Hiromasa Mazaki

High-Tc Phase Promoted and Stabilized in the Bi, Pb-Sr-Ca-Cu-O System

Partial substitution of Pb for Bi in the Bi-Sr-Ca-Cu-O system has been found to sharply increase the volume fraction of the high-Tc phase when both the starting material (coprecipitated oxalate being used in the present study) and the heating process are appropriate. The sharp powder X-ray diffraction pattern obtained from well-grown particles, 5~10 µm wide and 0.5 µm thick typically, was assigned to an orthorhombic cell with a=0.537 nm, b=2.682 nm, and c=3.726 nm. The electrical resistance dropped to zero at 107 K within the experimental limit of 10-6 Ω. A large diamagnetic response in the ac susceptibility due to the Meissner effect was seen below 120 K. The dominance of the high-Tc phase over the low-Tc phase was roughly estimated at 9/1 in volume.

Single-Crystal YBa2Cu3O7-x Thin Films by Activated Reactive Evaporation

By means of the activated reactive evaporation, YBa2Cu3O7-x single-crystal thin films with the c axis perpendicular to the substrate plane have been directly and epitaxially grown on the (100) surface of SrTiO3. The substrate temperature was kept below 600°C and the oxidation treatment was performed at below 500°C. The ac-resistive measurement for a film with 2000 A thickness gave a sharp superconducting transition, where the endpoint was 90.2 K and ΔTc(10–90%)=1.7 K. From the complex susceptibility measurement we confirmed the Meissner effect as well as the structural uniformity of the specimen.

Superconducting transition of multiconnected Josephson network

The response of superconducting Tc with multiconnected structure against a small ac magnetic field is studied in terms of complex susceptibility. It has been revealed that the fundamental susceptibility χ1′−iχ1″ is very sensitive to the amplitude h0 of the ac field; i.e., the superconducting transition width is appreciably broadened as h0 increases, but the onset temperature of the transition holds its value. In addition, χ1″ with respect to temperature has an asymmetric peak. We propose a phenomenological model on the assumption that the multiconnected network behaves like a single loop as a whole due to the coherent nature of the specimen. It has been revealed that the main features of observed characters are well reproduced by this model. Since our model predicts higher harmonics in susceptibility, the third harmonic response of the specimen against a small ac magnetic field is also studied. Measurements with respect to temperature are simultaneously performed for the amplitude of third harmonic suscep...

Polymerized complex synthesis and intergranular coupling of Bi‐Pb‐Sr‐Ca‐Cu‐O superconductors characterized by complex magnetic susceptibility

Superconducting Bi‐Pb‐Sr‐Ca‐Cu‐O ceramics have been synthesized by the polymerized complex method at the Bi1.8Pb0.2Sr2Ca2Cu3Oy, Bi1.6Pb0.4Sr2Ca2Cu3Oy and Bi1.68Pb0.32Sr1.75Ca1.82Cu2.83Oy compositions. The polymerized complex method includes three important major steps: (1) formation of stable citrate complexes uniformly dispersed into ethyleneglycol and water, (2) fabrication of polymerized complexes which are formed through dehydration (or esterification) between ethyleneglycol and metal‐citrate complexes, and (3) pyrolysis of the polymerized complex precursor and the subsequent heat treatments for producing the Bi‐Pb‐Sr‐Ca‐Cu‐O superconductor. The present technique offers more convenient and easier fabrication of (Bi,Pb)2Sr2Ca2Cu3Oy superconductors compared with other more elaborate preparation procedures based on wet‐chemical techniques, as it eliminates many steps such as ageing, filtration, washing, and pH control. ‘‘X‐ray‐clean’’ (Bi,Pb)‐2223 superconductors have been obtained for all the three comp...

The High-Tc Phase with a New Modulation Mode in the Bi, Pb-Sr-Ca-Cu-O System

The preparation and characterization of Bi1.73Pb0.35Sr1.89Ca1.86Cu3O9.91, a lead-rich high-Tc phase, were studied (Tc(ρ=0)=105 K). This compound crystallized in a pseudotetragonal structure with at=0.541 nm and ct=3.709 nm, while a unique one-dimensional modulation with a wavelength of 4.7 nm (≃8.7a) was found along the b axis using transmission electron microscopy. Such a lead-rich phase and a lead-free (or very poor) phase crystallizing in the well-known modulated structure (2.7 nm≃5a) were intergrown at intermediate compositions, such as Pb/Bi=1/9. Effects of annealing under different oxygen pressures were studied.

