Superdiamagnetism

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Superdiamagnetism (or perfect diamagnetism) is a phenomenon occurring in certain materials at low temperatures, characterised by the complete absence of magnetic susceptibility and the exclusion of the interior magnetic field. Superdiamagnetism is a feature of superconductivity. It was identified in 1933, by Walter Meissner and Robert Ochsenfeld (the Meissner effect).

Superdiamagnetism established that the superconductivity was a stage of phase transition. Superconducting magnetic levitation is due to the Superdiamagnetism (which repels a permanent magnet) and flux pinning, which stops the magnet from sliding away.

Diagram of the Meissner effect. Magnetic field lines, represented as arrows, are excluded from a superconductor when it is below its critical temperature.

[edit] Theory

Fritz London and Heinz London developed the theory that the exclusion of magnetic flux is brought about by electrical "screening currents" that flow at the surface of the superconducting metal and which generate a magnetic field that exactly cancels the externally applied field inside the superconductor. These screening currents are generated whenever a superconducting metal is brought inside a magnetic field. This may be understood from the fact that a superconductor has zero electrical resistance (so the "eddy currents" induced by motion of the metal inside a magnetic field will not decay). Fritz, at the Royal Society in 1935, stated that the thermodynamic state would be described by a single wave function.

Screening currents also appear in a situation where an initially normal conducting metal is placed inside a magnetic field, as soon as the metal is cooled below the transition temperature (such that it becomes superconducting). This expulsion of magnetic field on cooling down the metal cannot be explained any more by merely assuming zero resistance and is called the Meissner effect. It shows that the superconducting state does not depend on the history of preparation (only on the present values of temperature, pressure and magnetic field), and therefore is a true thermodynamic state.