Why do superconductors conduct electricity without resistance? Many authors have suggested that persistent currents in superconducting rings arise from the energy gap in the single-particle spectrum. Indeed, the argument has been put forward many times on this site! It is usually suggested that because there is an energy gap, Cooper pairs are prevented from scattering out of the condensate.
However, this cannot be correct. For one, high temperature superconductors have d-wave symmetry, which implies a node (i.e. it takes zero energy to excite an electron along this direction). This seems to suggest that a complete gap is not necessary for persistent currents. Furthermore, it has been shown by Abrikosov and Gorkov that when one introduces magnetic impurities into an s-wave superconductor, the gap closes before persistent currents are destroyed.
Therefore, the single-particle gap is not a necessary condition for superconductivity and any attempt to explain persistent currents by appealing to an energy gap in the single-particle spectrum cannot be correct.
Is there therefore a simple way to understand why persistent currents exist in a superconductor intuitively? What are the necessary requirements?
 A: A superconductor is characterized by two main properties:


*

*zero resistivity, and

*the Meissner effect.


Equivalently, these can be stated more succinctly as


*

*$E = 0$ (remember that resistivity is defined as $\frac{E}{j}$), and

*$B = 0$.


So even more succinctly: superconductors are characterized by no internal electromagnetic fields!
What is the intuitive reason for this? It can be understood from the fundamental/microscopic property of superconductors: superconductors can be described in terms of superpositions of electrons and holes. Note that these two components have different electric charges, hence such a superposition can only be coherent if nothing couples to the charges inside a SC! Indeed, if there were an electromagnetic field inside the SC, it would couple differently to the electron and hole, decohering the superposition and destroying the SC. [Of course this doesn't do full justice to the theory of superconductivity, since this reasoning doesn't explain why we have superpositions of holes and electrons. Rather, my point is that once we start from that, then the aforementioned is hopefully intuitive.]
A: A superconductor conducts electricity without resistance because the supercurrent is a collective motion of all the Cooper pairs present.
In a regular metal the electrons more or less move independenly. Each electron carries a current $-e \textbf{v}(\textbf{k})$, where $\textbf{k}$ is its momentum and $\textbf{v}(\textbf{k}) = \partial E(\textbf{k})/\partial \textbf{k}$ is the semiclassical velocity. If an electron gets scattered from momentum $\textbf{k}$ to $\textbf{k}'$ it gives a corresponding change in the current. A sequence of such processes can cause the current to degrade.
In a superconductor, the story is totally different because the Cooper pairs are bosons and are condensed. This means that Cooper pairs self-organize into a non-trivial collective state, which can be characterized by an order parameter $\langle \Psi(\textbf{x}) \rangle = \sqrt{n} e^{i\theta(\textbf{x})}$  (where $\Psi$ is the annihilation operator for Cooper pairs.) which varies smoothly in space. Since the current operator can be written in terms of $\Psi$ it follows that gradients of $\theta$ give rise to currents of the condensate: $\textbf{j} = n(\nabla \theta + \textbf{A})$. All the small-scale physics (such as scattering) gets absorbed into the effective macroscopic dynamics of this order parameter (Landau-Ginzburg theory).
One should think of every single Cooper pair in the system taking part in some kind of delicate quantum dance, with the net effect being a current flow. But this dance is a collective effect and so it's not sensitive to adding or removing a few Cooper pairs. Therefore, scattering processes don't affect the current.
A: This can be explained by the BCS THEORY.
According to this theory, a Cooper pair of two electrons can be formed due to the net attractive force between the two electrons. It would be very surprising that how can two electrons attract each other because we know that like charges repel each other. But it can be made possible because of electron - lattice - electron interaction , i. e, lattice ions acts as a mediator for providing net attractive force between the two electrons.
Since the net spin of Cooper pair is zero, all the electrons that form the cooper pair can  be accommodated in the same ground state (Pauli exclusion principle), so electrons condensate to a single state.
We know that resistance occurs due to scattering of electrons by the lattice vibrations. But here since all the electrons are in a single state so the lattice have to scatter all the electrons simultaneously. It's all or none situation.
A large energy would be required to scatter huge number of electrons so lattice ions are not able to scatter the electrons and hence due to this resistivity drops to zero. So Resistivity of superconductors is zero for temperature below $T_c$.
A: While I don't disagree with much of what is said, I think it is important to take note of Landau's argument for superfluidity.    They key question is why a supercurrent can't decay and lose energy by exciting low energy quasiparticles.     This is  quite subtle because circulation quantization (quantization of the fluxoid) is not enough to prevent this decay process from occurring.   One could maintain circulation quantization and still slowly reduce the current by reducing the magnitude of the order parameter (i.e., generating heat). 
If there is a gap it is obvious that one cannot dissipate energy into low energy excitations.   However, for gapless superconductivity, it is less clear.   For nodal superconductors, as long as the dispersion around the node is linear, Landau's argument shows that below a critical velocity, one cannot have a process that reduces the supercurrent and excites quasiparticles.  
A: The superconductivity has a universal cause — the Bose-Einstein-Condensation (BEC) of bosons, because BEC-bosons have a minimum and quantized kinetic energy and, thus, cannot transfer their energy to other particles by arbitrarily small portions. Each boson in superconductors is an electron pair with zero total spin. Thus, the universal requirement for persistent currents in a superconductor is stable electron pairs below BEC temperature.  
