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-T c 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-T c phase over the low-T c phase was roughly estimated at 9/1 in volume.

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 compositions after sintering at 850 °C in air for 94–235 h, depending upon the nominal composition of the starting batch. Noticeable is that a small deficiency of Sr, Ca, and Cu relative to the ideal stoichiometric composition of the (Bi,Pb)2Sr2Ca2Cu3Oy significantly accelerates the formation of the (Bi,Pb)-2223 phase. Complex magnetic susceptibility measurements show that Tc(onset) of the bulk (Bi,Pb)-2223 phase remains almost constant at around 108 K, independent of the compositions. However, it is stressed that low-level and/or amorphous impurity phases possibly concentrated on grain boundaries can have dramatic effects on the overall magnetic properties even for materials which look like very ‘‘clean’’ by x-ray diffraction. The best superconducting properties were obtained with samples with a nominal composition of Bi1.6Pb0.4Sr2Ca2Cu3Oy after sintering in air at 850 °C for 180–280 h. The onset temperature of superconducting transition obtained by complex susceptibility was 108.0 K and the transition width (10%–90%) of full diamagnetism was 5.2 K for a measuring field of 100 mOe.

Using V/Ag superconducting multilayers prepared by vacuum evaporation we have observed dimensional crossover from anisotropic three-dimensional to two-dimensional behavior in terms of the upper critical field versus temperature.At a higher temperature we have also observed an anomalous turning in the curve of the critical field, which implies the existence of a commensurability effect between the lattice of vortices and the periodic superstructure.Superconducting layered compounds such as transitionmetal dichalcogenides and their intercalation complexes have aroused interest because of their large anisotropy and the possible quasi-two-dimensional characteristics.Viewed from the upper critical field H, 2, the anisotropy is caused by the different superconducting coherence lengths g& and g~~, perpendicular and parallel to the layer. In particular, if gq becomes comparable to the layer repeat distance, unusual behavior not seen in conventional superconductors could be expected.Theoretically, the system composed of Josephson-coupled superconducting layers was initially investigated in the Ginzburg-Landau regime. ' This investigation revealed that near the transition temperature, the system behaves like an anisotropic three-dimensional superconductor.However, the theory advanced by Klemm and co-workers4 sho~ed that when interlayer coupling is appropriate, the upper critical field parallel to the layer (H, 2~~) drastically increases at a certain temperature, and it shifts to the value typical of two-dimensional superconductors. This is called "dimensional crossover. "Recently, as a model system of layered structure, synthetic layered superconductors, which are prepared by alternate deposition of two different elements, have been developed, 5 i4 and systematic experimentation to examine this unusual behavior has become possible. "' Ruggiero, Barbee, and Beasley" studied Nb/Ge superlattices and confirmed the temperature-dependent dimensional crossover expected in Josephson-coupled quasi-two-dimensional superconductors. Using Nb/Cu superlattices, Schuller and his co-workers'6 observed the dimensional crossover, and pointed out that the qualitative ideas put forward in theories of Josephson-coupled superconductors are valid also for proximity-coupled ones.In the present work, V/Ag proximity-coupled superlattices have been investigated by means of upper critical field measurements. Observations showed that dimensional crossover clearly appears in H, 2))~I n addition, at a higher temperature, we also found anomalous turning in the curve of H, 2~i, depending on the period of multilayer.Multilayered V/Ag films were prepared in an ultrahighvacuum deposition system, where two electron guns alternately evaporate V and Ag sources. The pressure during deposition is in the range of 10 9 torr and the deposition rate is about 0.3 A/s. Individual thicknesses are regulated with shutters, located in front of each source and controlled by thickness monitors. We prepared 13 samples with different V-to-Ag thickness ratios (1:1, 1:2,2:1,3:I)...