- What is it that causes the matter in superconducting state expel magnetic flux? What exactly happens at the atomic level which causes the expulsion of flux and super-conductance of electrons?
- Do all superconductors expel magnetic flux?
- Does the superconductor absolutely block all of flux that try to pass through it, no matter how strong the flux is?
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1$\begingroup$ First a remark, you should try to avoid asking several questions in one post. The expel of the magnetic field from the bulk superconductor is called Meissner effect. I suggest you to look for this keyword elsewhere on this website, as well as on Wikipedia and other websites. Then you might want to precise your question further in an other post. All superconductors repel the magnetic field: it's even their definition. Superconductors are not defined as perfect conductors, they are defined as perfect diamagnetic. The perfect conduction is a consequence of the perfect diamagnetism. (...) $\endgroup$– FraSchelleCommented Jan 16, 2015 at 8:13
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1$\begingroup$ (...) Nevertheless, some superconductors expel the magnetic field globally (they are called type I superconductors), whereas others expel the magnetic field only locally, whereas some tubes of magnetic flux can penetrate the bulk. These tubes are called vortices, and the associated superconductors are called type-II superconductors. Anyhow, as long as the superconductor repels the magnetic field, it behave as a superconductor. But for too strong magnetic field, there is a phase transition back into the normal phase, when there is no more Meissner effect. @IgnacioVergar answered this point $\endgroup$– FraSchelleCommented Jan 16, 2015 at 8:15
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I cannot answer the first question, but I can do the other two.
Yes, all superconductors expel magnetic flux (Meisner-Ochsenfeld effect). It is part of the definition of superconducting state. Without this property, the material would be just a perfect conductor ($\rho_e \approx 0$) which is different than a superconductor.
The field marginally penetrates the superconductor. This penetration is affected by the geometry of the system and the intensity of the external magnetic field applied. In some cases the presence of impurities in the material will trap the magnetic flux within the material in regions where it is not a superconductor anymore. This is important for applications like levitating superconducting rails.
Given a high/strong enough magnetic field, called critical field strength $H_c$, the superconducting state will be destroyed and the material will make a phase transition into a normal state (normal state would have a lower free energy than the superconducting state). Thereby, losing the properties of field expulsion and zero resistance electrical conduction.
answered Jan 15, 2015 at 16:56
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1$\begingroup$ The first question is easily answred: alsmost nothing. Superconductance is a bulk phenomenon of crystals. At the level of singel atoms nothing relevant happens. $\endgroup$– GeorgCommented Jan 15, 2015 at 18:04
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$\begingroup$ @Georg ...and what is it at the bulk level that expels the magnetic flux? I've been googling around, arriving to this question as well, but there seems to be a) no explanation and b) no admittance that an explanation doesn't exist (with at least a list of attempts at it). There are instead weasel explanations invoking the London equations, which are themselves just phenomenological. $\endgroup$ Commented Aug 10, 2017 at 16:16