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Superconductors are materials that exhibit zero electrical resistance when cooled below a certain critical temperature. This phenomenon, known as superconductivity, was first discovered in 1911 by Heike Kamerlingh Onnes while studying mercury. When a material becomes superconducting, electrical current can flow through it indefinitely without any loss of energy as heat. This means that once a current is established in a superconducting loop, it can persist for years.

The critical temperature, denoted as Tc, varies significantly among different superconducting materials. Early superconductors like mercury and lead had very low critical temperatures, requiring cooling with liquid helium. The discovery of high-temperature superconductors in the 1980s, which could superconduct at temperatures achievable with liquid nitrogen, was a major breakthrough. These materials, often complex ceramic oxides, opened up new possibilities for practical applications.

Superconductivity has several unique properties beyond zero resistance, including the Meissner effect, where a superconductor expels magnetic fields from its interior. This expulsion allows for magnetic levitation, a phenomenon widely demonstrated with superconducting magnets. Applications of superconductors are diverse, ranging from powerful electromagnets used in MRI machines and particle accelerators to potential uses in energy transmission, high-speed trains (maglev), and advanced computing. Research continues to discover new superconducting materials and to understand the underlying physics of superconductivity, aiming to achieve superconductivity at even higher temperatures and pressures.