Glossary · Basics
The Meissner effect: magnetic screening beyond zero resistance
The Meissner effect is the exclusion of magnetic field from the interior of a superconductor in its equilibrium low-field state.
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- DefinitionMeissner effect
- The expulsion of magnetic flux from the interior of a superconductor in its equilibrium low-field state, apart from a thin surface penetration region.
What magnetic screening means
When a material enters the superconducting state in a sufficiently weak applied field, surface currents oppose field penetration into its interior. The field still extends through a thin surface region, characterized by the penetration depth. The bulk response is diamagnetic: it opposes the applied field. This behavior is a thermodynamic property of superconductivity, rather than merely the electrical consequence of having no resistance. [1]
A hypothetical perfect conductor would prevent a change in the magnetic flux already inside it. Its response would therefore depend on whether the field was present before the resistance vanished. A superconductor instead has an equilibrium state with magnetic screening. That distinction is why a resistance measurement alone cannot demonstrate the Meissner effect, and why magnetic evidence matters when establishing that a new sample is superconducting. [1]
What changes in a strong field
Type II superconductors have a low-field Meissner state, but they can also admit magnetic flux as vortices. In the mixed state, field penetrates through these localized structures while superconductivity persists around them. Magnetic-field exclusion is therefore not complete under every superconducting operating condition. Strong-field magnet conductors work in this mixed state, where their ability to restrain vortex motion becomes important for carrying current with low dissipation. [2]
Defects can hold vortices in place through flux pinning. A sample cooled in a magnetic field may then retain trapped flux. Trapping and magnetic hysteresis complicate a simple picture of complete field expulsion, particularly in ceramic superconductors. A magnetic measurement should identify the cooling history, applied field, sample geometry, and background subtraction. These details help distinguish screening, pinning, and artifacts in the reported response. [2]
Why levitation needs more explanation
Magnetic screening can create repulsive forces between a superconductor and a nearby magnet. Whether those forces support a stable levitating arrangement depends on geometry and constraints. The Meissner effect explains a magnetic response; it does not mean that every magnet placed above every superconducting sample will float. The weight, field distribution, spacing, and superconducting operating condition all affect the mechanical outcome. [1][2]
The familiar demonstration in which a magnet remains fixed above, below, or beside a cooled ceramic also involves flux pinning. Pinned vortices resist changes in the relative position of the magnet and sample. That behavior is distinct from simple field exclusion, although both arise in a superconducting system. A levitation photograph is consequently useful illustration, but a careful reference distinguishes its forces from the evidence needed to establish superconductivity. [2]
Sources
- Theory of SuperconductivityPhysical Review, 1957
- Vortices in high-temperature superconductorsReviews of Modern Physics, 1994
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