Why superconductors repel magnets

While superconductors may appear to react the same when in an applied magnetic field, there are differences that separate superconductors into two categories depending on how they react. For all superconductors, there is a maximum magnetic field, called the critical magnetic field, Bc, that can be applied before the magnetization opposing the magnetic field reaches a maximum and the superconductor reverts to its nonsuperconducting state. For a Type I superconductor, this direct relationship between the applied magnetic field and the opposing magnetization follows until the critical magnetic field is reached and superconduction no longer occurs.

When this point is reached, the Meissner effect, which would occur within the superconductor up until the critical magnetic field, vanishes, and the magnetic field is able to pass through the superconductor unhindered. For Type II superconductors, there is an additional state that occurs between the Meissner state and the normal state. This state, called the mixed state or the vortex state, is noted by the mixing of the normal and Meissner states.

The magnetic field is allowed to pass through the superconductor at specific parts where the normal state is occurring, while the rest of the superconductor exhibits the Meissner effect and expels the magnetic field.

The shift from the Meissner effect to the vortex state occurs at a lower critical magnetic field, B c1. This vortex state continues to occur to the upper critical magnetic field, B c2 , where the magnetic field becomes too strong for the superconductor to expel and the superconductor allows all magnetic fields to pass through it, returning it to its normal state. Between these two extremes the vortex state occurs, where small tubular regions develop within the superconductor, inside which the superconductor is in the normal state.

The range of states for a Type II superconductor can be seen in Figure 4. Through these flux tubes or vortices, the magnetic field is allowed to pass.

As with the edge of the superconductor, currents develop on the inside of the flux tubes, preventing the magnetic field from passing into the Meissner state sections of the superconductor. It is within this vortex state that flux pinning occurs. While the effects of flux pinning may appear similar to the levitating magnets caused by the Meissner effect, the cause behind flux pinning differs in some ways. For flux pinning to occur, the superconducting material either needs to be very thin or it needs to be a Type II superconductor.

If it is thin or Type II, some of the magnetic field is allowed to pass through the superconductor, but only in specific spots, called flux tubes or a vortex. The reason the magnetic field is allowed to pass through the superconductor at these flux tubes is because there is no superconductivity within those regions.

The superconductor tries to keep the flux tubes pinned to weaker parts of the superconductor, such as grain boundaries or other imperfections.

Active 6 years, 10 months ago. Viewed 2k times. 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?

Improve this question. Nagendra Rao Nagendra Rao 3 3 bronze badges. 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. 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.

Add a comment. Interestingly, the conductivity, s, is infinite for superconductors! This scenario makes it possible to have infinite conductivity but finite current density. Figure 1: Current flowing in a superconducting ring. The red arrow indicates electric current.

Blue arrows represent magnetic field lines flowing through the ring. Equation 2. Certainly, this equation looks a bit daunting. Equation 3. Ah, this is much simpler now! Equation 3 assures us that the magnetic flux through a superconductor cannot change with time. So according to Equations 2 and 3, current flowing in a superconducting ring produces magnetic flux that cannot change with time.

Now notice that if the current stops, the magnetic flux will as well. Because of this, the electric current must continue forever superconduction to keep the magnetic flux constant! When a Type 2 superconductor Figure 2 is placed in a magnetic field, the magnetic field penetrates at microscopic impurities embedded in the superconductor Video 1, — The term flux tube is used to describe the system of ring current, impurity, and penetrating magnetic field.

Figure 2: A Type 2 superconductor grey rectangular solid in a magnetic field blue arrows. The magnetic field penetrates impurities white rings in the superconductor. The flow of field lines through the impurities must stay constant, which locks the superconductor in place. Flux tubes that are penetrated by constant magnetic field explain why a Type 2 superconductor can hover undisturbed above or below a magnetic field.