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22-457: The German physicists Walther Meißner (anglicized Meissner ) and Robert Ochsenfeld discovered this phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples. The samples, in the presence of an applied magnetic field, were cooled below their superconducting transition temperature , whereupon the samples cancelled nearly all interior magnetic fields. They detected this effect only indirectly because

44-481: A permanent magnet which approaches the superconductor, and flux pinning , which prevents the magnet floating away. Superdiamagnetism is a feature of superconductivity . It was identified in 1933, by Walther Meissner and Robert Ochsenfeld , but it is considered distinct from the Meissner effect which occurs when the superconductivity first forms, and involves the exclusion of magnetic fields that already penetrate

66-627: A reciprocal range , λ M := h / ( M c ) {\displaystyle \lambda _{M}:=h/(Mc)} where h is the Planck constant and c is the speed of light ) for a gauge field . In fact, this analogy is an abelian example for the Higgs mechanism , which generates the masses of the electroweak W and Z gauge particles in high-energy physics . The length λ M {\displaystyle \lambda _{M}}

88-435: A superconductor as it is cooled below its critical temperature does. The persisting currents that exist in the superconductor to expel the magnetic field is commonly misconceived as a result of Lenz's Law or Faraday's Law . A reason this is not the case is that no change in flux was made to induce the current. Another explanation is that since the superconductor experiences zero resistance, there cannot be an induced emf in

110-515: 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 of the material 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 wherein an initially normal, conducting metal

132-748: Is identical with the London penetration depth in the theory of superconductivity . 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 the Technische Hochschule in Charlottenburg (now Technische Universität Berlin ), his doctoral supervisor being Max Planck . He then entered

154-462: 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 resistance and is called the Meissner effect . It shows that the superconducting state does not depend on the history of preparation, only upon

176-473: The London penetration depth , the magnetic field is not completely canceled. Each superconducting material has its own characteristic penetration depth. Any perfect conductor will prevent any change to magnetic flux passing through its surface due to ordinary electromagnetic induction at zero resistance. However, the Meissner effect is distinct from this: when an ordinary conductor is cooled so that it makes

198-732: The Physikalisch-Technische Bundesanstalt in Berlin. From 1922 to 1925, he established the world's third largest helium-liquifier, and discovered in 1933 the Meissner effect , damping of the magnetic field in superconductors . One year later, he was called as chair in technical physics at the Technical University of Munich . After World War II , he became the president of the Bavarian Academy of Sciences and Humanities . In 1946, he

220-414: The magnetic flux is conserved by a superconductor: when the interior field decreases, the exterior field increases. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconductor state. The ability for the expulsion effect is determined by the nature of equilibrium formed by the neutralization within

242-494: The unit cell of a superconductor. A superconductor with little or no magnetic field within it is said to be in the Meissner state. The Meissner state breaks down when the applied magnetic field is too strong. Superconductors can be divided into two classes according to how this breakdown occurs. Most pure elemental superconductors, except niobium and carbon nanotubes , are type I, while almost all impure and compound superconductors are type II. The Meissner effect

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264-419: The complete absence of magnetic permeability (i.e. a volume magnetic susceptibility χ V {\displaystyle \chi _{\rm {V}}} = −1) and the exclusion of the interior magnetic field . Superdiamagnetism established that the superconductivity of a material was a stage of phase transition . Superconducting magnetic levitation is due to superdiamagnetism, which repels

286-433: The generation of a spontaneous magnetization of a material which directly opposes the direction of an applied field. However, the fundamental origins of diamagnetism in superconductors and normal materials are very different. In normal materials diamagnetism arises as a direct result of the orbital spin of electrons about the nuclei of an atom induced electromagnetically by the application of an applied field. In superconductors

308-536: The illusion of perfect diamagnetism arises from persistent screening currents which flow to oppose the applied field (the Meissner effect); not solely the orbital spin. The discovery of the Meissner effect led to the phenomenological theory of superconductivity by Fritz and Heinz London in 1935. This theory explained resistanceless transport and the Meissner effect, and allowed the first theoretical predictions for superconductivity to be made. However, this theory only explained experimental observations—it did not allow

330-469: The magnetic field H induces magnetization M within the London penetration depth from the surface. These surface currents shield the internal bulk of the superconductor from the external applied field. As the field expulsion, or cancellation, does not change with time, the currents producing this effect (called persistent currents or screening currents) do not decay with time. Near the surface, within

352-451: The microscopic origins of the superconducting properties to be identified. This was done successfully by the BCS theory in 1957, from which the penetration depth and the Meissner effect result. However, some physicists argue that BCS theory does not explain the Meissner effect. The Meissner superconductivity effect serves as an important paradigm for the generation mechanism of a mass M (i.e.,

374-406: The object. 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 material and which generate a magnetic field that exactly cancels the externally applied field inside the superconductor. These screening currents are generated whenever a superconducting material

396-470: The superconductor. The persisting current therefore is not a result of Faraday's Law. Superconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism , meaning that the total magnetic field is very close to zero deep inside them (many penetration depths from the surface). This means that their volume magnetic susceptibility is χ v {\displaystyle \chi _{v}} = −1. Diamagnetics are defined by

418-434: The surface. This exclusion of magnetic field is a manifestation of the superdiamagnetism emerged during the phase transition from conductor to superconductor, for example by reducing the temperature below critical temperature. In a weak applied field (less than the critical field that breaks down the superconducting phase), a superconductor expels nearly all magnetic flux by setting up electric currents near its surface, as

440-436: The transition to a superconducting state in the presence of a constant applied magnetic field, the magnetic flux is expelled during the transition. This effect cannot be explained by infinite conductivity, but only by the London equation. The placement and subsequent levitation of a magnet above an already superconducting material does not demonstrate the Meissner effect, while an initially stationary magnet later being repelled by

462-573: Was appointed director of the academy's first low temperature research commission. Laboratories were located in Herrsching am Ammersee until 1965, when they were moved to Garching . Meißner lived alone with his two dogs for the last several years of his life. Meißner died in Munich in 1974. Superdiamagnetism Superdiamagnetism (or perfect diamagnetism ) is a phenomenon occurring in certain materials at low temperatures , characterised by

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484-412: Was given a phenomenological explanation by the brothers Fritz and Heinz London , who showed that the electromagnetic free energy in a superconductor is minimized provided where H is the magnetic field and λ is the London penetration depth . This equation, known as the London equation , predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at

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