The Meissner Effect

When a material makes the transition from the normal to superconducting state, it actively excludes magnetic fields from its interior; this is called the Meissner effect.

This constraint to zero magnetic field inside a superconductor is distinct from the perfect diamagnetism which would arise from its zero electrical resistance. Zero resistance would imply that if you tried to magnetize a superconductor, current loops would be generated to exactly cancel the imposed field (Lenz's law). But if the material already had a steady magnetic field through it when it was cooled trough the superconducting transition, the magnetic field would be expected to remain. If there were no change in the applied magnetic field, there would be no generated voltage (Faraday's law) to drive currents, even in a perfect conductor. Hence the active exclusion of magnetic field must be considered to be an effect distinct from just zero resistance. A mixed state Meissner effect occurs with Type II materials.

One of the theoretical explanations of the Meissner effect comes from the London equation. It shows that the magnetic field decays exponentially inside the superconductor over a distance of 20-40 nm. It is described in terms of a parameter called the London penetration depth.

The Meissner Effect

Perfect Diamagnet If a conductor already had a steady magnetic field through it and was then cooled through the transition to a zero resistance state, becoming a perfect diamagnet, the magnetic field would be expected to stay the same. 如果一个导体原本已通过它有稳定的磁场，然后被冷却至零电阻状态，成为完美抗磁体，那么磁场预计会保持不变。	Superconductor Remarkably, the magnetic behavior of a superconductor is distinct from perfect diamagnetism. It will actively exclude any magnetic field present when it makes the phase change to the superconducting state. 超导体 Remarkably, 超导体的磁行为与完美抗磁性不同。当它转变为超导态时，它会主动排斥任何存在的磁场。

Perfect Diamagnetism

A conductor will oppose any change in externally applied magnetic field. Circulating currents will be induced to oppose the buildup of magnetic field in the conductor (Lenz's law). In a solid material, this is called diamagnetism, and a perfect conductor would be a perfect diamagnet. That is, induced currents in it would meet no resistance, so they would persist in whatever magnitude necessary to perfectly cancel the external field change. A superconductor is a perfect diamagnet, but there is more than this involved in the Meissner effect. 导体将反对任何外部施加的磁场变化。会感应出电流以反对导体中磁场的建立（楞次定律）。在固体材料中，这种现象称为顺磁性，而一个完美的导体将是一个完美的顺磁体。也就是说，其中感应出的电流将遇到零电阻，因此会持续存在，无论其大小如何，以完美地抵消外部磁场的变化。超导体是一个完美的顺磁体，但迈斯纳效应涉及的远不止于此。

Mixed-State Meissner Effect

In Type II superconductors the magnetic field is not excluded completely, but is constrained in filaments within the material. These filaments are in the normal state, surrounded by supercurrents in what is called a vortex state. Such materials can be subjected to much higher external magnetic fields and remain superconducting.

Spin Alignment vs Electron Pairs