The Meissner effect is distinct from this—it is the spontaneous expulsion which occurs during transition to superconductivity. Suppose we have a material in its normal state, containing a constant internal magnetic field. When the material is cooled below the critical temperature, we would observe the abrupt expulsion of the internal magnetic field, which we would not expect based on Lenz's law. The Meissner effect 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.
This equation, which is known as the London equation , predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at the surface.
Decoding the Secrets of Superconductivity
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 large. Superconductors can be divided into two classes according to how this breakdown occurs. In Type I superconductors , superconductivity is abruptly destroyed when the strength of the applied field rises above a critical value H c.
Depending on the geometry of the sample, one may obtain an intermediate state  consisting of a baroque pattern  of regions of normal material carrying a magnetic field mixed with regions of superconducting material containing no field. In Type II superconductors , raising the applied field past a critical value H c 1 leads to a mixed state also known as the vortex state in which an increasing amount of magnetic flux penetrates the material, but there remains no resistance to the flow of electric current as long as the current is not too large.
At a second critical field strength H c 2 , superconductivity is destroyed. The mixed state is actually caused by vortices in the electronic superfluid, sometimes called fluxons because the flux carried by these vortices is quantized. Conversely, a spinning superconductor generates a magnetic field, precisely aligned with the spin axis. This experiment measured the magnetic fields of four superconducting gyroscopes to determine their spin axes. This was critical to the experiment since it is one of the few ways to accurately determine the spin axis of an otherwise featureless sphere.
Superconductivity was discovered on April 8, by Heike Kamerlingh Onnes , who was studying the resistance of solid mercury at cryogenic temperatures using the recently produced liquid helium as a refrigerant. At the temperature of 4. The precise date and circumstances of the discovery were only reconstructed a century later, when Onnes's notebook was found.
Great efforts have been devoted to finding out how and why superconductivity works; the important step occurred in , when Meissner and Ochsenfeld discovered that superconductors expelled applied magnetic fields, a phenomenon which has come to be known as the Meissner effect. The first phenomenological theory of superconductivity was London theory. It was put forward by the brothers Fritz and Heinz London in , shortly after the discovery that magnetic fields are expelled from superconductors. A major triumph of the equations of this theory is their ability to explain the Meissner effect ,  wherein a material exponentially expels all internal magnetic fields as it crosses the superconducting threshold.
By using the London equation, one can obtain the dependence of the magnetic field inside the superconductor on the distance to the surface. The first equation follows from Newton's second law for superconducting electrons. During the s, theoretical condensed matter physicists arrived at an understanding of "conventional" superconductivity, through a pair of remarkable and important theories: the phenomenological Ginzburg-Landau theory and the microscopic BCS theory In , the phenomenological Ginzburg-Landau theory of superconductivity was devised by Landau and Ginzburg.
Abrikosov and Ginzburg were awarded the Nobel Prize for their work Landau had received the Nobel Prize for other work, and died in The four-dimensional extension of the Ginzburg-Landau theory, the Coleman-Weinberg model , is important in quantum field theory and cosmology.
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Also in , Maxwell and Reynolds et al. The complete microscopic theory of superconductivity was finally proposed in by Bardeen , Cooper and Schrieffer. For this work, the authors were awarded the Nobel Prize in The BCS theory was set on a firmer footing in , when N. Bogolyubov showed that the BCS wavefunction, which had originally been derived from a variational argument, could be obtained using a canonical transformation of the electronic Hamiltonian. Generalizations of BCS theory for conventional superconductors form the basis for understanding of the phenomenon of superfluidity , because they fall into the lambda transition universality class.
The extent to which such generalizations can be applied to unconventional superconductors is still controversial. The first practical application of superconductivity was developed in with Dudley Allen Buck 's invention of the cryotron. Soon after discovering superconductivity in , Kamerlingh Onnes attempted to make an electromagnet with superconducting windings but found that relatively low magnetic fields destroyed superconductivity in the materials he investigated.
Much later, in , G. Yntema  succeeded in constructing a small 0. Then, in , J. Kunzler, E. Buehler, F. Hsu, and J. Wernick  made the startling discovery that, at 4. Despite being brittle and difficult to fabricate, niobium-tin has since proved extremely useful in supermagnets generating magnetic fields as high as 20 tesla. In T. Berlincourt and R. Hake   discovered that alloys of niobium and titanium are suitable for applications up to 10 tesla. Promptly thereafter, commercial production of niobium-titanium supermagnet wire commenced at Westinghouse Electric Corporation and at Wah Chang Corporation.
