Superconductivity is the complete disappearance of all electrical resistance at low temperatures. We described this property at the end of Section 26-3 and the magnetic properties of type I and type II superconductors in Section 30-9. In this section we’ll relate superconductivity to the structure and energy-band model of a solid. Although superconductivity was discovered in 1911, it was not well understood on a theoretical basis until 1957. In that year, the American physicists John Bardeen, Leon Cooper, and Robert Schrieffer published the theory of superconductivity, now called the 0 BCS theory, that was to earn them the Nobel Prize in 1972. (It Bardeen’s second Nobel Prize; he shared his first for his work on the development of the transistor.) The key to the BCS theory is an interaction between pain of conduction electrons, called Cooper pairs, caused by an interaction with the positive ions of the crystal. Here’s arough qualitative picture of what happens. A free electron exerts attractive forces on nearby positive ions, pulling them slightly closer together. ‘The resulting slight concentration of positive charge then exerts an attractive force on another free electron with momentum opposite to the first. At ordinary temperatures this electron-pair interaction is very small in comparison to energies of thermal motion, but at very low temperatures it becomes significant.
Bound together this way, the pairs of electrons cannot individually gain or lose very small amounts of energy, as they would ordinarily be able to do in a partly filled conduction band. Their pairing gives an energy gap in the allowed electron quantum levels, and at low temperatures there is not enough collision energy to jump this gap. Therefore the electrons can move freely through the crystal without any energy exchange through collisions, that is, with zero resistance. Researchers in the field have not yet reached a consensus on whether or not some modification of the BCS theory can explain the properties of the high-Tc superconductors that have been discovered since 1986. There is evidence for pairing. but possibly of holes rather than electrons. Furthermore, the original pairing mechanism of the BCS theory seems too weak to explain the high transition temperatures and critical fields of these new superconductors.