| Meissner effect | |
|---|---|
| Name | Meissner effect |
| Caption | Schematic of magnetic flux expulsion from a superconductor |
| Discoverer | Walther Meissner |
| Discovered | 1933 |
| Field | Quantum physics; Condensed matter physics |
Meissner effect
The Meissner effect is the expulsion of magnetic flux from the interior of a material when it is cooled below its superconducting transition temperature. It distinguishes superconductors from perfect conductors by demonstrating an active rearrangement of currents that cancel interior magnetic fields, and it underpins key phenomena in BCS theory, London equations and applications such as magnetic levitation and superconducting magnets.
The Meissner effect refers to the macroscopic exclusion of static magnetic fields from the bulk of a superconducting specimen. When a normal conductor subject to an applied magnetic field is cooled through its critical temperature Tc into the superconducting state, screening currents develop within a characteristic length scale — the London penetration depth — that produce a magnetic moment opposing and nearly canceling the applied field inside the material. The result is a region of suppressed magnetic induction B ≈ 0 in the superconducting bulk for ideal Type I behavior, and quantized vortices in Type II materials. This phenomenon demonstrates spontaneous breaking of electromagnetic gauge symmetry in the superconducting phase and is central to superconductivity as a quantum coherent macroscopic state.
The effect was reported in 1933 by Walther Meissner and Robert Ochsenfeld after experiments on tin and lead samples cooled in an applied magnetic field. Their measurements showed the sudden disappearance of magnetic flux from the interior at the superconducting transition, distinguishing the superconducting state from ordinary perfect conductivity anticipated by classical electrodynamics. Subsequent experiments employed magnetometers, fluxgate sensors, and later SQUIDs (superconducting quantum interference devices) invented by Clifford Shull and teams to map flux expulsion and quantify the critical field Hc and temperature dependence. High-resolution imaging techniques such as magnetic force microscopy and muon spin rotation (muSR) have further detailed vortex structures and penetration profiles in modern studies at laboratories including CERN and national labs such as Los Alamos National Laboratory and Argonne National Laboratory.
Early theoretical description came with the phenomenological London equations (1935) by Fritz and Heinz London, which modify Maxwell's equations to include a relation between superconducting current and vector potential, producing exponential field decay over the London penetration depth. The London model captures the Meissner effect without microscopic detail. A microscopic foundation arrived with the BCS theory (1957) developed by John Bardeen, Leon Cooper, and Robert Schrieffer, explaining superconductivity as condensation of Cooper pairs and the emergence of a complex order parameter. In BCS theory the Meissner effect follows from collective response of paired electrons and results in a finite superfluid density; the stiffness of the phase of the order parameter enforces phase coherence and leads to the expulsion of magnetic flux. Extensions include Ginzburg–Landau theory by Vitaly Ginzburg and Lev Landau, which bridges phenomenology and microscopic theory and predicts characteristic lengths (coherence length ξ and penetration depth λ) and the dimensionless Ginzburg–Landau parameter κ that classify superconductors.
Superconductors split into two regimes distinguished by κ = λ/ξ. Type I superconductors, typically elemental metals like lead and mercury, exhibit a complete Meissner state up to a single thermodynamic critical field Hc, above which superconductivity is lost abruptly. Type II superconductors, including transition-metal alloys and ceramic high-Tc materials such as YBCO and BSCCO, allow partial flux penetration between lower and upper critical fields (Hc1 and Hc2) in the form of quantized flux lines or vortices carrying one flux quantum Φ0 = h/2e. Vortex physics — studied extensively by researchers like Abrikosov (Abrikosov vortex lattice) — governs mixed-state properties, pinning phenomena, and dissipation under current. Pinning centers from defects or engineered nanostructures suppress vortex motion and thereby maintain effective expulsion for transport applications.
The Meissner effect has deep connections to quantum field theory and concepts in quantum electrodynamics (QED) and spontaneous symmetry breaking. The acquisition of an effective mass by the photon in a superconductor (leading to exponential screening of electromagnetic fields) is analogous to the Anderson–Higgs mechanism articulated by Philip Anderson and formalized in particle physics via the Higgs mechanism in the Standard Model. Superconductors therefore provide condensed-matter realizations of gauge symmetry breaking and mass generation, informing theoretical approaches across disciplines. Experiments probing nonlocal electrodynamics, quasiparticle excitations, and topological superconducting states also test aspects of QED-like descriptions at low energies, while engineered hybrid systems coupling superconductors to semiconductor nanowires or topological insulators explore Majorana modes and quantum coherence relevant to quantum computing.
The Meissner effect enables magnetic levitation demonstrations and practical devices: superconducting magnetic bearings, maglev prototypes, and frictionless rotor systems exploit stable flux exclusion and pinning. In electrical engineering, superconducting magnets in magnetic resonance imaging (MRI) and particle accelerators (e.g., Large Hadron Collider) rely on persistent supercurrents and controlled flux behavior. SQUID magnetometers exploit flux quantization and Meissner screening for ultra-sensitive measurements used in geophysics, biomagnetism, and materials characterization. Ongoing developments in high-temperature superconductors and thin-film technologies aim to optimize flux pinning and reduce losses for power transmission, fault-current limiters, and superconducting qubits in quantum information science.