Superconductivity

Superconductivity is a property exhibited by some materials at very low temperatures. It was discovered by Kamerlingh Onnes in 1911. Superconductivity is characterized by two phenomena. First, the disappearance of all electrical resistance below a certain temperature (known as the critical temperature). Second, the exclusion of any magnetic fields from the bulk of the material below the same temperature. This second property is known as the "Meissner Effect".

There are two types of superconductors. Type I superconductors are usual pure metals or alloys. Lead (critical temperature 7.2K) and aluminium (critical temperature 1.2K) are both type one superconductors. A microscopic theory explaining the behaviour of type one super conductors was developed by Bardeen, Cooper and Schrieffer.

Type II superconductors are usually metal oxide alloys or complex oxide ceramics. They can have significantly higher critical temperatures than type one super conductors, with the highest recorded temperature of a type two superconductor being 138K. As yet these is no widely accepted theory to describe the microscopic behaviour of type two superconductors.