BCS theory Bardeen Cooper Schreiffer theor. A theory that explains superconductivity, where 2 electrons—a Cooper pair—of opposite spin and momentum are bound together so that they have zero net spin and momentum. The attractive force behind this pairing is a subtle interaction between the negative charge of electrons and the positive charge of ion cores in the superconducting material. These ion cores are simply atoms that have lost one or more of their outermost electrons, which become free to conduct electricity. The ion cores are pulled in toward an electron as it moves through the lattice of a solid, creating a region of enhanced positive charge. The theory is named after J Bardeen, LN Cooper and JR Schreiffer.