Superconductivity

Superconductivity is a phenomenon in which a material's resistance suddenly drops to zero as its temperature is lowered past a certain point. This point, called the critical temperature, is different for each material. Superconductors are divided broadly into two classes, "low temperature" and "high temperature", based on their critical temperature. The high temperature superconductors, discovered more recently, are not completely understood.

The low temperature superconductors were first discovered in 1911 when Heike Kammerlingh Onnes cooled solid mercury past 4.2 Kelvin, leading to an abrupt loss of resistance. Many materials like this, most individual metals and all with critical temperatures below 30 Kelvin, have been found. It took until 1957 for a model which explained the behaviour of these materials to emerge; this model is known as the BCS theory after its authors Bardeen, Cooper, and Schrieffer.