Superconductivity

The various causes of resistance to the flow of electrons in an electric current were mentioned-crystal imperfections, impurities and thermal vibrations of the component atoms. In a pure, perfect crystal at low temperature it would therefore be expected that the electrical resistance would be small and this is observed experimentally. However in 1911 Kammerlingh Onnes, a Dutch physicist, using liquid helium as a coolant  found that below the extremely low temperature of 4.15K (i.e. 4.15 degrees above absolute zero) the electrical resistance of mercury actually became zero rather than just small. This phenomenon was subsequently observed in many other substances at low temperatures. It manifests itself most dramatically if a current is set flowing in a closed loop of superconducting wire, for example by moving a magnet near it. Such a current should flow for ever since there is no resistance and there are, indeed, examples where currents have been kept flowing in such loops for years. This sudden drop of resistance to zero below a certain critical temperature cannot be accounted for in terms of the physical processes of conduction we have considered so far and heralds a new phenomenon referred to as superconductivity. An understanding of what was happening was not forthcoming until 1957 when Bardeen, Cooper and Schrieffer proposed a theory which hinged on the fact that, when moving through a material, electrons experience not only a repulsive force between each other due to their negative charges but also an attractive force due to their interaction with the atoms in the material. This latter force arises because one electron moving near an atom displaces it slightly and that displacement, in turn, can have an effect on another electron. The net result is that the two electrons experience a small attraction to each other. At sufficiently low temperatures that the thermal vibrations of the atoms do not disturb the situation, this attractive force overcomes the repulsive force and results in electrons coordinating their motion in pairs. In the superconducting state the resistive scattering of one electron by an atom is exactly cancelled by the scattering of the other electron in the pair and there is no net effect. All the electrons move together in a coherent way and there is no resistance to their motion. Of course this is a very rough and ready account of a very sophisticated quantitative theory. What should be clear, however, is that the phenomenon of superconductivity is a quantum effect: no classical explanation is possible. Another interesting effect observed with superconductors the Meissner effect-is that if a magnetic field is applied it does not penetrate into it. The electrons flowing in the superconductor change their motion in such a way as to create an exactly opposite field which just cancels the applied field inside it. However, if this applied field is above a certain strength-the critical field the cancellation cannot be sustained. The magnetic field penetrates the superconductor and it becomes a normal conductor. In recent years it has been found that superconductivity in certain alloys can be achieved at much higher temperatures, of the order of 150K, and the search is on for ‘room-temperature’ superconductivity which would enable large-scale loss-free transmission of electric power. Superconducting wire is, however, already used extensively in powerful electromagnets. For such magnets, large electric currents are needed and, in a superconductor, the current flows continuously without any heating and energy loss due to electrical resistance as with a normal electromagnet. The only expenditure is that required to keep the superconducting wire below its critical temperature.