A theory of superconductivity formulated by John Bardeen, Leon Cooper, and Robert Schrieffer. It explains the phenomenon in which a current of electron pairs flows without resistance in certain materials at low temperatures. This can happen, so the theory says, when a single negatively charged electron slightly distorts the lattice of atoms in the superconductor, drawing toward it a small excess of positive charge. This excess, in turn, attracts a second electron. It is this weak, indirect attraction that binds the electrons together, into a Cooper pair.