Superconductivity

Some materials, including several metals and alloys, exhibit a phenomenon called superconductivity. As the temperature decreases, the resistivity at first decreases smoothly like that of any metal. But then at a certain critical temperature T_C, a phase transition occurs and the resistivity drops suddenly to zero. And electric current established in a super conducting ring continues indefinitely without the presence of any driving field. Superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingle Onnes.

Superconductors display the Meissner effect which is discussed below.

Sub Topics

Meissner Effect
High Tc Superconductivity

Meissner Effect

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In 1933, Meissner and Ochsenfeld found that if a superconducting material is cooled to below its transition temperature, in a magnetic field, then the lines of induction B are pushed out. This is shown in the figure below.

Meissner effect in a superconductor

The Meissner effect shows that a bulk superconductor behaves as if inside it, B is zero.

High Tc Superconductivity

This table shows that some materials, mainly oxides, have a fairly high critical temperature T_c. The 125K bulk super conducting oxide was discovered in 1988. High T_c superconductors have promising commercial applications in thin film devices, levitated vehicles and long distance power transmission.

In MRI (magnetic resonance imaging) instruments, superconducting magnets made with these high T_c materials are being used. These can be cooled with liquid nitrogen. Earlier, liquid helium was required which was much more expensive.