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BCS theory places a maximum critical temperature below 40K, which
caused many scientists to believe that superconductivity could never
be very practical as it required sub-liquid nitrogen temperatures.
All this changed in 1987 when Paul Chu and Maw-Kuen Wu found materials
that became superconducting at temperatures as high at 90K. A new
class of superconductors was born: high-temperature superconductors.
BCS theory is just as accurate as it was in 1957, but it applies only
to type-I superconductors, but high-temperature superconductors are
type-II.25. Formally, type-I superconductors have a coherence length (
length
of attractive electron-electron interaction) longer than the penetration
depth and type-II superconductors are the opposite. There is no complete
theory for high-temperature superconductors and many question if the
mechanism for type-I superconductivity (Cooper pairs) is the same
in high-temperature superconductors. Besides their range in critical
temperatures, one major difference between type-I and type-II superconductors
is their magnetic properties, an example of which is shown in figure
12.
![\includegraphics[scale=0.5]{/home/lueyb/Sync/Comps/fig/Ibach-t1-t2-v3.eps}](img276.png)
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The potential scientific and technological benefits of superconductors are immense. While the theory of superconductivity discussed in this paper describes low-temperature superconductors with prohibitively high cooling costs for wide-scale use, recent advancements in high-temperature superconductors, with critical temperatures above that of liquid nitrogen, offer the hope of cheaper, more practical applications. In the future maybe superconductors will be used to transmit electricity from generators to homes and offices with no energy loss from Joule heating. But superconductors have practical uses today, mostly involving magnetic fields. Superconductors are useful in creating strong magnetic fields because the magnetic field produced by running electricity through a wire is proportional to the current, and the resistance of the wire is proportional to the power necessary to produce that current. Another problem with producing large magnetic fields with resistive wire, is that the wire heats up as high current is run though it and the heat raises the resistance of the wire, requiring more energy to maintain the same magnetic field and accelerating the heating process. Superconductors are used to produce the magnetic fields used in Magnetic Resonance Imaging (MRI) and CATSCANs. Superconductors are also used in places like Fermilab in Chicago to produce the high magnetic fields necessary to accelerate particles for high energy particle physics research. Some designs of magnetic levitating (maglev) trains are also based on superconductors. Also, superconductors can be used to make very sensitive measurements of magnetic fields and were used in the only experiment to give evidence for the existence of magnetic monopoles (Cabrera: 1982). Superconductivity also has possibilities for electronics and solid state devices where the dissipation of Joule heat often limits how close together transistors can be placed together, and thus the speed and size of the chip.