Superconductivity is the flow of electric current without energy losses; in other words current flows with zero electrical resistance. The superconductive state is also characterized by the absence of a magnetic field within the superconducting material.
The electrical resistance of metals falls with temperature. In some metals, at temperatures close to absolute zero, a critical temperature exists below which the metal becomes superconductive: resistance falls to zero. A current flowing in a superconductive metal circuit will flow for as long as the metal remains at low enough a temperature to maintain its superconductive state.
Metals can be regarded as a lattice of positively charged metal ions surrounded by a 'sea' of mobile electrons that are not bound to any particular metal nucleus. These electrons are described as occupying the metal's conduction band.
Electrical resistance arises normally because the flow of mobile electrons is impeded by thermal vibrations of the metal's lattice. These lattice vibrations are quantum mechanical waves known as phonons.
At very low temperatures, from the perspective of electron flow, interaction between electrons and phonons can change from discord to cooperation: a quantum mechanical electron-phonon interaction results in electrons forming pairs known as Cooper pairs. These pairs are very easily broken up by thermal energy, so they can exist only at temperatures close to absolute zero.
Cooper pairing fundamentally changes the electrons' properties - rather than behaving as fermions with a spin of ½, they behave as bosons with spin 0 or (sometimes) 1. Electron pairs pass through the metal lattice with no energy losses: electrical resistance is zero. The metal becomes a superconductor.
Not all metals become superconductors. Some that do, with their critical temperatures are:
|Element||Critical Temperature / K|
‡ β-mercury's lattice has body-centered tetragonal symmetry
Some materials have been discovered that are superconductive at temperatures too high for phonon mediated Cooper pairs to exist. These materials are known as high-temperature superconductors and the cause of their superconductivity is still a subject of debate.