The discovery of superconductivity dates back to 1911, when Dutch physicist Heike Kamerlingh Onnes observed that mercury lost all electrical resistance when cooled to near absolute zero (4.2 kelvin). Subsequent research revealed that superconductivity arises due to the formation of Cooper pairs—bound pairs of electrons that move through the crystal lattice of the material without scattering, thus avoiding resistance. This phenomenon is described by the BCS theory, proposed in 1957 by John Bardeen, Leon Cooper, and John Robert Schrieffer, which explains superconductivity in conventional materials.
Superconductors are classified into two main types: Type I and Type II. Type I superconductors, such as pure mercury or lead, exhibit a sharp transition to the superconducting state and expel all magnetic fields below a critical magnetic field strength. Type II superconductors, often alloys or complex compounds, display two critical magnetic field values: the lower critical field (Hc1), where vortices begin to penetrate, and the upper critical field (Hc2), beyond which superconductivity is lost. Type II superconductors are more practical for applications due to their ability to carry higher currents.
High-temperature superconductors, discovered in 1986 by Karl Alexander Müller and Johannes Georg Bednorz, operate at temperatures significantly above traditional superconductors, though still below room temperature. These materials, typically ceramic compounds, hold promise for more energy-efficient technologies but remain challenging to produce and integrate into devices. Despite ongoing research, the exact mechanisms governing high-temperature superconductivity are not fully understood.
Practical applications of superconductivity include magnetic resonance imaging (MRI) machines, particle accelerators like the Large Hadron Collider, and experimental magnetic levitation trains. Challenges such as the need for cryogenic cooling and material fragility limit widespread adoption, but advancements continue to expand the potential of superconducting technology.