Just a simple question. Why is it, that when a material becomes superconducting, and by that gets zero resistivity, the electrons don't hit impurities in the material? For the material to have zero resistivity, that means that the electrons can just flow without any disturbance at all?
Is it because of the Cooper pair creation? In that case, why exactly?
Answer
When the electrons pair up this opens an energy gap between the energy of the Cooper pairs and the energy of the lowest quasiparticle excitation. There is a nice article discussing this effect here (NB it's a PDF).
The gap means that you cannot scatter a Cooper pair by an arbitrarily small energy. If the energy is less than the gap energy it won't scatter because there are no available energy states for it to scatter into. That's why the electrons in a superconductor don't scatter off impurities, defects, etc. If you apply enough energy, e.g. a very high voltage, the collisions are energetic enough to scatter the Cooper pairs and the superconductivity breaks down.
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