迈斯纳效应

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.

当施加的磁场过强时，迈斯纳效应会失效。对于I型超导体，超导性在超过临界磁场[latex]H_c[/latex]后会突然消失。对于II型超导体，在超过较低临界磁场[latex]H_{c1}[/latex]后，磁场开始以量子化磁通涡旋的形式渗透到材料内部，而材料则保持超导状态直至超过更高的较高临界磁场[latex]H_{c2}[/latex]。