MeissnerOchsenfeld

The Meissner-Ochsenfeld effect, commonly called the Meissner effect, is the expulsion of magnetic flux from the interior of a material when it transitions into the superconducting state. It was discovered in 1933 by Walther Meissner and Robert Ochsenfeld, who observed that a superconductor cooled in a magnetic field excludes the field from its bulk.

Mechanism and implications: When a material becomes superconducting below its critical temperature, surface screening currents arise that generate a magnetic field opposing the applied field. As a result, the magnetic field inside the superconductor is driven to zero (up to the appropriate critical field), making the material a perfect diamagnet in its superconducting state. This flux expulsion is a thermodynamic property of the superconducting phase, not merely a consequence of zero electrical resistance.

Type I and type II behavior: In type I superconductors, complete flux expulsion occurs up to a single critical field Hc, above which superconductivity is abruptly destroyed. In type II superconductors, the Meissner state persists up to a lower critical field Hc1; beyond that, magnetic flux penetrates in quantized vortices, creating a mixed state until the upper critical field Hc2 is reached.

Key parameters: The magnetic penetration depth lambda and the coherence length xi describe how fields and superconducting order vary spatially. The ratio kappa = lambda/xi determines the classification as type I (low kappa) or type II (high kappa). Typical penetration depths are on the order of tens of nanometers.

Applications and significance: The Meissner effect underpins magnetic levitation and the diamagnetic behavior of superconductors, and it is central to the functioning of superconducting magnets used in MRI and other technologies. The effect is described by London theory and is consistent with microscopic BCS theory.