The Meissner Effect
A magnet is suspended over a liquid nitrogen cooled high-temperature superconductor (-200°C).
The Meissner effect (also known as the Meissner-Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor. Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the flux distribution outside of tin and lead specimens as they were cooled below their transition temperature in the presence of a magnetic field. They found that below the superconducting transition temperature the specimens became perfectly diamagnetic, cancelling all flux inside. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconducting state.In a weak applied field, a superconductor expels all magnetic flux. Although the magnetic field is completely expelled from the interior of the superconductor, there is not a sharp transition at the edges of a sample, but rather a rapid decay of field into the sample over a distance, the penetration depth. Each superconducting material has its own characteristic penetration depth.
When the temperature of a superconductor in a weak magnetic field is cooled below the transition temperature, surface currents arise that generate a magnetic field which yields zero net magnetic field within the superconductor. These currents do not decay in time, thus establishing that perfect diamagnetism implies zero electrical resistance. Called persistent currents, they only flow within a depth equal to the penetration depth, whose theory was given in the London equations by the brothers Fritz and Heinz London.
