（画像はイメージです）
Discovered in 1933 by Walther Meissner and Robert Ochsenfeld, the Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. When a material is cooled below its critical temperature ([latex]T_c[/latex]) in the presence of a weak external magnetic field, it actively cancels all magnetic flux inside it, becoming a perfect diamagnet.
The Meissner effect is a defining characteristic of superconductivity, distinguishing it from a hypothetical perfect conductor. A perfect conductor, according to Lenz’s law, would trap any existing magnetic field inside it as it cools, because changing the flux would induce currents to oppose the change. In contrast, a superconductor actively expels the field. This expulsion is achieved by the generation of screening currents on the surface of the material. These currents create a magnetic field that perfectly cancels the external field within the bulk of the superconductor. The magnetic field penetrates only a small distance into the surface, known as the London penetration depth ([latex]\lambda[/latex]). This discovery was crucial because it showed that superconductivity is a true thermodynamic phase transition, not just a case of infinite conductivity. It provided a key piece of the puzzle that any microscopic theory of superconductivity would have to explain. The effect is the basis for the dramatic levitation demonstrations where a magnet floats above a high-temperature superconductor cooled with liquid nitrogen. The expulsion of the magnet’s field by the superconductor creates a repulsive force strong enough to counteract gravity.
マイスナー効果は、印加磁場が強すぎると破綻します。タイプI超伝導体では、臨界磁場[latex]H_c[/latex]を超えると超伝導が突然破壊されます。タイプII超伝導体では、下限臨界磁場[latex]H_{c1}[/latex]を超えると、量子化された磁束渦の形で磁場が物質に浸透し始めますが、物質ははるかに高い上限臨界磁場[latex]H_{c2}[/latex]に達するまで超伝導状態を維持します。
マイスナー効果
（日付が不明または関連性がない場合、例えば「流体力学」などでは、その注目すべき出現時期の概算値が提示されます。）
フルサイズの画像とダウンロードは、登録会員のみが100％無料で利用できます。
> ログイン <
