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).
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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).
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| 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Ginzburg-Landau theory has been largely superseded by BCS theory, which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.
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