Discovery of Perfect Diamagnetism
In 1933, Walther Meissner and Robert Ochsenfeld made a discovery that fundamentally changed the understanding of superconductivity. They found that a superconductor cooled below Tc in an applied magnetic field actively expels the field from its interior — not just preventing new flux from entering, but ejecting flux that was already there. This is qualitatively different from perfect conductivity and established superconductivity as a distinct thermodynamic phase.
London Equations
Fritz and Heinz London proposed two equations in 1935 that capture the electrodynamics of the Meissner effect. The second London equation relates the supercurrent density to the vector potential, predicting that magnetic fields decay exponentially inside the superconductor with a characteristic length λ — the London penetration depth. For most elemental superconductors, λ is tens of nanometers.
Temperature Dependence
The penetration depth increases with temperature as λ(T) = λ₀ / √(1 - (T/Tc)⁴). Near Tc, λ diverges — the screening currents weaken and flux begins to penetrate deeper. At exactly Tc, the shielding vanishes and the material transitions to the normal state. This simulation shows how the field profile inside the superconductor evolves as you sweep temperature.
Levitation and Applications
The Meissner effect is the physics behind dramatic levitation demonstrations with high-temperature superconductors. YBCO cooled in liquid nitrogen hovers stably above permanent magnets. Beyond demonstrations, Meissner screening is essential for superconducting RF cavities in particle accelerators, where surface currents must flow without loss at microwave frequencies, and for shielding sensitive quantum devices from stray fields.