Meissner Effect and London Theory

A superconductor expels magnetic flux from its interior, the Meissner effect, and the London equations describe how a surface screening current confines any residual field to a thin penetration layer.

Definition

The Meissner effect is the active expulsion of magnetic flux from the interior of a superconductor cooled below its critical temperature; London theory accounts for it by postulating equations for the supercurrent that confine any magnetic field to a surface layer of thickness equal to the London penetration depth.

Scope

This topic covers the defining magnetic property of superconductivity: the Meissner-Ochsenfeld effect of complete flux expulsion that distinguishes a superconductor from a mere perfect conductor, and the phenomenological London theory whose two equations yield the London penetration depth and perfect diamagnetism. It treats flux quantization and the macroscopic coherence implied by the rigidity of the superconducting wavefunction, providing the electromagnetic foundation for the Ginzburg-Landau and BCS theories.

Core questions

• Why is the Meissner effect, not zero resistance alone, the hallmark of superconductivity?
• How do the London equations produce perfect diamagnetism and a finite penetration depth?
• What does the penetration depth tell us about the superconducting state?
• How does flux quantization reveal the macroscopic quantum coherence of a superconductor?

Key concepts

• Meissner-Ochsenfeld effect
• London equations
• London penetration depth
• Perfect diamagnetism
• Flux quantization

Key theories

London theory of superconductivity

The London brothers proposed phenomenological equations relating the supercurrent to the magnetic field that, combined with Maxwell's equations, force the field to decay exponentially into the superconductor, explaining the Meissner effect and defining the penetration depth.

Clinical relevance

Flux expulsion and the penetration depth underlie magnetic levitation, superconducting magnetic shielding, and the field-screening behavior exploited in superconducting magnets and microwave cavities; the Meissner effect is also the experimental signature used to confirm superconductivity in new materials.

History

Meissner and Ochsenfeld discovered the active expulsion of magnetic flux in 1933, showing superconductivity is a distinct thermodynamic state; Fritz and Heinz London supplied the phenomenological electromagnetic theory in 1935 that captured the Meissner effect and introduced the penetration depth.

Key figures

• Walther Meissner
• Fritz London
• Heinz London

Seminal works

• london1935
• tinkham2004

Frequently asked questions

How does a superconductor differ from a perfect conductor in a magnetic field?

A perfect conductor merely keeps whatever flux it had when it became perfectly conducting, but a superconductor cooled in a field actively pushes the flux out; this Meissner expulsion is a thermodynamic property that a perfect conductor would not show.

What is the London penetration depth?

It is the short distance, typically tens of nanometers, over which an applied magnetic field and the screening supercurrent decay into the surface of a superconductor; beyond it the interior is essentially field-free.