When a superconductor’s temperature drops below its critical value, some of the most loosely bound electrons assemble into a single, Bose-Einstein ground state. Locked together, the electrons flow through the lattice with unimpeded ease.
To reach that remarkable state, electrons, being fermions, must pair up to form bosons whose total spin S is an integer. Pairs of spin-½ electrons have two choices of S: 0 or 1, antiparallel or parallel. Because a pair of identical fermions must have an antisymmetric wavefunction, fixing S also constrains the pair’s total orbital angular momentum L: If S = 0, L must be an even integer; if S = 1, L must be an odd integer.
How electrons follow those rules and actually pair up depends on the symmetry of the lattice and on what fluctuations polarize and nudge the electrons together. In ordinary, Bardeen-Cooper-Schrieffer superconductors, lattice vibrations mediate the pairing and S...
