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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 임계 온도 ([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 초전도성, 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.
마이스너 효과는 인가된 자기장이 너무 강할 때 깨집니다. 1종 초전도체의 경우, 임계 자기장 [latex]H_c[/latex] 이상에서 초전도성이 갑자기 소멸됩니다. 2종 초전도체의 경우, 더 낮은 임계 자기장 [latex]H_{c1}[/latex] 이상에서 양자화된 자속 소용돌이 형태로 자기장이 물질 내부로 침투하기 시작하며, 물질은 훨씬 더 높은 상위 임계 자기장 [latex]H_{c2}[/latex]까지 초전도 상태를 유지합니다.
마이스너 효과
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