Materials issues, superconductor superconductors

Discovery of the 90+ K Superconductor "Paul" Chu and coworkers at the University of Houston (during October 1986) carried out the synthesis of (La1.xBax)CuOs.y (Type I) and (La1.xBax)2 Cu04.y (Type II) compounds and isolated superconducting phases exhibiting a sharp decrease in resistivity at 32 K. The best materials, however, showed only a 2% Meissner fraction. By applying pressure to one such product, their forte in superconductor research, they observed an increase in transition temperature of 8 degrees at 14 kbar pressure (see Figure 29). Chu, et al., submitted (156) these results to Physical Review Letters on 15 December 1986, and the publication appeared in the January 26, 1987 issue. 

As for the current status of the researches on these materials, a rather comprehensive review was given by Bersuker in Sect. 8.4 of [3] from the standpoint of elucidating the roles of the JT effect. Therefore it would not be necessary to reiterate a similar kind of review here, particularly for the issue of HTSC for which Bersuker made a very detailed account, but it might be appropriate for us to make some supplementary comments or remarks on the issues of the CMR and the fullerene superconductors from our perspective that is reflecting the experience of one of the authors (Y.T.) who was engaged in the studies on those issues in 1990s. 

R. W. McCallum, J. D. Verhoeven, M. A. Noack, E. D. Gibson, F. C. Laabs and D. K. Finnemore, Problems in the Production of YBa2Cu3Ox Superconducting Wire, in Ceramic Superconductors, special issue of Advanced Ceramic Materials, ed. D. R. Clarke and D. W. Johnson, to be published July 1987. 

B. W. Veal, in Ceramic Superconductors, special issue of Advanced Ceramic Materials, ed. D. R. Clarke and D. W. Johnson, to be published July 1987. 

It is a great pleasure for me to introduce volume 44 of Annual Reports on NMR Spectroscopy and to welcome my longstanding co-worker, Professor I. Ando of the Tokyo Institute of Technology as co-editor. This is another in the series of special issues of Annual Reports on NMR Spectroscopy and covers advances in applications of NMR to materials science, an area previously visited in volume 28 of this series. Three quite distinct topics are included in this volume, namely Applications of NMR Spectroscopy to the Structure and Ionic Aggregates of lonomers by H. Yoshimizu and Y. Tsujita, Applications of NMR Techniques to Coal Science by K. Saito, K. Kanehashi and I. Komaki and An NMR Study of Strongly Correlated Superconductors by K. Asayama, Y. Kitaoka, G-q. Zheng, K. Ishida and Y. Tokunaga. 

It is safe to state, in conclusion, that progress with our understanding of issues related to improve flux pinning in R-123 superconductors has been breathtaking over the past decade, and will certainly remain in the focus of further research, particularly with respect to all micro- and nano-structural aspects of material properties and defect configurations. 

Spectroscopic measurements of the energy gap are based on the fundamental fact that absorption of radiation with frequency less than the magnitude of the gap is prohibited in a superconductor, and therefore the dissipative (real) part of the conductivity vanishes for 0) < 2A. A clear threshold in the conductivity spectra at tu = 2A is found only in dirty superconductors where 1/r > 2A. In clean materials where 1/r normal state, and unlike other spectroscopies where a sharp feature can be seen at the gap energy, there is little change in the overall optical properties at the gap frequency in a clean superconductor. A detailed discussion of this issue can be found in a review article by Timusk and Tanner (1989). 

Much of the interest in superconductor components revolves around the central issue that below a characteristic temperature, Tc, superconductors exhibit zero resistance to the flow of electricity figure 1 A). Above this temperature, the material behaves as a normal metal wherein isolated electrons (or holes) carry the charge with finite resistance. Below Tc, however, the electrons form loosely associated pairs which are responsible for all the superconducting properties. At temperatures close to Tc, only a minute fraction of the conduction electrons form the Cooper pairs (Figure IB). Under such circumstances, superconductivity is easily disrupt by heati light, and magnetic fields. Creation of weakly coupled superconductor structures such as Josephson junctions, serves to further increase the sensitivity of the superconductor components. It is this sensitivity to external stimuli that provides the basis for the preparation of a variety of superconductor-based detectors and devices. 

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