Part of the answer in an energy gap, which keeps charges energetically separated in interaction (like friction) from other matter.

Another is Cooper pairs being bosons, allow any ocupation in a given quantum state, this allows collective motion.

Another component is perfect diamagnetism, for the expulsion of a magnetic field.

However understanding is hardly complete, one keeps trying.

Superconductivity involves the macroscopic quantum condensation of paired electrons. Such pairing can originate from a variety of quantum interactions. There are various kinds of interactions electron-phonon interaction, spin–spin interaction, charge density waves, spin density waves, and so on.

https://www.insidescience.org/video/superconductors-powering-our-future#:~:text=Superconductors%20are%20materials%20where%20electrons,to%20well%20below%20room%20temperature.&text=They%20stop%20showing%20any%20electrical,them%20ideal%20for%20conducting%20electricity.

Dear Roman,

Your question is insightful, since conventional theories cannot answer it.

Question 1: Electronic pairs, as bosons, can fall in the same bosonic ground state – the Bose-Einstein-Condensate (BEC). The BEC bosons have an energy uncertainty ∆Eb ≈ kB·TBEC, (TBEC is the temperature of BEC formation). The energy spectrum of electrons is quasi-continuous, i.e. the distance between neighboring energies is much smaller than ∆Eb, so within the ∆Eb can exist N electrons, N=S(E)·∆Eb , where S(E) is the DOS of electrons.

Question 2: In the BEC ground state every boson cannot absorb/emit any energy quant, smaller than its energy uncertainty ∆Eb ≈ kB·TBEC . So below TBEC the total energy (and, thus, total momentum) of bosons cannot be changed. Free electrons can absorb/emit much smaller energy quants, because the neighboring energy states are very close to each other. So electrons do energy/momentum dissipation.

@ Stanislav

Thank you for the answer. Okay, lets assume BEC condensate of paired electrons is still a very good approximation, even though pairs are not perfectly the same. Moreover, lets assume BEC somehow forces electrons to concentrate in a same state. However, the current through a semiconductor can change more or less gradually. That means, either BEC ground state is a degenerate state, that covers all possible electric current values, and so nothing prevents a set of bosons from tunneling between these substates, or these states of different electric current value are not the ground states, except very few of them, and still, bosons would eventually fall into a most ground one with some preferred electric current value, say, zero. So, what's the catch here?

The plausible answer follows from the Meissner effect. The non-dissipative random fluctuations of pairs can be redistributed into non-dissipative aligned fluctuations (=supercurrents), screening the additional magnetic energy in bulk. Thus, the total kinetic energy of pairs (ground state) doesn't change, but becomes aligned for the energy minimization in the crystal. The same is valid for external electric fields; they don't accelerate the pairs, but give them preferable directions, for screening the field energy in bulk.

Dear Roman Maltsev,

One difficulty about superconductivity concern the valence.

We know since almost sixty years that the xenon and some other gas call “inert” has valence and bonds properties see “Chernick C.L., Rec. Chem. Prog., 24, 139-155, (1962)”. As a result, in superconductors as La2-xBaxCuO4 where the bivalent barium replace the trivalent lanthanum it is possible to consider that the xenon shell of Ba or La contributes to the valance, see “From Ln valence to that of 3d”. The substitution of La by Ba introduces break in the bond allowing the electrical conduction in a similar way to that of the semiconductors see “Conductivity and statistic: an alternative view”.

Dear Dr. Roman Maltsev

The Cooper pairs are not stable, they lose their momentum under impurity scattering. The life time-bandwidth of bound states decreases as impurities are added.

Article Impurity Scattering in Superconductors

Ginzburg, Vitaly L., and Lev D. Landau. "On the theory of superconductivity." In On Superconductivity and Superfluidity, pp. 113-137. Springer, Berlin, Heidelberg, 2009.

One can ask about the pair scattering on impurities : if the pairs lose the momentum, how in some experiments the supercurrent can live for many months (assumed there is no ideally pure compounds) ?

always keep away the word quantum....that will help u a lot better.