What is a Meissner effect in superconductors?
1 Answer
The Meissner effect is a fascinating and essential phenomenon observed in superconductors. When a material becomes a superconductor, it exhibits zero electrical resistance and expels magnetic fields from its interior. This expulsion of magnetic flux is known as the Meissner effect. It was first discovered by physicists Walther Meissner and Robert Ochsenfeld in 1933.
When a superconductor is cooled below its critical temperature, it undergoes a transition to the superconducting state. In this state, the material forms pairs of electrons known as Cooper pairs, which behave collectively rather than independently. These Cooper pairs exhibit a unique property known as macroscopic quantum coherence, allowing them to flow through the material without scattering, hence the absence of electrical resistance.
As a magnetic field is applied to a superconducting material, the magnetic flux lines attempt to penetrate it. However, the Cooper pairs oppose this penetration and generate opposing magnetic fields to cancel out the external magnetic field within the superconductor. As a result, the magnetic field is expelled from the bulk of the material, and it only persists on the surface in the form of thin surface currents.
The expulsion of magnetic fields gives rise to several remarkable properties of superconductors, such as the ability to levitate above a magnet (quantum levitation) or the complete shielding of the interior from external magnetic fields. This effect is crucial for various practical applications, including superconducting magnets used in MRI machines, particle accelerators, and maglev trains.
It's important to note that the Meissner effect occurs only in type I superconductors, which have a single critical temperature below which they become superconducting. Type II superconductors, on the other hand, exhibit a mixed state where some magnetic flux can penetrate the material in the form of quantized vortices, allowing for a wider range of applications in high magnetic fields.
When a superconductor is cooled below its critical temperature, it undergoes a transition to the superconducting state. In this state, the material forms pairs of electrons known as Cooper pairs, which behave collectively rather than independently. These Cooper pairs exhibit a unique property known as macroscopic quantum coherence, allowing them to flow through the material without scattering, hence the absence of electrical resistance.
As a magnetic field is applied to a superconducting material, the magnetic flux lines attempt to penetrate it. However, the Cooper pairs oppose this penetration and generate opposing magnetic fields to cancel out the external magnetic field within the superconductor. As a result, the magnetic field is expelled from the bulk of the material, and it only persists on the surface in the form of thin surface currents.
The expulsion of magnetic fields gives rise to several remarkable properties of superconductors, such as the ability to levitate above a magnet (quantum levitation) or the complete shielding of the interior from external magnetic fields. This effect is crucial for various practical applications, including superconducting magnets used in MRI machines, particle accelerators, and maglev trains.
It's important to note that the Meissner effect occurs only in type I superconductors, which have a single critical temperature below which they become superconducting. Type II superconductors, on the other hand, exhibit a mixed state where some magnetic flux can penetrate the material in the form of quantized vortices, allowing for a wider range of applications in high magnetic fields.