What Causes Superconductivity ?

Following Kamerlingh Onnes’ discovery of zero resistance, it took a very long time to understand how superconducting electrons can move without hindrance through a metal. Attempts to explain from first principles how superconductivity comes about proved to be one of the most intractable problems of physics. Progress required more than just new data; it needed an innovative theoretical framework built around radical new ideas. There are inherent difficulties in achieving this. Fundamentally new concepts are not discovered by observation alone but require new modes of imaginative thought, which by their very nature are unpredictable and elusive. One day in 1955, when John Bardeen, Bernd Matthias and Theodore Geballe were driving between Murray Hill and Princeton, the question was raised: “What are the most important unsolved problems in solid state physics?” After a characteristic lengthy pause, Bardeen suggested that superconductivity must be a candidate. Later in 1957, in collaboration with Leon Cooper and Robert Schrieffer, he was to provide a most ingenious, and generally accepted, explanation of superconductivity founded on quantum mechanics. The physical principles underlying this BCS theory are the concern of this chapter. It has become evident that while the BCS theory gives a reasonable description of superconductivity in the “conventional” superconductors, known at that time, that is not the case for more recently discovered “unconventional” materials, such as the high temperature superconducting cuprates (which will be discussed separately in Chapter 11). BCS theory has established that superconductivity in conventional materials arises from interactions of the conduction electrons with the vibrations of the atoms. This interaction enables a small net attraction between pairs of electrons. Before insight into this electron pairing and a subsequent ordering can be gained, some characteristics of superconductors need to be brought to mind. Superconductivity is a common phenomenon; at low temperatures many metals, alloys and compounds are found to show no resistance to flow of an electric current and to exclude magnetic flux completely. When a superconductor is cooled below its critical temperature, its electronic properties are altered appreciably, but no change in the crystal structure is revealed by X-ray crystallographic studies. Furthermore, properties that depend on the thermal vibrations of the atoms remain the same in the superconducting phase as they were in the normal state. Superconductivity is not associated with any marked change in the behavior of the atoms on the crystal lattice. However, although superconductivity is not a property of particular atoms, it does 6