Superconductor
Superconductivity History And Theory
Superconductivity was first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes (1853–1926). After succeeding in liquefying helium (He), Onnes observed that the electrical resistance of a mercury filament dropped abruptly to an experimentally undetectable value at a temperature near -451.84°F (-268.8°C, 4.2K), the boiling point of helium. Onnes wrote: "Mercury has passed into a new state, which, because of its extraordinary electrical properties, may be called the superconductive state."
The temperature below which the resistance of a material = zero is referred to as the superconducting transition
Figure 1. The curve of resistance R(Ω) versus temperature T (K) of a mercury filament (after H. Kamerlingh Onnes, 1911).
Superconductors are categorized as type I (soft) and type II (hard). For type I superconductors (e.g., most pure superconducting elements, including lead, tin, and mercury), diamagnetism and superconductivity break down together when the material is subjected to an external magnetic field whose strength is above a certain critical threshold Hc, the thermodynamic critical field. For type II superconductors (e.g., some superconducting alloys and compounds such as Mb3Sn), diamagnetism (but not superconductivity) breaks down at a first threshold field strength Hc1 and superconductivity persists until a higher threshold Hc2 is reached. These properties arise from differences in the ways in which microscopic swirls or vortices of current tend to arise in each particular material in response to an external magnetic field.
No unified or complete theory of superconductivity yet exists. However, the basic underlying mechanism for
Figure 3. Increase in Tc with time since superconductivity was discovered.
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