Ülijuhtimise

Ülijuhtimine refers to the phenomenon of superconductivity, a state in which certain materials exhibit zero electrical resistance when cooled below a critical temperature. This means that electric current can flow through them indefinitely without any energy loss due to heat. The discovery of superconductivity was made by Heike Kamerlingh Onnes in 1911 when he observed that mercury lost all electrical resistance at a temperature of 4.2 Kelvin (-269 degrees Celsius).

The theoretical explanation for superconductivity was provided by the Bardeen-Cooper-Schrieffer (BCS) theory in 1957. This theory describes superconductivity as a quantum mechanical phenomenon where electrons pair up to form "Cooper pairs." These Cooper pairs can then move through the material's crystal lattice without scattering, thus experiencing no resistance.

Superconductors are broadly classified into two types: Type I and Type II. Type I superconductors, like mercury and lead, exhibit a sharp transition to the superconducting state and are fully expelled of magnetic fields (Meissner effect). Type II superconductors, often alloys and compounds, have a more gradual transition and can tolerate higher magnetic fields, although they eventually lose their superconductivity above a critical field strength.

The applications of superconductivity are vast and include powerful electromagnets used in MRI machines and particle accelerators, high-speed magnetic levitation trains, lossless power transmission lines, and advanced electronic devices like sensitive magnetometers and quantum computers. Ongoing research aims to discover and develop high-temperature superconductors that operate at more accessible temperatures, which would revolutionize many technological fields.