However, both niobium-tin and niobium-titanium find wide application in MRI medical imagers, bending and focusing magnets for enormous high-energy-particle accelerators, and a host of other applications. In , Josephson made the important theoretical prediction that a supercurrent can flow between two pieces of superconductor separated by a thin layer of insulator.
Coupled with the quantum Hall resistivity , this leads to a precise measurement of the Planck constant. Josephson was awarded the Nobel Prize for this work in In , it was proposed that the same mechanism that produces superconductivity could produce a superinsulator state in some materials, with almost infinite electrical resistance. This temperature jump is particularly significant, since it allows liquid nitrogen as a refrigerant, replacing liquid helium. Also, the higher temperatures help avoid some of the problems that arise at liquid helium temperatures, such as the formation of plugs of frozen air that can block cryogenic lines and cause unanticipated and potentially hazardous pressure buildup.
Many other cuprate superconductors have since been discovered, and the theory of superconductivity in these materials is one of the major outstanding challenges of theoretical condensed matter physics. In February , an iron-based family of high-temperature superconductors was discovered.
In May , hydrogen sulfide H 2 S was predicted to be a high-temperature superconductor with a transition temperature of 80 K at gigapascals of pressure. In , a research team from the Department of Physics, Massachusetts Institute of Technology , discovered superconductivity in bilayer graphene with one layer twisted at an angle of approximately 1. Even if the experiments were not carried out in a high-temperature environment, the results are correlated less to classical but high temperature superconductors, given that no foreign atoms need to be introduced.
Superconducting magnets are some of the most powerful electromagnets known. They can also be used for magnetic separation, where weakly magnetic particles are extracted from a background of less or non-magnetic particles, as in the pigment industries. In the s and s, superconductors were used to build experimental digital computers using cryotron switches. More recently, superconductors have been used to make digital circuits based on rapid single flux quantum technology and RF and microwave filters for mobile phone base stations.
Superconductors are used to build Josephson junctions which are the building blocks of SQUIDs superconducting quantum interference devices , the most sensitive magnetometers known. Series of Josephson devices are used to realize the SI volt. Depending on the particular mode of operation, a superconductor-insulator-superconductor Josephson junction can be used as a photon detector or as a mixer. The large resistance change at the transition from the normal- to the superconducting state is used to build thermometers in cryogenic micro-calorimeter photon detectors.
The same effect is used in ultrasensitive bolometers made from superconducting materials. Other early markets are arising where the relative efficiency, size and weight advantages of devices based on high-temperature superconductivity outweigh the additional costs involved. For example, in wind turbines the lower weight and volume of superconducting generators could lead to savings in construction and tower costs, offsetting the higher costs for the generator and lowering the total LCOE.
Promising future applications include high-performance smart grid , electric power transmission , transformers , power storage devices , electric motors e. However, superconductivity is sensitive to moving magnetic fields so applications that use alternating current e. Compared to traditional power lines superconducting transmission lines are more efficient and require only a fraction of the space, which would not only lead to a better environmental performance but could also improve public acceptance for expansion of the electric grid.
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Main article: History of superconductivity. Main article: High-temperature superconductivity. Main article: Technological applications of superconductivity. Andreev reflection BCS theory Bean's critical state model Color superconductivity in quarks Conventional superconductor Covalent superconductors Flux pumping Heavy fermion superconductor High-temperature superconductivity Homes's law Iron-based superconductor List of superconductors Little-Parks effect Magnetic levitation Macroscopic quantum phenomena Organic superconductor Oxypnictide Persistent current Proximity effect Room-temperature superconductor Rutherford cable SU 2 color superconductivity Superconducting RF Superconductor classification Superfluidity Superstripes Technological applications of superconductivity Superconducting wire Timeline of low-temperature technology Type-I superconductor Type-II superconductor Unconventional superconductor.
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Finally, IISc team confirms breakthrough in superconductivity at room temperature - The Hindu
Physics Today. Bibcode : PhT Ochsenfeld Bibcode : NW London They were first discovered in by Heike Kammerlingh Onnes, who was also the first person to liquefy helium Superconductor Week is designed for scientists, executives, engineers, investors, analysts, and administrators and policy-makers in government—anyone seeking to track the rapidly changing field of superconductivity. Skip to content. The Voice of Superconductivity Since For over three decades, Superconductor Week has been the leading newsletter providing global coverage of the technology and commercialization of low- and high-temperature superconductors, and cryogenics for large- and small-scale applications.
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