Bednorz

Superconductivity The physical state in which all resistance to the flow of direct-current electricity disappears is defined as superconductivity. The Bardeen-Cooper-Schriefer (BCS) theoiy has been reasonably successful in accounting for most of the basic features observed of the superconducting state for low-temperature superconductors (LTS) operating below 23 K. The advent of the ceramic high-temperature superconductors (HTS) by Bednorz and Miller (Z. Phys. B64, 189, 1989) has called for modifications to existing theories which have not been finahzed to date. The massive interest in the new superconductors that can be cooled with liquid nitrogen is just now beginning to make its way into new applications. 

G. Bednorz and K. A. Miiller (Zurich) for their important breakthrough in the discovery of superconductivity in ceramic materials. 

High Temperature Superconductors developed byj. Georg Bednorz and Karl A. Muller. 1769... 

Some positive steps were made in this direction through the use of niobium-based alloys. In 1973 the alloy Nb3Ge was found to have a Tc = 23 K, which remained the highest attainable critical temperature until 1986. In 1986 two IBM scientists in Zurich, the German, Johannes Georg Bednorz and the Swiss, Karl Alex Muller found that a ceramic oxide based on lanthanum, barium and copper (of stoichiometry... 

Equation 25 fits Eq. 9, being the criteria of quantum ferroelectricity given by Morf et al. [2]. In Fig. 3a, the solid line shows the relationship between x and Tc determined by Bednorz and Muller for the SCT system [17]. In this figure. 

The chemistry of copper, especially in oxide compounds, is presented from a crysto-chemical viewpoint. The known binary and ternary compounds are reviewed and the important features of geometry, covalency, ionic radii, and bond length are discussed in great depth. This section concludes with a brief description of the Bednorz and Muller discovery, followed by Chu and co-workers important discovery of superconductivity at 95 K in the so-called "1-2-3" copper-oxide system. 

Figure 26 The resistivity of Ba0 75La4 2BCu5015 gy versus absolute temperature (T/K), as published by Bednorz and Muller in Reference 148.

Paul" Chu, and others at University of Houston, also reproduced Zurich s I.B.M. research results (156). Bell Lab s confirmation of Bednorz and Muller s discovery of high Tc superconductivity in copper oxide compounds was published (157) in the Jan. 1987 issue of Physical Review Letters. The electrical resistivity data from their work showing an onset of superconductivity at 36.5 K for the composition Lax gSr0 2Cu04 is plotted as Figure 28. This product also showed a 60-70% Meissner effect. 

In a previous section, it was shown how the substitution of Y3+ for Las+ was an important step towards the formation of new compounds for superconductivity research. This substitution attempt led to the discovery of the 1-2-3 (or 90 K) superconducting compound. Substitutions similar to that proposed by Muller and Bednorz are now outlined. The type of chemical substitutions, based on atomic radii, which were proposed for La2Cu04, are presented in the following Table 11. 

K. Alex Muller and J. Georg Bednorz, "The Discovery Of A Class Of High-temperature Superconductors , Science, 237 1133-1139 (1987). 

B. Raveau, C. Michel, M. Hervieu, J. Provost, and F. Studer in Earlier and Recent Aspects of Superconductivity J.G. Bednorz and K.A. Mueller, Eds. Springer-Verlag (1989). 

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