Superconductivity

A superconductor is a material in the superconducting state. Below a material-dependent critical temperature, its direct-current electrical resistance drops to zero, so a current can persist without resistive energy loss. A second defining property is the Meissner effect: as a material enters this state, it expels magnetic fields from its interior, provided the applied field is within the relevant limits. This distinguishes superconductivity from simply having very low resistance. Zero resistance also does not mean that a complete device uses no energy, because cooling equipment and other components may still consume power.

Superconductivity exists only within operating limits. A material has a critical temperature, and it can also lose its superconducting state if the magnetic field or electric current becomes too large. Type I superconductors lose the state abruptly when an applied magnetic field passes a threshold. Type II materials can tolerate stronger fields: magnetic flux penetrates them in localized regions while superconducting behavior persists elsewhere, up to a higher limit. This mixed-state behavior makes Type II materials especially useful for high-field magnets.

In conventional superconductors, the Bardeen–Cooper–Schrieffer (BCS) theory explains the transition using paired electrons. At sufficiently low temperatures, interactions involving vibrations of the crystal lattice, called phonons, help bind electrons into Cooper pairs. The paired electrons collectively move through the material without the scattering that ordinarily produces electrical resistance. BCS theory, established in 1957, successfully describes conventional superconductors, but it does not explain every higher-temperature or unconventional superconductor; understanding those materials remains an important research problem.

In 1911, Dutch physicist Heike Kamerlingh Onnes and his collaborators observed that mercury's electrical resistance disappeared when it was cooled to very low temperatures. The phenomenon became known as superconductivity. In 1933, Walther Meissner and Robert Ochsenfeld observed the expulsion of magnetic fields, later called the Meissner effect. A major theoretical explanation followed in 1957, when John Bardeen, Leon Cooper and Robert Schrieffer developed the theory now known as BCS. These discoveries established superconductivity as both a fundamental subject in condensed-matter physics and a basis for specialized technologies.

Superconducting wires can carry large currents, making them useful for coils that generate powerful magnetic fields. Such magnets are used in magnetic resonance imaging and in particle accelerators, including accelerator systems at CERN. The technology depends on keeping materials below their critical temperature and within current and field limits; refrigeration and the engineering needed to manage these conditions are central parts of a practical system. Research seeks materials and designs that operate at warmer temperatures or under more demanding conditions, which could broaden the range of applications.