Shigemitsu Nakanishi

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.

Some Optical Properties of CuCl Single Crystals

The fluorescence emission was investigated in single crystal specimens of CuCl at low temperatures. Strong emission lines of instrumentally limitted width are observed at wavelengths in the range of exciton absorption below 20°K. These lines are associated with recombination radiations from free and bound excitons with or without phonon cooperation, and they shift toward higher energy with increasing temperature as are observed in the exciton absorption bands. The average temperature coefficient of all the peak energy at low temperature is +2.5×10 -4 eV/°K. From the temperature dependence of the emission intensity, the activation energies of the various processes are estimated; the ionization energy of the exciton is 0.16 eV and the binding energies of the bound exciton complexes ranged between 0.03 and 0.007 eV.

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.

Orthorhombic-Tetragonal Phase Boundary in the LnBa2Cu3Oy Compound Systems (Ln=Tm, Gd, Sm)

The orthorhombic-tetragonal phase boundaries were determined for TmBa2Cu3Oy (TBC), GdBa2Cu3Oy (GBC) and SmBa2Cu3Oy (SBC) by means of high temperature (in situ) X-ray diffaction. To cover the wide temperature and compositin regions, two different methods previously reported in YBa2Cu3Oy (YBC) were employed. The results obtained for the TBC, GBC and SBC revealed the same characteristic in shape as that in the YBC: the phase boundary consisted of two curves being upwardly convex. The Rietveld refinement of phases appearing on the low temperature side was also made for the samples with various ys obtained by long-period annealing at low temperature. The percolation limit was confirmed to be located at the position proposed previously for these systems.

ESR and Nonresonant Microwave Absorption of ErBa2Cu3O7-δ and HoBa2Cu3O7-δ Single Crystals

ESR measurements were performed for ErBa2Cu3O7-δ and HoBa2Cu3O7-δ single crystals from 77 K to room temperature. The ESR signals of Er2BaCuO5 and Ho2BaCuO5 were observed, and their temperature variations were investigated. A nonresonant microwave absorption was also observed below the superconducting critical temperature of 93 K, and was investigated. The principal values of lower critical field were determined.

Pb-Amount Dependence of Copper and Oxygen Valence in Pb-Doped Bi-Sr-Ca-Cu-O Superconductors

X-ray photoelectron spectroscopic (XPS) studies are done on superconducting (Bi1-xPbx)2Sr2Ca2Cu3Oy (x=0-0.4) containing various volume fractions of the 80 K and 110 K phases. Systematic changes in the Cu 2p and O 1s core-level emission spectra are observed as a function of x. The analyses of these XPS spectra reveal that Cu+, Cu2+, and O (off-plane), O (on-plane) ions exist in the superconductors. With increasing volume fraction of the 110 K phases in the samples, the ratios of Cu+/Cu2+ and O (off-plane)/O(on-plane) increase.

ESR and Nonresonant Microwave Absorption of YBa2Cu3O7-δ Single Crystal

ESR measurements were performed for a single crystal of YBa2Cu3O7-δ over a temperature range from 77 K to room temperature. ESR signals of Y2BaCuO5 and CuO were observed and investigated. Nonresonant microwave absorption was observed below the superconducting critical temperature of 93 K and was also investigated. The principal values of lower critical field were determined.