I Why superconductors have zero resistance

Hi I know that BCS theory predicts Cooper pairs which move as a Bose condensate in superconductors. But why would scattering lead to a zero resisitance state because surely the Cpairs would provide resistance. I realise that superconductors have zero resistance but just can't really seee why?
 

ZapperZ

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Hi I know that BCS theory predicts Cooper pairs which move as a Bose condensate in superconductors. But why would scattering lead to a zero resisitance state because surely the Cpairs would provide resistance. I realise that superconductors have zero resistance but just can't really seee why?
What 'scattering' leads to this zero resistance?

It is the LACK of scattering that leads to zero resistance. The superconducting state opens a gap in the density of states around the Fermi energy. So low energy scattering of the Cooper pairs will not break up these pairs because there are no available states for the electrons to go into.

Zz.
 

DrDu

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In deed, Cooper pairs can break up in scattering events. However, the electrons sooner or later will recombine to form again a Cooper pair. The point is the following: Consider first an ordinary electron gas carrying some current. Electrons with high current can get scattered to the other end of the Fermi surface whence the current reduces continuously. In a superconductor which carries current, all cooper pairs have the same momentum. Breaking up a Cooper pair only increases the energy as the electrons cannot decay into a condensate of lower momentum (and energy) as this would require a condensate of low momentum to be already present.
See the illuminating article by B Hansen, Infinite conductivity of ordinary and gappless superconductors:
http://www.sciencedirect.com/science/article/pii/0031891468901699
 
why would a superconductor revert to its normal state above T_c, is it because eddy currents can't form so nothing opposes the B field so it just becomes a normal metal, why though can't eddy currents form?
 

f95toli

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No, superconductors become normal conductors above Tc because there is enough thermal energy available to break all the Cooper pairs, which in turn means that all you are left with are normal electrons.
 
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. One key to the understanding of the BCS theory is accepting the existence of a pair of electrons(cooper pair) that has a lower energy than two individual electrons.
Imagine an electron in a metal with no lattice vibrations. This electron perturbs the lattice slightly in its neighborhood. When such an electron drifts through a crystal the perturbation is only momentary and after passing a displaced ion reverts back into its orginal position and oscillates around its rest position. So a phonon is created. This phonon in turn interacts quickly with a second electron which takes advantage of the deformation and lowers its energy. Electron two finally emits a phonon by itself which interacts with the first electron and so on. This passing back and forth of phonons couples the two electrons together and brings them into a lower energy state. One can visualize that all electrons on the Fermi-surface having opposite spin from those cooper pairs. Thus the superconducting state is an ordered state of the conduction electrons. The scattering on the lattice atoms is eliminated, thus causing a zero resistance.
 

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