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Phonons in BCS theory (1 Viewer)
- Thread starter Kimyia
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someth1ng
Retired Nov '14
1. An electron moves through the lattice, the negative charge of the electron attracts the positive metal ions of the lattice.
2. This creates a region of net positive charge, attracting a second electron.
3. These two electrons have formed a weak attraction between the electrons even through their charges make them naturally repel.
--> These two electrons are said to be a Cooper pair.
--> For Cooper pairs to form and remain intact, the binding energy between two electrons must be higher than the energy of lattice vibrations.
--> As lattice vibrations increase with temperature, below a certain temperature, the binding energy of a Cooper pair will be greater than energy of lattice vibrations allowing them to stay intact.
4. It is said that the first electron emitted a phonon and the second electron absorbed a phonon.
5. If a Cooper pair can be formed, the two electrons can move through the lattice unimpeded where lattice distortions assist the current flow rather than opposing current flow.
Always include diagrams in these responses.
2. This creates a region of net positive charge, attracting a second electron.
3. These two electrons have formed a weak attraction between the electrons even through their charges make them naturally repel.
--> These two electrons are said to be a Cooper pair.
--> For Cooper pairs to form and remain intact, the binding energy between two electrons must be higher than the energy of lattice vibrations.
--> As lattice vibrations increase with temperature, below a certain temperature, the binding energy of a Cooper pair will be greater than energy of lattice vibrations allowing them to stay intact.
4. It is said that the first electron emitted a phonon and the second electron absorbed a phonon.
5. If a Cooper pair can be formed, the two electrons can move through the lattice unimpeded where lattice distortions assist the current flow rather than opposing current flow.
Always include diagrams in these responses.
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So can we say that after the first electron travels through, the lattice distorts and creates a positive region which attracts the second electron. The two electrons then swap phonons to form a Cooper pair to keep them moving through the lattice together, past this positive region and unimpeded by vibrations?
someth1ng
Retired Nov '14
I'd say they're exchanged rather than swapped but personally, I say that they're transferred because an exchange implies that both emit a phonon and both absorb a phonon from each other but apart from that, there's also the idea that the transfer of phonons creates a weak attraction between the electrons which allows the formation of Cooper pairs.
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someth1ng
Retired Nov '14
It still does distort but there's much more lattice vibrations which don't allow Cooper pairs to remain intact.
someth1ng
Retired Nov '14
The Meissner effect is NOT due to eddy currents, it's due to superconductors expelling all magnetic flux at attempts to penetrate it. It means that there are persistent currents in superconductors that repel all magnetic flux that attempts to penetrate it.
A side consequence of this is that the magnetic field generated due to induced current is STRONG enough to just balance the gravitational force acting on the magnet, thus causing magnetic levitation.