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A Brief History of Superconductivity

The Discovery of Superconduction

Before the discovery of superconduction, it was already known that cooling a metal increased its conductivity - due to decreased electron-phonon interactions (detailed in the Theory section).
After the 'discovery' of liquified helium, allowing objects to be cooled to within 4K of absolute zero, it was discovered (by Onnes, 1911) that when mercury was cooled to 4.15K, its resistance suddenly (and unexpectedly) dropped to zero (i.e. it went superconducting).

Variation of resistivity with temperature for mercury Left: When Onnes cooled mercury to 4.15K, the resistivity suddenly dropped to zero

In 1913, it was discovered that lead went superconducting at 7.2K. It was then 17 years until niobium was found to superconduct at a higher temperature of 9.2K.
Onnes also observed that normal conduction characteristics could be restored in the presence of a strong magnetic field.

The Meissner Effect

It was not until 1933 that physicists became aware of the other property of superconductors - perfect diamagnetism. This was when Meissner and Oschenfeld discovered that a superconducting material cooled below its critical temperature in a magnetic field excluded the magnetic flux. This effect has now become known as the Meissner effect (- you can see a QuickTime video of this in action from this link).

The Meissner effect - exclusion of a magnetic field
Above: The Meissner effect - a superconducting sphere in a constant applied magnetic field excludes the magnetic flux

The limit of external magnetic field strength at which a superconductor can exclude the field is known as the critical field strength, Bc.
Type II superconductors have two critical field strengths; Bc1, above which the field penetrates into the superconductor, and Bc2, above which superconductivity is destroyed, as per Bc for Type I superconductors.

Theory of Superconduction

Fritz and Heinz London proposed equations to explain the Meissner effect and predict how far a magnetic field could penetrate into a superconductor, but it was not until 1950 that any great theoretical progression was made, with Ginzburg-Landau theory, which explained superconductivity and provided derivation for the London equations.

Ginzburg-Landau theory has been largely superseded by BCS theory, which deals with superconduction in a more microscopic manner.
BCS theory was proposed by J. Bardeen, L. Cooper and J. R. Schrieffer in 1957 - it is dealt with in the Theory section. BCS suggests the formation of so-called 'Cooper pairs', and correlates Ginzburg-Landau and London predictions well.
However, BCS theory does not account well for high temperature superconduction, which is still not fully understood.

High Temperature Superconduction

The highest known temperature at which a material went superconducting increased slowly as scientists found new materials with higher values of Tc, but it was in 1986 that a Ba-La-Cu-O system was found to superconduct at 35K - by far the highest then found. This was interesting as BCS theory had predicted a theoretical limit of about 30-40K to Tc (due to thermal vibrations).
Soon, materials were found that would superconduct above 77K - the melting point of liquid nitrogen, which is far safer and much less expensive than liquid helium as a refrigerant. Although high temperature superconductors are more useful above 77K, the term technically refers to those materials that superconduct above 30-40K.
In 1994, the record for Tc was 164K, under 30GPa of pressure, for HgBa2Ca2Cu3O8+x.


What Is Superconductivity? Electron Conduction Theory