Superconductor Meissner Effect

Zero resistance is not the whole story

Cool certain materials below a characteristic critical temperature and their electrical resistance drops, abruptly and completely, to zero — a current started in a superconducting loop will circulate for as long as anyone has ever measured, with no detectable decay. That alone would make a fascinating conductor, but in 1933 Walther Meissner and Robert Ochsenfeld discovered something stranger: a superconductor does not merely resist letting new magnetic field in, it actively pushes existing field out as it crosses the critical temperature, regardless of whether that field was already present before cooling began. This active expulsion, the Meissner effect, cannot be explained by zero resistance alone and is really the defining signature of true superconductivity.

Cooper pairs: electrons that stop repelling each other

The microscopic explanation, worked out by Bardeen, Cooper and Schrieffer in 1957 (BCS theory), starts from an unlikely idea: below the critical temperature, electrons in the material can form loosely bound pairs, called Cooper pairs, despite their mutual electrical repulsion. The pairing is mediated by the crystal lattice itself — one electron slightly distorts the lattice as it passes, briefly pulling nearby positive ions closer together, and a second electron is attracted to that transient region of extra positive charge before the lattice relaxes back. The net effect is a weak, phonon-mediated attraction that can outweigh direct Coulomb repulsion at low enough temperature.

Because a Cooper pair has integer total spin, it behaves as a boson rather than a fermion, and a macroscopic number of pairs can condense into the same single quantum ground state — a superconducting condensate. That collective state has an energy gap separating it from the lowest available excited states, so weak scattering events that would normally degrade an ordinary current simply cannot happen: there is nowhere nearby in energy for a scattered electron to go, and resistance vanishes.

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The London penetration depth

Field expulsion is not a perfectly sharp cutoff at the surface. The Cooper-pair condensate responds to an external field by setting up persistent screening currents in a thin surface layer, and those currents generate their own field that cancels the external one inside the bulk. The thickness of that surface layer is the London penetration depth, typically tens to a few hundred nanometres, and within it magnetic field decays roughly exponentially with distance from the surface:

B(x) = B_surface * exp(-x / lambda_L)     (field decaying into the surface)

lambda_L = the London penetration depth, a material property
x        = depth measured from the superconductor's surface

Beyond a few penetration depths into the material, the field is essentially zero — the bulk of a superconductor genuinely excludes magnetic flux, exactly as the Meissner effect predicts.

Flux pinning: from repulsion to stable levitation

Pure Meissner expulsion alone would only repel a nearby magnet, which is an unstable configuration — the classic demonstration of a magnet hovering unaided above a purely diamagnetic repeller is famously impossible to balance. Real type-II superconductors, the kind used in demonstrations and applications, allow magnetic flux to penetrate in discrete, quantised filaments called vortices once the field exceeds a lower critical value, rather than excluding it completely. Crucially, material defects and impurities can physically pin these flux vortices in place, locking the superconductor's position and orientation relative to the magnet's field lines rather than simply pushing it away. That pinning is what lets a cooled superconducting puck float stably above a magnet track, resist being nudged sideways, and even hang suspended beneath a magnet — a behaviour pure repulsion could never produce on its own.

Frequently asked questions

Is the Meissner effect the same as just having zero electrical resistance?

No, and this distinction is historically important. A hypothetical perfect conductor with merely zero resistance would trap whatever magnetic field was present when it was cooled, freezing the flux in place. A real superconductor actively pushes field out regardless of the field's history, which is a genuinely different and stronger phenomenon that zero resistance alone cannot explain.

Why do Cooper pairs behave so differently from ordinary conduction electrons?

Because they form a phonon-mediated bound state, Cooper pairs have integer total spin and behave as bosons, so at low enough temperature a macroscopic number of them condense into the very same quantum ground state. That collective state has no low-energy excitations available for a small disturbance to scatter into, which is what removes electrical resistance and enables the persistent screening currents behind the Meissner effect.

What lets a superconducting puck float stably instead of just being pushed away?

Flux pinning. In a real type-II superconductor, magnetic flux threads through in discrete quantised filaments that get physically pinned at material defects, locking the puck's position and orientation relative to the magnet's field rather than merely repelling it. This is why a pinned superconductor can levitate stably above or even hang stably below a magnet.

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Superconductor Meissner Effect: Why a Cold Puck Floats