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Discovery of Superconductivity
Hieke Kamerlingh Onnes discovered superconductivity when researching the resistance of solid mercury at cryogenic temperatures. When the mercury was below 4.15K, the resistance of an applied current through it disappeared. This property was deemed superconductivity, and the maximum temperature a substance would have to be in order to reach this state would be called the characteristic and/or critical temperature for that substance. -
Meissner Effect
Walther Meissner and Robert Ochsenfeld measured the magnetic field distribution outside superconducting tin and lead samples below their characteristic critical temperatures. They found that the interior magnetic field was canceled in the transition of the material to its superconductive state. 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 "Meissner Effect". -
Phenomenological Theory of Superconductivity
The discovery of the Meissner effect led to the phenomenological theory of superconductivity by Fritz and Heinz London in 1935. This theory explained resistance-less 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 the microscopic origins of the superconducting properties to be identified. -
Ginzburg–Landau theory
The Ginzburg-Landau theory is a mathematical physical theory used to describe superconductivity. The geometric setting that this theory takes place in allows for a discrete explanation of superconductivity, from the coherence depth and the penetration length, this theory provided a deep mathematical description of superconductors before the BCS theory. This theory also proposed that the electrons that contribute to superconductivity form a superfluid. -
BCS theory
The first widely-accepted theoretical understanding of superconductivity was advanced in 1957 by American physicists John Bardeen, Leon Cooper, and John Schrieffer. The mathematically-complex BCS theory explained superconductivity at temperatures close to absolute zero for elements and simple alloys. However, at higher temperatures and with different superconductor systems, the BCS theory has subsequently become inadequate to fully explain how superconductivity is occurring. -
Alternate Formalism for the BCS Theory
The BCS theory was set on a firmer footing in 1958, when Nikolay 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. -
Lev Gor'kov's Special Case
Lev Gor'kov showed that the BCS theory reduced to the Ginzburg-Landau theory close to the critical temperature. Gor'kov was the first to derive the superconducting phase evolution equation. This allowed for the BCS theory to have an even stronger argument. -
Discovery of Noibium-Tin's Superconductive Properties
J. E. Kunzler, E. Buehler, F. S. L. Hsu, and J. H. Wernick made the startling discovery that at 4.2 kelvins, a compound consisting of three parts niobium and one part tin was capable of supporting a current density of more than 100,000 amperes per square centimeter in a magnetic field of 8.8 teslas. Despite being brittle and difficult to fabricate, niobium-tin has since proved extremely useful in super magnets generating magnetic fields as high as 20 teslas. -
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Commercial Use of Superconductors
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. Conectus, a European superconductivity consortium, estimated that in 2014, global economic activity for which superconductivity was indispensable amounted to about five billion euros, with MRI systems accounting for about 80% of that total. -
Discovery of Noibium-Titanium's Superconductive Properties
In 1962, Ted Berlincourt and Richard Hake discovered that less brittle alloys of niobium and titanium are suitable for applications up to 10 teslas. Promptly thereafter, commercial production of niobium-titanium super magnet wire commenced at Westinghouse Electric Corporation and at Wah Chang Corporation. niobium-titanium has become the most widely used “workhorse” super magnet material, in large measure a consequence of its very high ductility and ease of fabrication. -
Brian Josephson's Theoretical Prediction
In 1962, Brian Josephson made the important theoretical prediction that a supercurrent can flow between two pieces of superconductor separated by a thin layer of insulator. This phenomenon, now called the Josephson effect, is exploited by superconducting devices such as SQUIDs. It is used in the most accurate available measurements of the magnetic flux quantum h/2e, and thus (coupled with the quantum Hall resistivity) for Planck's constant h. -
Advent of High-Temperature Superconductive Materials
A lanthanum-based cuprate perovskite material was found to have a critical temperature at 35K, the first high-temperature superconductor. It was then found that replacing the lanthanum with yttrium raised the critical temperature to 92 K, which was important because liquid nitrogen could be used as a refrigerant, allowing for commercial use. Liquid nitrogen can be produced cheaply on-site with no raw materials and is not prone to some of the problems (solid air plugs, etc.) of helium in piping. -
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High Temperature Superconductors
Since lanthanum barium copper oxide's high-temperature superconductive properties were uncovered, many other materials were found to have a high characteristic critical temperature: yttrium barium copper oxide, lanthanum oxygen fluorine iron arsenide, and bilayer graphene were all discovered to have a high characteristic critical temperature. -
Present Day
The theory of superconductivity in these cuprate high-temperature superconductors is one of the major outstanding challenges of theoretical condensed matter physics. There are currently two main hypotheses – the resonating-valence-bond theory, and spin fluctuation which have the most support in the research community. The second hypothesis proposed that electron pairing in high-temperature superconductors is mediated by short-range spin waves known as paramagnons.
