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Proximity effect or Holm-Meissner effect is a term used in the field of superconductivity to describe phenomena that occur when a superconductor (S) is placed in contact with a "normal" (N) non-superconductor. Typically the critical temperature of the superconductor is suppressed and signs of weak superconductivity are observed in the normal material over mesoscopic distances. The proximity effect is known since the pioneering work by R. Holm and W. Meissner. They have observed zero resistance in SNS pressed contacts, in which two superconducting metals are separated by a thin film of a non-superconducting (i.e. normal) metal. The discovery of the supercurrent in SNS contacts is sometimes mistakenly attributed to B. Josephson 1962 work, yet the effect was known long before his publication and was understood as the proximity effect. ==Origin of the effect== Electrons in a superconductor in the superconducting state are ordered in a very different way than in a normal metal, i.e. they are paired into cooper pairs. Furthermore, electrons in a material cannot be said to have a definitive position because of the momentum-position complementarity. In solid state physics one generally chooses a momentum space basis, and all electron states are filled with electrons until the fermi surface in a metal, or until the gap edge energy in the superconductor. Because of the nonlocality of the electrons in metals, the properties of those electrons cannot change infinitely quickly. In the case of a superconductor and a normal metal, we have the superconducting cooper-paired-electrons order in the superconductor, and the gapless filled-up-to-the-fermi-surface electron order in the normal metal. if we bring the two together, the electron order in the one system cannot infinitely quickly (in real space) change into the other order at the border. The paired state in the superconducting layer is carried over to the normal metal, and there the pairing is destroyed by scattering events causing the paired electrons (cooper pairs) to lose coherence. For very clean metals like Cu it can be several hundreds of micrometers before the pairing is destroyed. Conversely, the (gapless) electron order present in the normal metal is also carried over to the superconductor in that the superconducting gap is lowered near the interface. The microscopic model describing this behavior in terms of single electron processes is called Andreev reflection. It describes how electrons in one material "pick up" the order of the layer they are proximate to by taking into account which states are present in the other material to scatter from and taking into account effects as interface transparency. 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「Proximity effect (superconductivity)」の詳細全文を読む スポンサード リンク
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