Superdiamagnetism (or perfect

diamagnetism Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnet ...

) is a

phenomenon A phenomenon ( phenomena), sometimes spelled phaenomenon, is an observable Event (philosophy), event. The term came into its modern Philosophy, philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be ...

occurring in certain materials at low temperatures, characterised by the complete absence of

magnetic permeability In electromagnetism, permeability is the measure of magnetization produced in a material in response to an applied magnetic field. Permeability is typically represented by the (italicized) Greek letter ''μ''. It is the ratio of the magnetic ...

(i.e. a volume

magnetic susceptibility In electromagnetism, the magnetic susceptibility (; denoted , chi) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the applied magnet ...

\chi_

= −1) and the exclusion of the interior

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

. Superdiamagnetism established that the superconductivity of a material was a stage of

phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

. Superconducting

magnetic levitation Magnetic levitation (maglev) or magnetic suspension is a method by which an object is levitation (physics), suspended with no support other than magnetic fields. Lorentz force, Magnetic force is used to counteract the effects of the gravitation ...

is due to superdiamagnetism, which repels a permanent magnet which approaches the superconductor, and flux pinning, which prevents the magnet floating away. Superdiamagnetism is a feature of

superconductivity Superconductivity is a set of physical properties observed in superconductors: materials where Electrical resistance and conductance, electrical resistance vanishes and Magnetic field, magnetic fields are expelled from the material. Unlike an ord ...

. It was identified in 1933, by

Walther Meissner Fritz Walther Meißner (anglicized: ''Meissner'') (16 December 1882 – 16 November 1974) was a German technical physicist. Meißner was born in Berlin to Waldemar Meißner and Johanna Greger. He studied mechanical engineering and physics at ...

and Robert Ochsenfeld, but it is considered distinct from the

Meissner effect In condensed-matter physics, the Meissner effect (or Meißner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. Th ...

which occurs when the superconductivity first forms, and involves the exclusion of magnetic fields that already penetrate the object.

Theory

Fritz London Fritz Wolfgang London (March 7, 1900 – March 30, 1954) was a German born physicist and professor at Duke University. His fundamental contributions to the theories of chemical bonding and of intermolecular forces (London dispersion forces) are to ...

and Heinz London developed the theory that the exclusion of

magnetic flux In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the we ...

is brought about by

electrical Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

screening currents that flow at the surface of the superconducting material and which generate a

that exactly cancels the externally applied field inside the superconductor. These screening currents are generated whenever a superconducting material is brought inside a magnetic field. This can be understood by the fact that a superconductor has zero electrical resistance, so that eddy currents, induced by the

motion In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...

of the material inside a magnetic field, will not decay. Fritz, at the

Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...

in 1935, stated that the

thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...

state would be described by a single

wave function In quantum physics, a wave function (or wavefunction) is a mathematical description of the quantum state of an isolated quantum system. The most common symbols for a wave function are the Greek letters and (lower-case and capital psi (letter) ...

. "Screening currents" also appear in a situation wherein an initially normal, conducting metal is placed inside a magnetic field. As soon as the metal is cooled below the appropriate transition temperature, it becomes superconducting. This expulsion of magnetic field upon the cooling of the metal cannot be explained any longer by merely assuming

zero 0 (zero) is a number representing an empty quantity. Adding (or subtracting) 0 to any number leaves that number unchanged; in mathematical terminology, 0 is the additive identity of the integers, rational numbers, real numbers, and compl ...

resistance and is called the ''

''. It shows that the superconducting state does not depend on the history of preparation, only upon the present values of

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

and magnetic field, and therefore is a true thermodynamic state.

References

* Shachtman, Tom, ''Absolute Zero: And the Conquest of Cold''. Houghton Mifflin Company, December 1999. {{Superconductivity Magnetic levitation Superconductivity

Theory

See also

References