SUPERCONDUCTIVITY

complete disappearance of electrical resistance in various solids when they are cooled below a characteristic temperature. This temperature, called the transition temperature, varies for different materials but generally is below 20 K (-253 C). Superconductivity was first discovered in mercury by the Dutch physicist Heike Kamerlingh Onnes in 1911. Similar behaviour has been found in approximately 25 other chemical elements, including lead and tin, and in thousands of alloys and chemical compounds. All other materials that have been investigated to within fractions of a degree of absolute zero show normal resistance to the flow of electric currents. The use of superconductors in magnets is limited by the fact that strong magnetic fields above a certain critical value, depending upon the material, cause a superconductor to revert to its normal, or nonsuperconducting, state, even though the material is kept well below the transition temperature. Magnetization as a function of magnetic field for a type I superconductor and a type II Another basic property of superconductors, besides their lack of resistance, is their ability to prevent external magnetic fields from penetrating their interior: they are perfect diamagnets. All external magnetic fields less than the critical value are totally screened from the interior of type I superconductors, but strong fields are only partially screened from the interior of type II. Some type II superconductors have been found to retain their superconductivity in all but the strongest magnetic fields. In 1986 Karl Alex Mller and J. Georg Bednorz discovered that certain type II superconductors could retain their superconductivity at temperatures as high as 35 K. Compounds retaining their superconductivity at temperatures as high as 134 or 127 K were soon found. These high-temperature superconductors all contain copper and oxygen atoms that form planes or chains of atoms in the crystal. Their properties are anisotropici.e., dependent on the direction of current flow and of magnetic field with respect to the planes and chains of atoms. These new materials are ceramics, and their properties are sensitive to the amount of oxygen in them. Because they are superconducting at temperatures that can be inexpensively obtained with liquid nitrogen, these ceramic oxides hold great promise for practical applications. Problems of brittleness, instabilities in some chemical environments (such as moist air), and a tendency for impurities to segregate at surfaces of the crystals (where they interfere with the flow of high currents in the superconducting state) have yet to be overcome, however. Suggested uses for superconducting materials include medical magnetic-imaging devices, magnetic energy-storage systems, motors, generators, transformers, computer parts, and very sensitive devices for measuring magnetic fields, voltages, or currents. The main advantages of devices made from superconductors are low power dissipation, high-speed operation, and high sensitivity.