Glossary · Measures and cooling
London penetration depth sets the magnetic screening length
Magnetic-field exclusion has a finite surface region. London penetration depth describes the decay of the field and screening current into a superconductor in the local London model. It connects the Meissner effect to a measurable length scale. [1]
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- DefinitionLondon penetration depth
- The characteristic length over which a magnetic field decays inside a superconductor in the local London description of magnetic screening. It also sets the spatial scale of the associated screening currents.
Screening occurs over a finite distance
In the Meissner state, screening currents oppose magnetic-field penetration into the bulk. The field does not fall abruptly to zero at the surface. For a thick sample with a flat surface in the local model, it decays exponentially with depth. London penetration depth is the characteristic distance in that exponential dependence. [1]
This length is a material response parameter, not the thickness of a separate coating. The current and field occupy a surface region whose dimensions depend on the superconducting state. When a film or wire is comparable in size to that region, its geometry must be included in the screening calculation. [1]
The London equations describe the response
The London equations relate superconducting current to the electric and magnetic fields. Combined with Maxwell’s equations, the magnetic relation produces the field-decay equation. This adds equilibrium magnetic screening to a description of current without ordinary scattering, distinguishing a superconductor from a hypothetical perfect conductor that simply preserves its initial flux. [1]
Within this description, the penetration depth depends on superconducting carrier density and the relevant mass and charge. A reduced superfluid density gives weaker screening and a longer penetration depth. The length generally changes with temperature, so a reported measurement should identify the thermal conditions rather than present an unexplained constant. [1]
Coherence length supplies the complementary scale
Penetration depth describes how the magnetic field varies. Coherence length describes how the magnitude of superconducting order can vary in space. Their ratio forms the Ginzburg-Landau parameter, which determines the type I or type II classification in the simplest Ginzburg-Landau model. The lengths describe different aspects of the same state. [2]
In a strongly type II material, a vortex has a compact region of suppressed superconducting order surrounded by a broader magnetic-field distribution. Coherence length sets the core scale, while penetration depth sets the magnetic scale. Equating the vortex core radius with its entire magnetic footprint confuses these distinct quantities. [2]
Measurements need the model and sample conditions
Inductive methods can detect the flux admitted through the penetration region and infer a screening length. Temperature dependence supplies information about superconducting carriers. Impurities and the electron mean free path also affect the measured response. When current depends on fields over an extended region, the strictly local London approximation needs modification. [1]
A useful result identifies material composition, temperature, crystal direction when relevant, and sample geometry. It should also distinguish an intrinsic bulk penetration depth from a geometry-dependent screening parameter in a thin device. These qualifications make comparisons meaningful for films, resonators, and magnetic measurements without treating every fitted length as the same physical quantity. [1][2]
Sources
- Solid State Physics: Superconductivity II, A. H. HarkerUniversity College London
- Type II Superconductors and the Vortex LatticeNobel Prize, 2003
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