A Conversation for Super-cooling and a Brief History of Temperature
Jim diGriz Started conversation Jun 8, 2000
As far as I know, most (all?) superconductors are not metals. The ones I know about (the 'high-temperature' ones) are in fact ceramic, which isn't a conductor at all at room temperature.
For example, the flurry of work in the 1980s was based on the Yttrium-Barium-Copper-Oxide family.
Phil Posted Jun 8, 2000
And the superconductivity is all to do with pairs of electrons running round. Or at least that's what I was told anyway.
Jim diGriz Posted Jun 8, 2000
Aye, Cooper pairs I think they're called.
A distant memory is telling me that the pair of electrons exhibits the properties of a single zero-spin particle.
26199 Posted Jun 8, 2000
Hmmm. I think even the ceramic ones have metallic elements in them, although I may be wrong about this... but I'm fairly sure that most metals superconduct if cooled enough.
Would anybody who actually knows what they're on about like to comment?
Phil Posted Jun 8, 2000
Ok then, from my advanced electronic materials course at uni five years ago...
Metals if cooled become superconductors,for example Mercury (Hg) superconducts at below 4K, a bit low for any application really.
The high temperature superconductors have mainly been based arround CuO2 and some other elements like Yttrium and Barium.
Supercondutivity theory: BCS (Bardeen, Cooper and Schreiffer)
A pair of electrons become bound together. Each electron has opposite spin, so the total spin of the pair is 0. This means that the magnetic moments cancel (where the exclusion of magnetic fields comes in I guess) and the energy of the pair is lower than the sum of the unpaired energies. The distance between the two electrons can extend to up to 40nm (thats 0.00004mm, or about a quater of the size of the smallest feature on a current top end silicon chip).
Why do they do this? it's all down to phonon - phonon interactions between the electrons, via the atoms the electrons are with.
Clear as mud? perhaps we aught to try and find someone with some more knowledge and ask them
Munchkin Posted Jun 9, 2000
OK, bare in mind I have not actually played with superconductors, but have looked at very cold electrons.
What Phil said sounds good. All electrons like to act in pairs. This lets them work at lower energies, by the mentioned cancelling of spin. If you twiddle electrically with one of the electrons, the other one will react, trying to keep in an opposite state, as extra energy is required to move away from this lower state. Thus, fiddling with one electron (say passing a current) will cause the other one to react without any of that dull movement nonsense normal electrons have to do. It is the movement of electrons, and their banging into bits of the universe that causes electrical resistance. Hence you don't get as much under these circumstances.
Of course, this will only work if the electrons are well ordered and there is little spare energy for them to use. Thus it has to be cold. Also, the type of material is important. If these electrons were to find an impurity (big lump of electrical crud) inbetween them, they will stop working together. Hence why it only happens in special cases. It has to be said, normal metals are very impure.
Hope this helps, and if someone who knows better comes along, it wasn't me, a big boy did it and then ran away.
Phil Posted Jun 9, 2000
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