Superconductivity: the Meissner Effect and Cooper Pairs
Below a critical temperature some metals expel magnetic flux completely, which is a stronger claim than zero resistance. A weak attraction carried by the lattice binds electrons into pairs that share one quantum state, and a gap opens at the Fermi level.
What a learner can do afterwards
- Distinguishes the Meissner effect from what a perfect conductor would do
- Explains how a lattice-mediated attraction can outweigh Coulomb repulsion between electrons
- Connects the energy gap to the disappearance of resistance
1 · Read
Below a critical temperature some metals conduct with no resistance at all. Stronger still, they expel magnetic flux completely from inside: the Meissner effect, found by cooling in a field and watching the field leave.
No resistance alone would only freeze the field in place, which is what a perfect conductor does. Expelling it is a bolder claim, and that is why the Meissner effect proves superconductivity is more than perfect conduction.
A weak attraction carried by the lattice lets two electrons pair up despite their repulsion. They take turns: one electron pulls the lattice in, the other is drawn to the dent, so they rarely meet head-on. Many such Cooper pairs then share one quantum state, the way bosons may all crowd into a single state, and that shared state moves without scattering.
A gap opens at the Fermi level around the shared state, and breaking a pair costs that gap energy. Ordinary bumps cannot pay it, so resistance disappears with the gap.
Flux expelled, electrons paired by the lattice, one shared state protected by a gap: that is the superconductor.
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