Superconductors and Superconductivity
A superconductor is a material that conducts electrical current with no resistance. The phenomenon of superconductivity was first discovered by H. Kamerlingh Onnes in 1911, when he found that the resistance of mercury dropped to zero at a temperature of about 4K. Many other materials, including tin, lead, and niobium, were tested and also found to superconduct at extremely low temperatures, less than 10 K. In 1957, a theory of superconductivity was advanced that explained the phenomenon quite well, and it has been used in further research and development of other superconducting materials.
Superconductivity normally only occurs at extremely low temperatures. Each superconducting material has a different critical temperature; above the critical temperature, the material does not superconduct. As the temperature of the material is lowered, the material undergoes a phase transition from its normal state to the superconducting state. The critical temperature also depends on the external magnetic field present. As the strength of the magnetic field increases, the critical temperature decreases. At a certain field strength, called the critical magnetic field, the material can no longer superconduct at any temperature.
Superconductors have another interesting property: in the presence of a weak external magnetic field, all magnetic field lines will be expelled from the superconductor itself. This effect is called the Meissner Effect. It can be easily observed by trying to place a magnet on top of a piece of superconducting material; the magnet will levitate above the surface of the superconductor. Another interesting phenomenon involving superconductors is quantized magnetic flux. If a superconducting ring is placed in a magnetic field above its critical temperature, and then cooled, the magnetic flux will be expelled from the superconductor itself, but some flux will be trapped in the hole. It is found that the flux can only exist in quantized units; it turns out that this phenomenon is explained by requiring the wavefunction for electrons in the superconductor to have a single value at each spatial point.
In 1957, John Bardeen, Leon N. Cooper, and John Robert Schrieffer published a quantum mechanical theory of superconductors that came to be known as the BCS theory. In this theory, pairs of electrons attract each other by interacting through the crystal lattice of the solid material. These Cooper pairs are the charge carriers of the electric current, with charge -2e. The attractive potential energy of the electron-lattice interaction sets up an energy gap between electrons that interact collectively in pairs, and electrons that interact individually. The energy of the Cooper pairs cannot be dissipated by collisions, because the energy gap between levels is too large compared to the energy that would be lost in a collision. Therefore, there is no power loss and hence no resistance.
The possible uses of superconductors are quite exciting. For example, a current set up in a superconducting ring can continue to circulate for an extremely long time without dissipating. Theory predicts a lifetime of at least 100,000 years for a current circulating in a superconducting ring; experimenters have been able to measure the current dissipation in superconducting rings and have found it to be negligible. Power could be transmitted through superconducting power lines, making electrical transmission more efficient and saving energy. Superconducting magnets can be produced that act like permanent magnets. The Meissner effect could be used to produce efficient magnetic-levitation transportation or other technologies.
The obstacle standing in the way of widespread use of superconductors is that superconducting materials have to be cooled to extremely low temperatures. Common superconducting materials, such as mercury and lead, have to be cooled below 10K, around the boiling point of liquid helium. After much research, materials known as high-T superconductors have been created, but they still require temperatures close to the boiling point of nitrogen, 77K. These high-T superconductors are exotic compounds containing rare elements; for example, the compound YBaCuO has a critical temperature of 90K, but the amount of Yttrium found naturally is probably too small to allow mass production of superconducting power lines. It is safe to say that high-T superconductor research is one of the largest current research fields, and room-temperature superconductors are one of the most coveted materials in science research. The promising uses for these materials makes their development extremely desirable.
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