Kamerlingh Onnes (1853 – 1926), discovery 1911, Nobel Prize 1913!

In 1972, John Bardeen, Leon N. Cooper and J. Robert Schrieffer won the Nobel Prize for the theoretical description (1957) of the 1911 discovery of what we now call Type 1 Superconductivity, by Onnes.

We now understand electrical conductivity in metal solids to be comprehensible only in terms of quantum physics, but a classical theory of conductivity existed by 1900, created by Paul Drude. It was so successful, and the quantum theory of solids so difficult to work out, that his approach was not replaced until around 1933 by a satisfactory quantum theory... which did NOT explain superconductivity in any way! Type 1 superconductors were at first found only to operate near absolute zero, but continual search for new superconducting materials eventually resulted in the discovery of superconductivity in certain materials at somewhat below or at the temperature of liquid nitrogen, 77 K, efforts which depended upon the  discovery of a completely different type of superconductivity, type 2. Vigorous research at many laboratories has identified exotic materials, involving many different types of atoms in  very complex structures, materials which superconduct at higher and higher temperatures, but so far nowhere near room temperature, generally taken to be defined as 300 K.

This is how superconductivity is usually demonstrated to reporters and the general public. A semiconducting material excludes magnetic fields, so that a magnet brought down slowly above a superconductor will eventually be supported against gravity, in a phenomenon reporters call magnetic levitation, but physicists call the Meissner Effect.  Type 2 materials show a gradual dropoff of resistance instead of the abrupt drop seen in Type 1 materials, but do eventually show the Meissner levitation. Why do metals exhibit resistance anyway? The flow of electrons through a conductor is impeded by imperfections and defects in the metallic lattice, by the presence of “impurities” (atoms of other elements present in small quantities), and by the normal thermal oscillations of the metal atoms themselves --- which is why resistance increases with temperature. In short, the electrons interact in various ways with the atoms in the lattice, and lose kinetic energy steadily... which is why conductors become hot when current flows through them. So how does a type 1 superconductor avoid such fates for the electrons?

The answer is that at low temperature in relatively pure metals, in Type 1 superconductors, electrons can form Cooper pairs, which behave like bosons and have enough binding energy to be relatively stable. The rough idea is that the interaction of an electron with a lattice ion shifts its position slightly, which in turn affects another electron... the electrons form a bound state held together by phonon exchange. This state physically occupies an enormous amount of space, when compared to the typical spacing of atoms in the lattice.  The result is nearly a standing wave of sorts that stretches throughout the lattice. To see how this works, consider two electrons moving in opposite directions. One would be described by exp(+ikx) and the other by exp(-ikx). But remember that, for example, sin(kx) = [exp(ikx) - exp(-ikx)]/(2i). The Cooper pairs consist of electrons propagating in opposite directions. The pair is even more tightly bound if the electron spin states combine to S = 0. In fact this spin pairing is strong enough on its own to result in Type 2 superconductivity, without Cooper pairs ever forming in that case.  Here is the tragic story of how Type 2 superconductivity was discovered very early, and then lost, for decades!  Studies of so-called iron-based superconductors over the past 15 years have suggested that there is likely a third possible type of superconductivity, based on neighboring atoms of different elements having electron states at the same energy, with totally different and overlapping state functions.

Low temperature Type 2 superconductors, which remain superconducting even in enormously strong magnetic fields, have seen an equally enormous number of applications in modern technology, particularly in the creation of electromagnets used in medical diagnostics, particle accelerators, etc., and even in electric motors and generators. Type 1 superconductors are not suitable to use to construct powerful electromagnets because, in them, a strong magnetic field destroys the superconducting state! Therefore Type 2 materials have to be used for superconductive magnets. These electromagnets, achieving a magnetic field strength of about 20 Tesla or even more, use wire made from niobium alloys, and operate at a  temperature of 4 K (cooled by liquid helium).

Much research today goes into discovering properties of two-dimensional materials. These materials often exhibit low-temperature superconductivity, and there is no clear reason why. Really strange things happen when the materials are layered, with a slight offset. As mentioned above, the strong suspicion is that here we see a third kind of superconductivity, due to effects of electron bound states on conduction electrons.  However, finding materials that are not brittle but do superconduct at liquid nitrogen temperatures or above, at normal pressure,  also remains an elusive goal.