Hiroyuki Sasakura

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.

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.

PREPARATION OF SUPERCONDUCTING 1201 PHASE IN THE (PB1-YCUY)SR2-XLAXCUOZ SYSTEM

An almost-single-1201-phase sample has been synthesized in the (Pb1-yCuy)Sr2-xLaxCuOz system. The sample showed the superconducting transition with the onset temperature of about 36.5 K and a zero resistivity temperature of about 31.5 K. These values were about 7 K higher than those reported by Adachi et al. The X-ray powder diffraction study showed that the almost-single phase was attained at the nominal composition of x=0.9 and y=0.5, and the crystal structure had a tetragonal symmetry with a=0.3769 nm and c=0.8697 nm.

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.

Characteristics of Holes Investigated by ESR in High-Tc phase of Bi(Pb)-Sr-Ca-Cu-O System

Measurements of nonresonant and ESR absorption are made in the high-Tc phase of the Bi(Pb)-Sr-Ca-Cu-O system. Two components of ESR signals have been detected above the superconducting transition temperature Tc. One is anisotropic and the other is isotropic. The isotropic spectrum is not considered to arise from impurities. The nonresonant microwave absorption near zero magnetic field is greatly enhanced at Tc and the ESR signals are almost completely masked. The origin of the ESR is discussed in relation to doped holes.

New Pb-Based Cuprates with 1222 Structure Having No Tetravalent Ions, (Pb0.5Cu0.5)Sr2-xLn2-yLax+yCu2Oz (Ln: Gd, Dy, Ho and Er)

New Pb-based cuprates with 1222 structure have been synthesized in (Pb0.5Cu0.5)Sr2-xLn2-yLax+yCu2Oz (Ln:Gd, Dy, Ho and Er) systems having no tetravalent ions such as Ce4+ and Pr4+. The composition to obtain an almost-single phase sample for each system was found to be located within a narrow limited region. After oxygen annealing at 500°C for 100 h under partial pressure of 100 atm, all the almost-single-phase samples were semiconductors, but a sample with the nominal composition of (Pb0.5Cu0.5)Sr1.45Gd1.95La0.60Cu2Oz among them showed a diamagnetic signal starting from about 20 K.

Superconducting 2201 phase in the Bi2PrxCa2−xCuOz system

A new Sr-free Bi cuprate superconductor has been synthesized in the Bi2PrxCa2-xCuOz system. The superconducting phase was isostructural to the 2201 phase (Bi2Sr2CuOz). The almost-single phase was attained at the nominal composition of x=0.7~0.8. The sample with x=0.7 showed a superconducting onset at about 12 K and zero resistivity at about 6 K. The crystal structure had a pseudotetragonal symmetry with a=0.5381 nm and c=2.381 nm.

Reappearance of ESR in the Bi(Pb)-Sr-Ca-Cu-O System in the Superconducting State

The electron spin resonance of Cu2+ ions in the high-Tc phase of the Bi(Pb)-Sr-Ca-Cu-O system has been confirmed to appear above Tc. Two components of ESR, namely, the relatively sharp anisotropic and the very broad isotropic ones, are observed in accordance with a previous result. Both of these spectra apparently disappear at the transition temperature Tc. However, the anisotropic spectrum reappears at a temperature well below Tc. The result is discussed in relation to the layered structure of the high-Tc superconductor with three sheets of a CuO2 plane.

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).