Meissner effect

The Meissner effect is the expulsion of magnetic flux when a material is cooled through its superconducting transition in an applied magnetic field. Walther Meissner and Robert Ochsenfeld reported the phenomenon in 1933 while studying superconducting tin and lead; their measurements showed that cooling could drive magnetic field out of the sample rather than merely preserve the field already present.

That observation distinguished superconductivity from perfect conductivity. A hypothetical perfect conductor would simply freeze the initial magnetic flux into place, whereas a superconductor approaches a characteristic magnetic state with near-zero internal field. The expulsion is not perfectly complete at a surface: magnetic field penetrates a thin region described by the London penetration depth, and vortices can carry flux into many practical superconductors.

The effect arises from screening currents that form near the surface and oppose the applied magnetic field. In the simplest description, the London equations imply that magnetic field decays exponentially inside a superconductor over the penetration depth λ rather than ending abruptly at its surface.

The superconducting condensate is a coherent quantum state of paired charge carriers. Its rigidity makes the current persist without the ordinary dissipative mechanism of a normal metal, while the electromagnetic free energy favors field exclusion. The related Ginzburg–Landau theory describes the order parameter, coherence length, and surface behavior, and explains why the balance between penetration depth and coherence length separates type-I and type-II materials. The effect therefore reflects both thermodynamics and quantum coherence, not simply unusually good conductivity.

The Meissner effect has finite limits set by critical temperature, critical magnetic field, and critical current density. In type-I superconductors, increasing the field can produce an abrupt transition from the superconducting state to the normal state. Type-II superconductors instead admit magnetic flux through quantized vortices between a lower and an upper critical field; each vortex has a normal-like core surrounded by circulating supercurrent.

Vortex pinning can keep these flux lines from moving, allowing high-current magnets to operate without destructive energy loss from flux motion. This distinction underlies practical superconducting magnets in particle accelerators and magnetic-resonance imaging, as well as magnetic levitation demonstrations. Levitation is a consequence of field exclusion and, in suitable materials, flux pinning; it is not a separate force that operates independently of superconductivity.

The effect depends on how a sample is cooled and on its geometry, so complete field exclusion is an idealized description rather than a universal visual rule. Demagnetization fields can make the measured response differ substantially from the internal response, especially in thin films, needles, and irregular samples.

Flux quantization provides a more precise signature in many superconducting circuits: the magnetic flux associated with a closed superconducting path is quantized in units determined by the charge of a Cooper pair.¹ In weak links, the same coherent phase produces the Josephson effect, which permits supercurrent across a thin insulating barrier and supports sensitive magnetometers. The Meissner state can also be modified by surface roughness, impurities, weak links, and trapped vortices, while unconventional superconductors may show anisotropic or incomplete-looking screening without abandoning the underlying phenomenon.²

Critical field

A magnetic-field limit beyond which superconductivity is disrupted; type-II materials have lower and upper critical fields.

London penetration depth

The characteristic distance over which magnetic field decreases inside a superconductor.

Flux pinning

Immobilization of magnetic vortices by defects or inhomogeneities in a type-II superconductor.

Type-II superconductor

A superconductor that permits quantized vortices in an intermediate magnetic-field range.