Why does electrical resistance vanish completely in a superconductor?

In a superconductor, resistance vanishes because the electrons stop moving as lonely individuals and move as paired teams that pass through the metal without resistance. Below a characteristic critical temperature, the resistance of the material drops abruptly to zero, and a current in a loop of superconducting wire can persist indefinitely with no power source.

It was found at very low temperature. On 8 April 1911, Heike Kamerlingh Onnes was using liquid helium as a refrigerant to study solid mercury when he found its resistance had disappeared. Mercury's critical temperature is 4.2 K. Some scientists, such as Lord Kelvin, had believed metal resistivity would become infinitely large at absolute zero. The opposite happened.

The explanation came in 1957 from Bardeen, Cooper and Schrieffer: the current is a superfluid of Cooper pairs, pairs of electrons interacting through the exchange of phonons. A passing electron distorts the lattice of positive ions, moving them slightly toward it, and the extra positive charge nearby can attract another electron. Electrons are fermions, which refuse to share a quantum state, but a Cooper pair has integer total spin and so behaves as a composite boson. Many pairs can occupy the same quantum state, which is responsible for superconductivity. And that shared state is hard to disturb: the pair fluid has an energy gap, a minimum amount of energy needed to excite it, whereas an electron in a normal metal can be knocked into a new state by an arbitrarily small amount. When the gap is larger than the thermal energy of the lattice, the lattice cannot scatter the pairs, so they flow with no energy lost.

The catch is that the pairing is weak: thermal energy can easily break the pairs, so superconductors work only when cold. Liquid nitrogen boils at 77 K, so superconductors that work at higher temperatures make applications more practical, such as the magnets in hospital MRI machines.