Low Temperature Superconducting Materials

After the initial discovery by Onnes of superconductivity in mercury, tin, and lead, research focused on the discovery of new superconducting phases with even higher values. It was found that 25 % of the elements of the periodic table are superconductors and that a plethora of alloys exhibit superconductivity [16]. A theory to describe the phenomenon of superconductivity was introduced by Bardeen, Cooper, and Schrieffer (BCS) which, as originally formulated, placed an upper limit on Tc of about 35 to 40 K [19]. For a synopsis of the historical development of superconductor theory, see [20]. We shall use the term low temperature superconductor (LTS) as a reference to those materials which possess values less than the theoretical limit of 35 to 40 K imposed by the original BCS theory. 

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By 1959, the theory of superconductivity was firmly established, and even though there have been modifications, it has stood the test of time. But only, apparently, insofar as it relates to the conventional, metallic, low-temperature superconducting materials. The new high-temperature superconductors, the ceramics, pose another set of questions and may require an entirely new theory. 

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