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Superconductivity is a phenomenon occurring in certain materials where electrical resistance drops to zero below a critical temperature, allowing electric current to flow indefinitely without energy loss. This state is accompanied by the expulsion of magnetic fields from the material, known as the Meissner effect, which indicates the formation of a perfect diamagnet.

The discovery of superconductivity dates back to 1911 when Dutch physicist Heike Kamerlingh Onnes observed zero electrical resistance in mercury at temperatures near absolute zero (4.2 K). Since then, superconductors have been categorized into two main types: Type I and Type II. Type I superconductors exhibit a complete expulsion of magnetic fields below a critical magnetic field strength, while Type II superconductors allow partial penetration of magnetic fields in a mixed state before entering a fully superconducting phase.

Superconductivity is governed by the BCS (Bardeen-Cooper-Schrieffer) theory, which explains how electrons pair up (Cooper pairs) and move through the crystal lattice without scattering, enabling resistance-free conduction. However, high-temperature superconductors, discovered in 1986, defy conventional BCS explanations and operate at higher temperatures (up to around 138 K under pressure), though still requiring cryogenic cooling.

Practical applications of superconductivity include magnetic resonance imaging (MRI) machines, maglev trains, and energy-efficient power transmission lines. Challenges remain in achieving superconductivity at room temperature, which would revolutionize technology by eliminating energy losses in electrical grids and enabling more efficient devices. Research continues to explore new materials and mechanisms to unlock this potential.