Nobuo Fukuoka

Nuclear Quadrupole Resonance of Cu in High-Tc Bi–Pb–Sr–Ca–Cu–O

Nuclear quadrupole resonance (NQR) signals of Cu have been discovered in the high- T c superconducting oxide Bi–Pb–Sr–Ca–Cu–O with T c =109 K. It has been observed that there are two Cu sites with a different quadrupole coupling. The temperature dependence of the nuclear relaxation rate, 1/ T 1 , was similar to that of the CuO 2 plane in YBa 2 Cu 3 O 7 . It is found that the 1/ T 1 for the Cu site demonstrates the reflection of the strong Cu-d spin correlation.

Direct Observation of 18O Tracer Diffusion in a YBa2Cu3Oy Single Crystal by Secondary Ion Mass Spectrometry

Direct observation of 18O tracer diffusion by secondary ion mass spectrometry (SIMS) was made on a single crystal of YBa2Cu3Oy with an array of steps on the stairlike surface. The unique distribution of the diffused 18O in the ab-plane and in the c-axis direction was shown visually as a mapped image, which revealed some fundamental processes on the oxygen penetration through the surface and diffusion in the inner crystal. It was found that at 530°C, the diffusion coefficient in an ab-plane is larger than that in a c-axis direction by more than five orders of magnitude.

The Phase Change Due to Oxygen Content in the LnBa2Cu3Oy Compound Systems (Ln=Tm, Gd, Sm)

The phase change due to oxygen content y (7>y>6) was investigated for TmBa2Cu3Oy (TBC), GdBa2Cu3Oy (GBC) and SmBa2Cu3Oy (SBC) by means of X-ray diffraction and electrical resistivity. The phase boundary between orthorhombic and tetragonal phases moved toward the oxygen-rich side when the system changed as TBC→YBa2Cu3Oy(YBC)→GBC→SBC. The plateau in the critical temperature Tc was observed in the intermediate region of 6.7>y>6.4 for each system, supporting the existence of an orthorhombic-II phase. It seemed to exhibit a system dependence, i.e., a decrease in Tc in the order of TBC, YBC, GBC and SBC.

Superconductivity in the Pb2Sr2-xLaxCu2Oz System

Superconductivity has been discovered in the new Pb-based copper oxide system of Pb2Sr2-xLaxCu2Oz. The maximum onset and zero resistivity temperature was about 38 K and 33 K, respectively. The ceramics had a tetragonal symmetry with a=0.5422 nm and c=1.2695 nm, which became an almost-single phase in the composition x=1.0.

ESR of CuO Particles in a Bi2(Sr, Ca)3Cu2Oy Crystal

ESR measurements were performed at the X-band for a Bi2(Sr, Ca)3Cu2Oy crystal over the temperature range from 77 K to room temperature, and a clear ESR line of superparamagnetic particles of CuO was observed with the external field of ca. 3 kOe. The resonance field of the absorption peak became a minimum at 230 K, at which point the absorption intensity became a maximum. Furthermore, an anisotropy of the resonance field was observed for another broad absorption. Possible interpretations for the results are discussed in terms of magnetic anisotropies in CuO particles.

Anisotropic oxygen diffusion in YBa2Cu3Oy single crystal observed by sims

Abstract Oxygen diffusion in a single crystal of YBa 2 Cu 3 O y was studied using 18 O tracer and secondary ion mass spectrometry (SIMS). The samples used had a flaky configuration with a terraced surface. The three dimentional distribution of the 18 O atoms in the spacimens were observed after an annealing process at 530°C and 450°C in 18 O 2 gas of 1.3×10 3 Pa by SIMS. The diffusion coefficients in the ab -plane, D ab , and along the c -axis, D c , were determined simultaneously using a single sample. These values were larger at the higher annealing temperature, and agreed with results reported by previous researchers. The values of D ab / D c were about 5×10 5 at both temperatures.

Surface Analysis of Single Crystals in the Bi–Sr–Ca–Cu–O System by Low-Energy Electron Diffraction and Auger Electron Spectroscopy

The cleaved surfaces of low-Tc-phase single crystals in the Bi-Sr-Ca-Cu-O system were investigated by means of the LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy) techniques. The LEED patterns from the cleaved surfaces exhibited a modulated structure with a period of about 4.8a, as expected from the modulation in the bulk crystals. The incident angular dependence of the Auger signal intensity revealed that the crystals were exactly cleaved between the Bi double layers. Through detailed analysis of the dependence based on a model calculation, it was found that oxygen was absent or strongly deficient on the cleaved surfaces. It was also suggested that the weak bond of Bi-O is the principal reason for the cleavability between the Bi double layers.

Superconductivity in the Bi2LnxCa3-xCu2Oz (Ln=La, Pr, Nd, Sm, Eu and Gd) Systems

A new family of oxide superconductors of Bi2LnxCa3-xCu2Oz (Ln=La, Pr, Nd, Sm, Eu and Gd) has been examined by means of the electrical resistivity and the X-ray diffraction methods. All the ceramics exhibited superconductivity, and the highest zero resistivity temperature obtained among them was about 45 K for Ln=Pr. The X-ray powder diffraction study showed that these ceramics had a similar crystal structure to that of the 2212 phase (Bi2Sr2CaCu2Oy) and that the single phase was almost attained at the nominal composition of x=0.5 (Ln=La, Pr, Nd and Sm) and 0.6 (Ln=Eu and Gd).

Microwave magnetoabsorptions of ErBa2Cu3Oy and HoBa2Cu3Oy single crystals

Abstract Microwave magnetoabsorption measurements were performed for single crystals of ErBa 2 Cu 3 O y and HoBa 2 Cu 3 O y in a temperature range from 3 to 80 K. Microwave magnetoabsorptions due to superconductivity were observed, and the principal values of the lower critical field were determined as 140 or 150 ± 10 Oe and 430 ± 10 Oe at 5 K for the ab -plane and the c -axis, respectively. Their temperature variations were investigated. Further, ESR signals of Er 2 BaCuO 5 and Ho 2 BaCuO 5 were observed, and their superparamagnetic properties were investigated.

Lower Critical Fields in Bi-2212 and Bi-2223 Superconductors

Low-field microwave magnetoabsorptions were observed for Bi-2212 and Bi-2223 specimens, and the lower critical fields were investigated. The principal values of Hc1 were determined over a temperature range from 4 to 60 K. Their temperature variations may be related to the low dimensional character of superconductivity.