Meissner effect

Japanese: マイスナー効果 - マイスナーこうか（英語表記）Meissner effect

A phenomenon that characterizes the magnetic properties of superconductivity. When a superconductor is cooled in a weak magnetic field, below the superconducting transition temperature (critical temperature), magnetic flux is expelled from the superconductor and it becomes completely diamagnetic. This property is called the Meissner effect. It was discovered in 1933 by W. Meissner and R. Ochsenfeld. This effect cannot be explained by the perfect electrical conductivity that characterizes superconductivity alone. When a magnetic field is applied to a superconductor, a superconducting current flows on the surface, just as if to cancel out the applied magnetic field. The magnetic flux penetrates the superconductor to the same extent as the thickness of the layer through which this surface current flows. The thickness of this layer is called the penetration length. The penetration length is several tens of nanometers at absolute zero, but increases with temperature, becoming longer as the transition temperature is approached. When the magnetic field is strengthened, the superconducting state is destroyed. The figure shows the magnetization of superconductivity as a function of magnetic field. Superconductors made of pure materials other than niobium and vanadium are called type I superconductors, which show the Meissner effect up to a critical magnetic field H _C and undergo a first-order transition to the normal state at H _C. Superconductors made of niobium, vanadium, and other alloys and compounds are called type II superconductors. The Meissner region is reached up to the lower critical magnetic field H _C1 , but above H _C1 magnetic flux enters the superconductor as flux quanta. The region up to the upper critical magnetic field H _C2 is called the mixed state. The mixed state undergoes a second-order transition to the normal state at H _C2 . H _C varies depending on the material, but can be as large as 10 ^-2 T. Type II superconductors made of suitable alloys and compounds have a very large H _C2 , so they are used in superconducting magnets, etc.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

超伝導の磁気的性質を特徴づける現象。超伝導体を弱い磁場中で冷却してゆくとき，超伝導転移温度 (臨界温度) 以下では磁束が超伝導体の外へ追出されて完全反磁性になる性質をマイスナー効果という。 1933年 W.マイスナーと R.オクセンフェルトによって発見された。超伝導を特徴づける完全電気伝導性のみでは，この効果を説明できない。超伝導体に磁場をかけると，加えた磁場をちょうど打消すように超伝導電流が表面を流れる。この表面電流が流れる層の厚みと同じ程度に磁束が超伝導体にしみこむ。この層の厚みを浸入長という。浸入長は絶対零度では数十 nmであるが，温度とともに増大し，転移温度に近づくに従って長くなる。磁場を強くすると，超伝導状態は破壊される。図に超伝導の磁化を磁場の関数として示す。ニオブとバナジウム以外の純粋物質の超伝導体を第一種超伝導体といい，臨界磁場 H_C までマイスナー効果を示し，H_C で常伝導状態に一次転移する。ニオブ，バナジウム，および他の合金や化合物の超伝導体を第二種超伝導体という。下部臨界磁場 H_C1 まではマイスナー領域であるが，H_C1 をこえると磁束が磁束量子として超伝導体に入る。上部臨界磁場 H_C2 までの間を混合状態という。混合状態は H_C2 で常伝導状態へ二次転移する。 H_C は物質により異なるが，大きなもので 10^-2 T程度である。適当な合金や化合物の第二種超伝導体では H_C2 が非常に大きいので，超伝導磁石などに利用される。

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