What is Superconductivity?

Superconductivity is a phenomenon displayed by some materials when they are cooled below a certain temperature, known as the superconducting critical temperature, T_c.

Below T_c, superconducting materials exhibit two characteristic properties:

• Zero electrical resistance

• Perfect diamagnetism (the Meissner effect)

Zero electrical resistance means that no energy is lost as heat as the material conducts electricity - this has many applications, which you can read about in the Uses section.

The second of these properties, perfect diamagnetism, means that the superconducting material will exclude a magnetic field - this is known as the Meissner effect (after its discoverer), and can be used to display extraordinary physical effects:

Left: The Meissner effect is used to levitate a superconductor above a magnet. To re-create this effect yourselves, see Making your own superconductors. (Click on the image to see a QuickTime movie of the Meissner effect - Download Apple's QuickTime plugin from www.apple.com/quicktime/download if you do not already have it)

Superconducting materials can be categorised into one of two types:

• Type I Superconductors - which totally exclude all applied magnetic fields. Most elemental superconductors are Type I.

• Type II Superconductors - which totally exclude low applied magnetic fields, but only partially exclude high applied magnetic fields; their diagmagnetism is not perfect but mixed in the presence of high fields. Niobium is an example of an elemental Type II superconductor.

Both types exhibit perfect electrical conductivity, and can be restored to 'normal' conductors in the presence of a sufficiently strong magnetic field.
For further information about Type I and Type II superconductivity, you might like to look at the relevant section at Superconductor.org - Type I or Type II.

To learn why superconductors exhibit these extraordinary properties, go to the Theory section.