Complex Susceptibility of YBa2Cu3O6.86

The superconducting transition of YBa2Cu3O6.86 has been studied in terms of complex susceptibility. It is found that the transition has two phases. One is insensitive to the change in the amplitude, h0 (5-1000 mOe), of the external ac field. The other is very sensitive to h0, where the transition width is appreciably broadened as h0 increases. No frequency dependence is found below 320 Hz. These behaviors are qualitatively common to the other oxide superconductors, La1.9Sr0.1CuO4-x and ErBa2Cu3O6+x. Discussion is made in the framework of the phenomenological weak-link loop model.

High-quality ceramics of YBa2Cu4O8 from citrate sol-gel precursors sintered at one atmosphere oxygen pressure

Abstract Single phase ceramic material of the superconducting phase YBa2Cu4O8 has been synthesized at 780–790°C under one atmosphere oxygen pressure with a sol-gel powder precursor technique. The precursor was derived from citrate metal ion complexes uniformly dispersed in a solvent mixture of ethyleneglycol and water. The main advantages of the present method are, apart from the low oxygen pressure needed, the elimination of centrifugation, ageing and fine control of pH usually required for other solution techniques. X-ray diffraction analysis show that the material is virtually of single-phase character with only minor traces of CuO (∼1%). AC-susceptibility measurements indicate a high-quality sample with an onset of the superconducting transition at Tc=82.6 K. The superconducting transition is much sharper, ΔTc(10–90%)=4.5K, than for ceramic samples of previously reported synthesis procedures.

Optical absorption spectra of single-crystal YBa2Cu3Oy films

Abstract Optical transmittance spectra of single-crystal YBa 2 Cu 3 O y thin films ( y =6.0, 6.4, 6.8) were measured in a wavenumber range of 1500–50000 cm −1 (0.2–6 eV) at room temperature. The electric field of incident light was parallel to the Cu-O planes. An absorption spectrum for each oxygen content was obtained from the transmittance spectra of two samples with different thicknesses. In YBa 2 Cu 3 O 6.0 , an energy gap at 1.5 eV and an absorption peak at 4.1 eV were observed. When the oxygen content increases, the absorption within the energy gap increases and the 4.1 eV absorption peak becomes smaller. We found that the dependence of the 4.1 eV peak intensity on the oxygen content correlates with that of the absorption within the 1.5 eV gap. Some discussion is given in connection with hole doping.

Polymerized complex synthesis of a pure 93 K Y2Ba4Cu7O15−d superconductor without the need of high oxygen pressure and additive catalysts

High‐purity ceramic material of the superconducting phase Y2Ba4Cu7O14.82 (247) has been synthesized at 870 °C by the polymerized complex method using neither high oxygen pressure nor additive catalysts. The method is based on the formation of a polymer‐metal complex precursor which is prepared through polyesterification between metal citrate complexes and ethylene glycol. Apart from the fact that high oxygen pressure is unnecessary, the present preparation technique offers more convenient and easier fabrication of highly pure 247 material compared with other wet chemical routes since it eliminates many steps such as centrifugation, filtration, aging, and pH control. X‐ray diffraction and Raman scattering analyses show that the material is single‐phase without any indication of secondary phases. Zero‐resistance has been achieved at 88.0 K with a transition width narrower than 4 K. Complex ac magnetic susceptibility measurements confirm the presence of a single bulk superconducting 247 phase with Tc(onset)=...

Complex Susceptibility of Bi2Sr2Ca1-xYxCu2Oy from Citrate Sol-Gel Precursors

In order to examine the dependence of the superconducting property of the double Cu-O layer system on the hole concentration, we measured the superconducting transition of Bi2Sr2Ca1-xYxCu2Oy (0?x?0.4) compounds in terms of complex susceptibility, ?=??-i???. The samples were synthesized by a citrate sol-gel precursor technique. The main advantages of this method are the shorter reaction time, the elimination of some steps such as centrifugation, and a fine control of pH. X-ray diffraction analysis showed that each sample has a single phase, although ? for x=0.1 indicates the existence of different superconducting phases. It has been revealed that the superconducting transition temperature Tc apparently depends on the hole concentration, and the highest Tc (95.2 K) was achieved for around x=0.1. The profiles of ? for five samples with different values of x are discussed.