×105, 1.9×105, and 6.5×104 A/cm2 at 2, 10, and 27 T, respectively. The upper critical field defined by zero resistivity is estimated to be 35 and 180 T at 77.3 and 0 K, respectively. A peak effect in the critical current is observed at high fields.

Y‐Ba‐Cu‐O superconducting films prepared on SrTiO3 substrates by chemical vapor deposition

High Tc superconducting YBa2 Cu3 O7−x films were prepared by chemical vapor deposition using β‐diketonates of Y, Ba, and Cu as source materials. The films were epitaxially grown on SrTiO3 (100) substrates. The c‐axis orientation perpendicular to the plane of the substrates was observed. A sharp resistive superconducting transition with Tc (R=0) at 84 K was obtained. The maximum critical‐current density at 77 K was 2×104 A/cm2 .

Synthesis of bulk high Tc superconductors of TlBa2Can −1CunO2n + 3 (n = 2 − 5)

Abstract Bulk specimens of high Tc superconductors of TlBa2Can − 1CunO2n + 3 (n = 2 − 5) have been synthesized by heat treatment up to 10 hours. Crystal structures were analyzed by X-ray powder diffraction and high resolution electron microscopy, and superconducting properties were determined by resistivity and DC susceptibility measurements. Tc increases with increasing number of Cu layers up to n = 4, where it turns down.

Strong flux pinning centers in Y‐Ba‐Cu‐O films prepared by chemical vapor deposition

The Y‐Ba‐Cu‐O film prepared by chemical vapor deposition showed Tc=91.5 K and Bc2 (77.3 K)=60 T defined by zero resistance. The Jc values measured at 77.3 K were 7.8×105 A/cm2 at B=0 and 1.0×105 A/cm2 at 16 T, magnetic fields perpendicular to the c axis. Small disk‐shaped precipitates possibly regarded as strong flux pinning centers in the Y‐Ba‐Cu‐O films were observed using transmission electron microscopy. The evaluation of Jc related to the concentration and configuration of the precipitates was in reasonable agreement with the measured Jc values.

X-Ray and electron microscopic study of a high temperature superconductor Y0.4Ba0.6CuO2.22

A superconducting oxide Y0.4Ba0.6CuO2.22 with Tc higher than 88 K was synthesized. The crystal structure was found to be a derivative of the perovskite structure with orthorhombic unit cell dimensions of a=3.818 (1) A, b=3.888 (1) A, c=11.667 (4) A, and V=173.19 (8) A3. Basic unit cell of the cubic perovskite structure was tripled along the c axis, probably due to the ordering of Y3+ and Ba2+ ions and/or of oxygen vacancy. Apparent short bond length for Cu-O may partly be explained by oxygen vacancy amounting to a quarter, but the observed high Tc suggests actual contraction of Cu-O bonds.

Temperature and angular dependences of upper critical fields for the layer structure superconductor 2H-NbSe2

The temperature and angular dependences of upper critical fieldsHc2have been measured for several 2H-NbSe2 single crystals by use of an electrical conduction method in magnetic fields up to 150 kOe. As the temperature approaches the transition temperatureTc, the value ofHc2‖(parallel to the layer planes) decreases with a positive curvature, while the value ofHc2⊥(perpendicular to the layer planes) decreases almost linearly. The ratio ofHc2‖toHc2⊥increases monotonically from 2.4 nearTcwith decreasing temperature and reaches the constant value of 3.2 at the lowest temperature. It becomes clear that the simple effective mass model based on the anisotropic Ginzburg-Landau theory does not explain our experimental results. The anisotropic behavior ofHc2can be accounted for by the Takanaka theory, which includes anisotropies of both the Fermi velocity and the energy gap and the effect of nonlocality. Agreement between experimental results and the theoretical prediction is obtained by the use of values of 0.16≲ε12≲0.25 and −0.6≲ε2≲−0.3, where ε1 is the mass anisotropy parameter and ε2 the gap anisotropy parameter. The coupling strength between layers is too strong to be explained by the Josephson phase coupling model proposed for quasi-two-dimensional layer superconductors.

Preparation and Chemical Composition of Superconducting Oxide Tl2Ba2Can-1CunO2n+4 with n=1, 2 and 3

Superconducting oxides of Tl2Ba2Can-1CunO2n+4 with n=1, 2 and 3 were synthesized, and their structure and chemical composition were examined by X-ray powder diffraction, EPMA, Rietveld analysis and electron microscopy. Partial substitution of Tl with Ca was confirmed, but the production of holes in Cu layers expected from this substitution was rather small for a 120 K superconductor. Charge transfer of Tl3-δ-Cu2+δ would provide both the hole in the Cu-O bond required for high Tc and s-like electron of Tl atoms for stabilization of layered structures.

Superconductivity of high-Tc TlBa2Ca1−xRExCu2O7 (RE=Nd, Gd or Y) system

Abstract T c and the unit cell dimensions of TlBa 2 Ca 1− x RE x Cu 2 O 7 (where RE is Nd, Gd or Y) have been investigated for a wide substitution range. The tetragonal unit cell dimensions c and a decreased and increased, respectively, with the substitution of Ca with RE. An as-prepared specimen of TlBa 2 CaCu 2 O 7 with T c =80 K was found to be in an over-doping state and T c increased to 100 K with increasing x up to x =0.2-0.35, where it began to decrease and became nonsuperconducting at x =0.4-0.5.

Anisotropy and hysteresis of transport critical currents in high temperature Ln-Y-Ba-Cu-O superconductors

Abstract Following the measurements of anisotropy and hysteresis in transport critical currents with changing temperature and polarity of magnetic field, a.c. magnetic measurements by the Campbell method were performed for sintered Ln-Y-Ba-Cu-O superconductors. The information derived from them indicated that hysteresis does not occur in the intragrain current but in the intergrain one. The results are interpreted in terms of flux pinning, where the anisotropy originates from the texture structure and hysteresis is associated with the superconducting weak links of the sintered oxide pellets.

Anomalously large negative magnetoresistance of 1T-TaS2 at ultra low temperatures

Abstract The magnetoresistance of 1 T -TaS 2 in the commensurate CDW phase was measured from 8 K down to 50 mK. Very large negative magnetoresistance was observed below 0.5 K for both transverse and longitudinal configurations of the magnetic field with respect to the current. This result is explained by Fukuyama and Yosidas theory based on the variable-range hopping in the Anderson localized states.