A superconductor is a material that conducts electrical current with no resistance. The phenomenon of superconductivity was first discovered by H. Kamerlingh Onnes in 1911, when he found that the resistance of mercury dropped to zero at a temperature of about 4K. Many other materials, including tin, lead, and niobium, were tested and also found to superconduct at extremely low temperatures, less than 10 K. In 1957, a theory of superconductivity was advanced that explained the phenomenon quite well, and it has been used in further research and development of other superconducting materials.

Superconductivity normally only occurs at extremely low temperatures. Each superconducting material has a different critical temperature; above the critical temperature, the material does not superconduct. As the temperature of the material is lowered, the material undergoes a phase transition from its normal state to the superconducting state. The critical temperature also depends on the...