Y-Ba-Cu-0 System

After this manuscript was completed, another preprint arrived. In this study, Mohanran et al. found for YBa2Cu 0y and for Y 05 al.95 u3°7 (T) curves and type 1 (optimum superconductivity) ani type z (presence of the semiconductor component) respectively and additionally for Yq cl Cu Oy they found a variation in which R(T) decreases monotonically with decreasing temperature towards zero with no abrupt decrease. This behavior, which Mohanran et al. found to be curious and unique, is part of the model described herein. The variation in R(T) corresponds to that shown by curve d in Figure 5, i.e., the curve dominated by the semimetallic component. Hence, for the Y-Ba-Cu-0 system, curves which span the range from that for a semiconductor (curve a) to that dominated by the semimetal (curve d) have been observed. The observation of the existence of the two end members in which pair formation is negligible demonstrates the applicability of the model, and thereby makes the existence of the intermediate behavior (curves b and c) plausible, even though the behavior is not observable because it is accompanied by a more abrupt increase in a(T) through the conductivity attributable to the pairs. 

The composition, structure and superconducting properties of high Tc oxide superconductors, La-Sr-Cu-0 and Y-Ba-Cu-0 systems have been investigated. From comparative studies of effects of partial replacements of metal sites by other elements, a low dimensional nature of conduction path was suggested in both oxide systems. Critical relevance of oxygen deficiency and the conditions of heat treatments to the evolution of superconductivity, is discussed in terms of the relatively large nonstoichiometry found in these systems. 

Then the substitution on the La sites led Wu et al. (8) to find another superconductor, Y-Ba-Cu-0 system, and finally to overcome a long-waited technological barrier of liquid nitrogen temperature in February 1987. Independent discoveries of the same system were also reported (9-10) in a short period. The identification of the superconducting phase as Ba2YCu30y, a new crystal structure, was performed by Siegrist et al.(ll) and further refined through neutron diffraction studies (12-13). 

Replacement of Ba by Sr in the case of the La-Ba-Cu-0 system was thus favorable in decreasing aQ and increasing Tc. However the same is not true in the case of the Y-Ba-Cu-0 system. Figure 2 shows the orthorhombic lattice parameters of aQ, bo and cq for (Ba x rx)2 u3 7 f°r x=0 to 0.7. It is clearly seen that these parameters all decrease with Sr composition, x. Figure 3 shows the resistivity vs. temperature of these specimens. Quite unexpectedly from the discussions up to this point, Tc in this particular system decreases with increase in x. Although the difference in these two examples is not clear yet, it should be quite essential in understanding the mechanism of the superconductivity in the both systems. 

In the case of the Y-Ba-Cu-0 system, 4f magnetic ions may not be considered to be impurities as far as the superconductivity is concerned. As many investigators have discovered(18-24), most of the rare earth elements give 90 K superconductors without any appreciable difference in Tc. Kishio et al. (25) have shown that 90 K conductors can be also successfully prepared even from certain compositions of unseparated mixture of rare earths as starting materials. 

All of the effects of replacements of each metal site so far described can be understood within a framework of a low dimensional nature of conduction mechanisms. More specifically, superconduction takes place through the O-Cu-O bonds of the interconnected CuOft octahedra or CUO4 squares in the La-Sr-Cu-0 and Y-Ba-Cu-0 systems, respectively. The La or Y sites are less sensitive for the presence of magnetic impurities and seem to be rather away from the superconduction path. This tendency is more significant in the Y-Ba-Cu-0 system, because the conduction path would be along the one dimensional chains of O-Cu-O bonds which are most apart from the Y sites. 

Figure 3. Continued. Compositions of Y-Ba-Cu-0 system and summary of results for samples prepared at (c) 1200 °C.

The discovery by Wu et al. (1) of superconductivity at 9SK in a mixed-phase sample in the Y-Ba-Cu-0 system has stimulated an unprecedented amount of research effort directed at solving the structures of the phase(s) responsible for superconductivity. In this paper we will present the results of the first single-crystal structure analyses of the phases present in these samples to be carried out (2), and then review the additional structural information now available in relation to the previously determined structures of other oxygen defect perovskites. 

Preparation of the Bi-Sr-Ca-Cu-0 superconductors is essentially the same as in the Y-Ba-Cu-0 system, except the calcination temperature. The Bi (2223) phase is formed by firing at 850°C for 200 h in air atmosphere. To shorten the firing time, the conventional practice is to replace 10-20 atom% Bi by Pb. The replacement induces the formation of liquid-phase CaaPbOa, which accelerates the solid-state reaction. Lead atoms are incorporated in the Bi sites. The Bi (2212) phase is more stable than the Bi (2223) phase and is formed by calcin-... 

A good example of the power of synchrotron source radiation is found in the study of the thin-film superconductors that are required for applications of high-temperature superconductors (HTSC) in microelectronics technology [77]. An example of such work [78] involves the analysis of HTSC films produced by laser evaporation of elements in the Y-Ba-Cu-0 system. Ten films were simultaneously deposited with various target-to-substrate distances allowing study to be made of the laser plasma expansion in vacuo. A very important advantage of this method is that it allows extremely low detection limits to be achieved. As an example, in the referenced study [78] the authors claim a detection limit of 5 x lO atoms/cm. ... 

The Y-Ba-Cu-0 system is extensively studied because the YBa2Cu307 phase displays superconducting properties. The complete study of the quaternary system requires knowledge of the limiting binary systems, in particular of the Y-Ba and Y-Cu alloys. Studies are now being performed to obtain thermodynamic data for these alloys. 

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