Superconductivity

The discovery of supercon­ductivity - the property of certain conductors to display zero DC electrical resistance - by Heike Kamerlingh Onnes and co-workers in 1911 illustrates the point. It was a true discovery, and indeed a remarkable one, because there were no valid arguments around to predict such a phenomenon. Yet, to draw the conclusion that it was accidental would be unjustified. The necessary technical basis and opportunity for the discovery had been solidly established in the same group by the liquefaction of the inert gas helium in 1908. And research, both experimental and theoretical, on the electrical conductivity of metals at temper­atures approaching the absolute zero, was ongoing and considered an important issue by leading physicists. They were even puzzled by an apparent similarity between the temperature dependences of electrical conductivity and heat capac­ity in metals. On traveling down the infinite road towards zero degrees Kelvin, new discoveries could and should be expected, since temperature is the most universal variable by which the (equilibrium) state of matter is defined. In fact, the absence of discoveries along this route should have been the real surprise. Even the cryogen itself, liquid helium, was later to offer a number of opportuni­ties for important discovery. And many more were to come in other condensed matter systems below ambient temperatures.

A system composed of particles, whose quantum mechanical zero point en­ergy is large compared to their interaction energy, does not solidify even at absolute zero temperature and remains a so-called quantum liquid. Typi­cal examples of such systems are conduction electrons in metals and liquid helium. Many conductors undergo a phase transition at their critical temper­ature T_c and become superconducting below it. Similarly, liquid ⁴He under its vapour pressure becomes a superfluid at T_c = 2.17 K (T is often used instead of T_Q), and liquid ³He at T_Q = 0.9 mK. Although the phenomena are called superconductivity for a charged system like conduction electrons, and superfluidity for a neutral system like liquid helium, they are characterized by the same basic property that the wave nature of the particles manifests itself on a macroscopic scale.

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• What is superconductivity?