Meissner effect

   Meissner effect or 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.

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Explanation

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 superconductor will have a characteristic penetration depth dependent on the material properties. When a superconductor is cooled in a weak magnetic field and crosses below the transition temperature, persistent currents arise on the surface. These currents circulate, which generates a magnetic field opposed to the applied field, canceling out the flux inside the superconductor. These persistent currents only flow in a depth equal to the penetration depth.

Perfect diamagnetism

Superconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism, such that their magnetic susceptibility is -1. Diamagnetism is defined as the generation of a spontaneous magnetization of a material which directly opposes the direction of an applied field. However, the fundamental origins of the diamagnetism in superconductors and normal materials are very different. In superconductors the diamagnetism arises from the persistent screening currents which flow to oppose the applied field, in normal materials diamagnetism arises as a direct result of an orbital rotation of electrons about the nuclei of an atom induced electromagnetically by the application of an applied field.

Consequences

The discovery of the Meissner effect led to the phenomenological theory of superconductivity by F. and H. London in 1935. They successfully created a theory which explained the resistanceless transport and Meissner effect which allowed the first theoretical predictions for superconductivity to be made. However, their theory merely explained experimental observations - it did not allow the microscopic origins of the superconducting properties to be identified.

Observation

Observation of the Meissner effect is a very difficult experiment, as the applied fields have to be very small (the measurements need to be made a long way from the phase boundary). This is because the penetration depth is temperature dependent and tends to infinity close to the phase boundary.

References

• M. Tinkham, “Introduction to Superconductivity”, 2nd Ed., Dover Books on Physics (2004). ISBN 0-486-43503-2 (Paperback). A good technical reference.
• Fritz London, "Superfluids", Volume I, "Macroscopic Theory of Superconductivity", (1950). Reprinted by Dover. ISBN 0-486-600440. By the man who explained the Meissner effect. pp.34-37 gives a technical discussion of the Meissner effect for a superconducting sphere.
• Wayne M. Saslow, "Electricity, Magnetism, and Light", Academic (2002). ISBN 0-12-619455-6. pp.486-489 gives a simple mathematical discussion of the surface currents responsible for the Meissner effect, in the case of a long magnet levitated above a superconducting plane.
• W. Meissner and R. Ochsenfeld, Naturwissenschaften 21, 787 (1933)

See also

• Superdiamagnetism
• Superfluid
• Diamagnetism