The electrical resistivity of many metals and alloys drops suddenly to zero when the specimen is cooled to a sufficiently low temperature often a temperature in the liquid helium range. This phenomenon is called superconductivity. At a critical temperature Tc the specimen undergoes a phase transition from a state of normal electrical resistivity to a superconducting state Fig 17. This phenomenon was observed by Heike Kamerlingh Onnes in 1911.
The superconducting state is on ordered state of the conduction electrons of the metal. The order is in the formation of loosely associated pairs of electrons. The electrons are ordered at temperatures below the transition temperature and they are disordered above the transition temperature Tc.
Occurrence of superconductivity
Superconductivity occurs in many metallic elements of the periodic system and also in alloys, intermetallic compounds and doped semiconductors.
Tc values of some elements, alloys and compounds are given in table (2.2)
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