When a material becomes superconducting below some critical temperature Tc, its electrical resistivity drops abruptly to zero. That complete loss of resistance may seem impossible because current-carrying electrons scatter from atomic vibrations, impurities, and crystal imperfections of the lattice. Electrons also ordinarily repel each other because they have the same negative charge. But two electrons in a metallic lattice can experience an attractive interaction through polarization of the positive ionic lattice, as explained in box 1. Because of that interaction, pairs of electrons with opposite momenta and spin angular momenta can bind together. Those Cooper pairs coalesce into a coherent wavefunction known as the superconducting condensate.

First observed soon after helium was liquefied in 1908 (see the article by Dirk van Delft and Peter Kes, Physics Today, September 2010, page 38) and today open to simple tabletop demonstrations, the superconducting condensate holds a unique standing...

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