superconductivity

Superconductivity is a phenomenon in which certain materials conduct electric current with zero resistance when cooled below a characteristic critical temperature. In the superconducting state, magnetic fields are expelled from the interior of the material (the Meissner effect), and electrical currents can persist indefinitely in the absence of power loss. The transition to superconductivity was discovered in mercury by Heike Kamerlingh Onnes in 1911.

Conventional superconductors are explained by BCS theory, where electrons form Cooper pairs via lattice vibrations, moving without resistance. This pairing creates an energy gap at the Fermi surface. The London equations describe the electromagnetic response and the Meissner effect. Superconductors are classified as type I, which expel magnetic fields completely up to a single critical field, and type II, which permit flux vortices and remain superconducting up to a higher field.

Materials include elemental and alloy superconductors (eg, mercury, lead, NbTi) that become superconducting at very low temperatures, and high-temperature superconductors such as certain cuprates and iron-based compounds with higher Tc. The Josephson effect enables SQUIDs and precision magnetometry. Type II superconductors support vortices, important for high-field applications.

Applications include MRI, particle accelerators, and superconducting magnets for maglev systems and power cables. Research aims to raise Tc and improve materials, processing, and cost. Although some hydride systems show very high Tc under pressure, room-temperature superconductivity remains an open goal explored through unconventional materials and alternative pairing mechanisms.