superconductor

A superconductor is a material that exhibits zero electrical resistance and the expulsion of magnetic fields when cooled below a characteristic critical temperature. In the superconducting state, electric current can flow indefinitely without energy loss, and a material becomes perfectly diamagnetic due to the Meissner effect, which excludes magnetic flux from its interior.

Key properties include the transition temperature (Tc), a critical magnetic field (Hc), and a critical current density (Jc). Type I superconductors show complete flux expulsion up to a single critical field and transition to the normal state, while Type II superconductors tolerate higher fields by allowing magnetic vortices to penetrate in a mixed state, enabling practical magnets and devices.

Mechanism and materials vary. Conventional, or low-temperature, superconductivity is explained by BCS theory, where electrons form Cooper pairs via lattice vibrations (phonons) and condense into a superconducting ground state. High-temperature superconductors, discovered in the 1980s (notably cuprates), have higher Tc values but often exhibit unconventional pairing mechanisms and complex crystal structures; their full theory remains an area of active research.

History and discovery: Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes. The Meissner effect, demonstrating complete magnetic field expulsion, was reported in 1933. The subsequent discovery of high-temperature superconductors expanded potential applications and research directions.

Applications and challenges: Superconductors are used in magnetic resonance imaging (MRI) and NMR, high-field magnets for particle accelerators, and magnetic levitation systems. They hold promise for efficient power transmission, but cooling requirements, material brittleness, and cost limit widespread deployment. Advances in cryogenics and new materials continue to broaden their practical use.