The rigorous quantum mechanical explanation shows that the effect is due to electron–phonon interactions, with the phonon being the collective motion of the positively-charged lattice.
So only at low temperatures, in metal and other substrates, are a significant number of the electrons bound in Cooper pairs.
[5] Electrons have spin-1⁄2, so they are fermions, but the total spin of a Cooper pair is integer (0 or 1) so it is a composite boson.
Therefore, unlike electrons, multiple Cooper pairs are allowed to be in the same quantum state, which is responsible for the phenomenon of superconductivity.
[6] The tendency for all the Cooper pairs in a body to "condense" into the same ground quantum state is responsible for the peculiar properties of superconductivity.
When one considers the more realistic state of many electronic pair formations, as is elucidated in the full BCS theory, one finds that the pairing opens a gap in the continuous spectrum of allowed energy states of the electrons, meaning that all excitations of the system must possess some minimum amount of energy.
This is a fine distinction that John Bardeen makes: The mathematical description of the second-order coherence involved here is given by Yang.