The Meissner effect is a fascinating and fundamental phenomenon observed in superconductors, which are materials that exhibit zero electrical resistance and the expulsion of magnetic fields below a certain critical temperature. This effect was first discovered by Walter Meissner and Robert Ochsenfeld in 1933.
When a superconductor is cooled below its critical temperature (also known as the transition temperature), it undergoes a phase transition and enters the superconducting state. In this state, electrons form Cooper pairs due to the attractive interaction mediated by lattice vibrations (phonons). These Cooper pairs of electrons behave as bosons, which allows them to condense into a collective ground state, known as a superconducting condensate.
The Meissner effect occurs when an external magnetic field is applied to the superconductor while it is in its superconducting state. As a result of the Cooper pairs' condensation and their ability to form a macroscopic quantum state, the superconductor expels the magnetic field from its interior. The expelled magnetic field lines, also known as magnetic flux, are forced to travel around the surface of the superconductor in closed loops. As a result, there is no penetration of the magnetic field into the bulk of the superconducting material.
This expulsion of magnetic fields has some important consequences:
Zero resistance: The absence of magnetic field penetration means that there is no change in the magnetic flux threading the superconductor. Consequently, there is no electromagnetic induction occurring, and the resistance of the material drops to zero, allowing for lossless transmission of electrical currents.
Perfect diamagnetism: The expulsion of magnetic fields from the interior of the superconductor gives rise to a perfect diamagnetic response. Diamagnetic materials generate an induced magnetic field in the opposite direction to the applied field, effectively canceling out the external magnetic field within the material.
The Meissner effect is crucial in practical superconducting applications, such as in magnetic levitation (maglev) trains, magnetic resonance imaging (MRI) machines, and various experimental physics applications where the absence of resistance and magnetic interference is highly desirable.