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Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of ''perfect conductivity'' in classical physics. The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing through a loop of superconducting wire can persist indefinitely with no power source.〔 reprinted in Nikolaĭ Nikolaevich Bogoliubov (1963) ''The Theory of Superconductivity, Vol. 4'', CRC Press, ISBN 0677000804, p. 73〕〔 〕 In 1986, it was discovered that some cuprate-perovskite ceramic materials have a critical temperature above . Such a high transition temperature is theoretically impossible for a conventional superconductor, leading the materials to be termed high-temperature superconductors. Liquid nitrogen boils at 77 K, and superconduction at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures. ==Classification== (詳細はType I'', meaning it has a single critical field, above which all superconductivity is lost; or ''Type II'', meaning it has two critical fields, between which it allows partial penetration of the magnetic field. *By theory of operation: It is ''conventional'' if it can be explained by the BCS theory or its derivatives, or ''unconventional'', otherwise. *By critical temperature: A superconductor is generally considered ''high temperature'' if it reaches a superconducting state when cooled using liquid nitrogen – that is, at only ''Tc'' > 77 K) – or ''low temperature'' if more aggressive cooling techniques are required to reach its critical temperature. *By material: Superconductor material classes include chemical elements (e.g. mercury or lead), alloys (such as niobium-titanium, germanium-niobium, and niobium nitride), ceramics (YBCO and magnesium diboride), or organic superconductors (fullerenes and carbon nanotubes; though perhaps these examples should be included among the chemical elements, as they are composed entirely of carbon). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「superconductivity」の詳細全文を読む スポンサード リンク
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