| 单词 | superconductivity |
| 释义 | superconductivity The absence of measurable electrical resistance in certain substances at temperatures close to 0 K. First discovered in 1911 in mercury, superconductivity is now known to occur in some 26 metallic elements and many compounds and alloys. The temperature below which a substance becomes superconducting is called the transition temperature (or critical temperature ). Compounds are now known that show superconductivity at liquid-nitrogen temperatures. The theoretical explanation of the phenomenon was given by John Bardeen, L. N. Cooper (1930– ), and J. R. Schrieffer (1931– ) in 1957 and is known as the BCS theory . According to this theory an electron moving through an elastic crystal lattice creates a slight distortion of the lattice as a result of Coulomb forces between the positively charged lattice and the negatively charged electron. If this distortion persists for a finite time it can affect a second passing electron. In 1956 Cooper showed that the effect of this phenomenon is for the current to be carried in superconductors not by individual electrons but by bound pairs of electrons, the Cooper pairs . The BCS theory is based on a wave function in which all the electrons are paired. Because the total momentum of a Cooper pair is unchanged by the interaction between one of its electrons and the lattice, the flow of electrons continues indefinitely. Superconducting coils in which large currents can circulate indefinitely can be used to create powerful magnetic fields and are used for this purpose in some particle accelerators and in other devices. Superconductivity can also occur by a slightly more complicated mechanism than BCS theory in heavy-fermion systems. In 1986, Georg Bednorz (1950– ) and Karl Müller (1927– ) found an apparently completely different type of superconductivity. This is called high-temperature superconductivity , since the critical temperature is very much higher than for BCS superconductors; some high-temperature superconductors have critical temperatures greater than 100 K. A typical high-temperature superconductor is YBa2Cu3O1–7. At the present time a theory of high-temperature superconductivity has not been established in spite of a great deal of effort, which is still going on. The BCS mechanism, and minor modifications of it, almost certainly do not apply. One model for high-temperature superconductivity uses the concept of anyons, but at present there is no evidence for this mechanism |
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