Superconductivity is a state of matter in which certain materials exhibit zero direct-current electrical resistance and expel magnetic fields from their bulk when cooled below a critical temperature, subject to limits on magnetic field and current. It is a macroscopic manifestation of quantum mechanics and a major subject of condensed-matter physics. Its electrical and magnetic properties enable powerful magnets, sensitive detectors, and quantum electronic circuits. (energy.gov)
Discovery and development
Heike Kamerlingh Onnes discovered superconductivity in 1911 while investigating mercury at temperatures near 4 kelvin, using liquid helium for cooling. Unlike the gradual reduction of resistance normally observed in cooling metals, mercury underwent an abrupt transition to an immeasurably small resistance. Onnes received the 1913 Nobel Prize in Physics for his investigations of matter at low temperatures. (nobelprize.org)
Theoretical understanding developed through several complementary approaches. The London model described magnetic screening, while Ginzburg–Landau theory, introduced in 1950, represented superconductivity through a spatially varying order parameter. In 1957, John Bardeen, Leon Cooper, and John Robert Schrieffer formulated BCS theory, providing a microscopic explanation of conventional superconductivity. The discovery of superconducting copper-oxide ceramics in 1986 opened the field of high-temperature superconductivity. (cds.cern.ch)
Electrical and magnetic properties
Zero resistance means that a steady current can flow without the resistive heating associated with ordinary conductors. This property concerns appropriate direct-current conditions: superconductors can still dissipate energy under alternating fields or when magnetic vortices move. Superconductivity therefore does not imply that every superconducting device operates without energy consumption or loss. (energy.gov)
The Meissner effect is the expulsion of magnetic flux as a material enters its superconducting state. It distinguishes superconductivity from merely perfect electrical conduction: zero resistance alone would preserve existing magnetic flux rather than require its expulsion. Screening currents near the surface reduce the field within the material over a characteristic penetration depth. In the Meissner state, sufficiently far from the surface, the magnetic induction approaches zero. (arxiv.org)
Superconducting behavior occupies a restricted region of temperature, magnetic field, and current density. Excessive heating or field destroys the superconducting state. Excessive current can also break electron pairs, although practical current limits often arise earlier through vortex motion. Thus a material’s transition temperature alone does not determine its usefulness in an electrical application. (cds.cern.ch)
Microscopic explanation
In conventional superconductors, electrons form correlated pairs called Cooper pairs. An electron’s interaction with the crystal lattice can produce an effective attraction between electrons, mediated by quantized lattice vibrations, or phonons. Below the transition temperature, the paired electrons participate in a collective, phase-coherent quantum state rather than behaving as independent charge carriers. (energy.gov)
BCS theory connects pairing to an energy gap in the electronic excitation spectrum and explains major thermodynamic and electromagnetic properties of conventional superconductors. Ginzburg–Landau theory instead describes the collective state using a complex order parameter whose magnitude and phase vary in space. This phenomenological description is particularly useful near the transition and for understanding interfaces, magnetic screening, and vortices. (cds.cern.ch)
Pairing also occurs in unconventional superconductors, but a simple phonon-mediated explanation does not adequately describe many copper-oxide and iron-based materials. Their electronic correlations and competing magnetic or charge orders make a comprehensive microscopic account more difficult. Research on these materials examines both the formation of pairs and the establishment of long-range phase coherence. (energy.gov)
Type I and type II superconductors
Superconductors are classified by their response to magnetic fields. Type I materials have a single thermodynamic critical field separating the superconducting and normal states. Type II materials have lower and upper critical fields; between them lies a mixed state in which magnetic flux penetrates through vortices, while superconductivity persists around their cores. (cds.cern.ch)
An ordinary singly quantized vortex carries one flux quantum,
[ \Phi_0=\frac{h}{2e}, ]
where (h) is Planck’s constant and (e) is the magnitude of the electron charge. Flux quantization reflects the collective quantum state of paired charge carriers. Vortex motion produces dissipation; defects that immobilize vortices through flux pinning are therefore essential to many high-current superconductors. Type II materials support the strong fields required by practical superconducting magnets. (cds.cern.ch)
Materials and operating conditions
Superconducting materials include elemental metals, alloys, intermetallic compounds, and ceramic oxides. Niobium–titanium and niobium–tin are important magnet materials. Copper-oxide superconductors have substantially higher transition temperatures than many conventional metallic superconductors; some operate above liquid nitrogen’s boiling temperature, approximately 77 kelvin. “High-temperature” is consequently a relative term and does not mean ordinary room temperature. (fcc.web.cern.ch)
Pressure can profoundly alter superconducting behavior. Experiments on lanthanum hydride have demonstrated transitions near 250 kelvin under pressures around 150–170 gigapascals. Such results concern microscopic samples under extreme compression, not materials readily usable in ambient-pressure equipment. Practical operation depends on the combined requirements of cooling, pressure, current capacity, magnetic-field tolerance, and fabrication. (nature.com)
Applications and engineering constraints
Superconducting magnets provide strong, stable fields for magnetic resonance imaging, nuclear magnetic resonance instruments, and particle accelerators. Superconducting cables and related equipment have also been developed for electrical grids, where high current density can increase transmission capacity. These systems still require refrigeration and associated engineering infrastructure. (energy.gov)
The Josephson effect permits supercurrent across a weak link between superconductors. Devices incorporating Josephson junctions include the superconducting quantum interference device, or SQUID, used for sensitive magnetic measurements. Junction circuits also support precision voltage standards and superconducting qubits used in quantum computers. (nist.gov)
Large magnets require protection against a quench, a transition of part of the conductor into the resistive state. Stored magnetic energy can then generate substantial heat and voltage. Detection circuits, heaters, and energy-extraction systems limit damage by managing the transition and distributing or removing the stored energy. (cern-courier.web.cern.ch)