Superconductors early examples and basic theory

A superconductor is a material, the electrical resistance of which drops to zero when it is cooled below its critical temperature, T. A perfect superconductor exhibits perfect diamagnetic behaviour. 

A range of metals, alloys and metallic compounds are Type I superconductors (Table 28.1). However, to put the practical limitations of working with the materials listed in Table 28.1 (and others including the superconducting fullerides described in Section 14.4) into perspective, we must compare the values of with the boiling points of available coolants, e.g. liquid He (4.2 K), H2 (20.1 K) and N2 (77 K). The low values of limit the possible applications of these materials, and illustrate why the so-called high-temperature superconductors described below are more important than superconducting metals and alloys. 

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