History of superconductivity

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The number of patent families (in red) and non-patent publications (in blue) about superconductivity by year. Also shown as black vertical lines are the main breakthroughs in the field. SuperCond-ByYear-ALL.jpg
The number of patent families (in red) and non-patent publications (in blue) about superconductivity by year. Also shown as black vertical lines are the main breakthroughs in the field.

Superconductivity is the phenomenon of certain materials exhibiting zero electrical resistance and the expulsion of magnetic fields below a characteristic temperature. The history of superconductivity began with Dutch physicist Heike Kamerlingh Onnes's discovery of superconductivity in mercury in 1911. Since then, many other superconducting materials have been discovered and the theory of superconductivity has been developed. These subjects remain active areas of study in the field of condensed matter physics.

Contents

The study of superconductivity has a fascinating history, with several breakthroughs having dramatically accelerated publication and patenting activity in this field, as shown in the figure on the right and described in details below. Throughout its 100+ year history the number of non-patent publications per year about superconductivity has been a factor of 10 larger than the number of patent families, which is characteristic of a technology, that has not achieved a substantial commercial success (see Technological applications of superconductivity).

With the help of the Van der Waals' equation of state, the critical-point parameters of gases could be accurately predicted from thermodynamic measurements made at much higher temperatures. Heike Kamerlingh Onnes was influenced by the work of Van der Waals. Johannes Diderik van der Waals.jpg
With the help of the Van der Waals' equation of state, the critical-point parameters of gases could be accurately predicted from thermodynamic measurements made at much higher temperatures. Heike Kamerlingh Onnes was influenced by the work of Van der Waals.
In 1908, Heike Kamerlingh Onnes became the first to make liquid helium and this led directly to his 1911 discovery of superconductivity. Kamerlingh Onnes signed.jpg
In 1908, Heike Kamerlingh Onnes became the first to make liquid helium and this led directly to his 1911 discovery of superconductivity.
Heike Kamerlingh Onnes (right), the discoverer of superconductivity. Paul Ehrenfest, Hendrik Lorentz, Niels Bohr stand to his left. Ehrenfest Lorentz Bohr Kamerlingh Onnes.jpg
Heike Kamerlingh Onnes (right), the discoverer of superconductivity. Paul Ehrenfest, Hendrik Lorentz, Niels Bohr stand to his left.

Exploring ultra-cold phenomena (to 1908)

James Dewar initiated research into electrical resistance at low temperatures. Dewar and John Ambrose Fleming predicted that at absolute zero, pure metals would become perfect electromagnetic conductors (though, later, Dewar altered his opinion on the disappearance of resistance, believing that there would always be some resistance). Walther Hermann Nernst developed the third law of thermodynamics and stated that absolute zero was unattainable. Carl von Linde and William Hampson, both commercial researchers, nearly at the same time filed for patents on the Joule–Thomson effect for the liquefaction of gases. Linde's patent was the climax of 20 years of systematic investigation of established facts, using a regenerative counterflow method. Hampson's designs was also of a regenerative method. The combined process became known as the Hampson–Linde liquefaction process.

Onnes purchased a Linde machine for his research. On March 21, 1900, Nikola Tesla was granted a patent for the means for increasing the intensity of electrical oscillations by lowering the temperature, which was caused by lowered resistance.[ further explanation needed ] Within this patent it describes the increased intensity and duration of electric oscillations of a low temperature resonating circuit. It is believed that Tesla had intended that Linde's machine would be used to attain the cooling agents.

A milestone was achieved on July 10, 1908, when Heike Kamerlingh Onnes at Leiden University in the Netherlands produced, for the first time, liquified helium, which has a boiling point of 4.2 K (−269 °C) at atmospheric pressure.

Sudden and fundamental disappearance

Heike Kamerlingh Onnes and Jacob Clay reinvestigated Dewar's earlier experiments on the reduction of resistance at low temperatures. Onnes began the investigations with platinum and gold, replacing these later with mercury (a more readily refinable material). Onnes's research into the resistivity of solid mercury at cryogenic temperatures was accomplished by using liquid helium as a refrigerant. On April 8, 1911, 16:00 hours Onnes noted "Kwik nagenoeg nul", which translates as "[Resistance of] mercury almost zero." [4] At the temperature of 4.19 K, he observed that the resistivity abruptly disappeared (the measuring device Onnes was using did not indicate any resistance). Onnes disclosed his research in 1911, in a paper titled "On the Sudden Rate at Which the Resistance of Mercury Disappears." Onnes stated in that paper that the "specific resistance" became thousands of times less in amount relative to the best conductor at ordinary temperature. Onnes later reversed the process and found that at 4.2 K, the resistance returned to the material. The next year, Onnes published more articles about the phenomenon. Initially, Onnes called the phenomenon "supraconductivity" (1913) and, only later, adopted the term "superconductivity." For his research, he was awarded the Nobel Prize in Physics in 1913.

Onnes conducted an experiment, in 1912, on the usability of superconductivity. Onnes introduced an electric current into a superconductive ring and removed the battery that generated it. Upon measuring the electric current, Onnes found that its intensity did not diminish with the time. [5] The current persisted due to the superconductive state of the conductive medium.

In subsequent decades, superconductivity was found in several other materials; In 1913, lead at 7 K, in 1930's niobium at 10 K, and in 1941 niobium nitride at 16 K.[ citation needed ]

Enigmas and solutions (1933–1979)

The next important step in understanding superconductivity occurred in 1933, when Walther Meissner and Robert Ochsenfeld discovered that superconductors expelled applied magnetic fields, a phenomenon that has come to be known as the Meissner effect. In 1935, brothers Fritz London and Heinz London showed that the Meissner effect was a consequence of the minimization of the electromagnetic free energy carried by superconducting current.

In 1937, Lev Shubnikov discovered a new type of superconductors (later called type-II superconductors), that presented a mixed phase between ordinary and superconductive properties.

In 1950, the phenomenological Ginzburg–Landau theory of superconductivity was devised by Lev Landau and Vitaly Ginzburg. The Ginzburg–Landau theory, which combined Landau's theory of second-order phase transitions with a Schrödinger-like wave equation, had great success in explaining the macroscopic properties of superconductors. In particular, Alexei Abrikosov showed that Ginzburg–Landau theory predicts the division of superconductors into the two categories now referred to as type I and type II supeconductivity. Abrikosov and Ginzburg were awarded the 2003 Nobel Prize in Physics for their work (Landau having died in 1968). Also in 1950, Emanuel Maxwell and, almost simultaneously, C.A. Reynolds et al. found that the critical temperature of a superconductor depends on the isotopic mass of the constituent element. This important discovery pointed to the electron-phonon interaction as the microscopic mechanism responsible for superconductivity.

On the experimental side, collaborations of Bernd T. Matthias in the 1950s with John Kenneth Hulm and Theodore H. Geballe, led to the discovery of hundreds of low temperature superconductors using a technique based on the Meissner effect. Due to his experience, he came up with Matthias' rules in 1954, a set of empirical guidelines on how to find these types of superconductors. [6]

BCS theory

The complete microscopic theory of superconductivity was finally proposed in 1957 by John Bardeen, Leon N. Cooper, and Robert Schrieffer. This BCS theory explained the superconducting current as a superfluid of Cooper pairs, pairs of electrons interacting through the exchange of phonons. For this work, the authors were awarded the Nobel Prize in Physics in 1972. 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. In 1959, 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 .

Little–Parks effect

The Little–Parks effect was discovered in 1962 in experiments with empty and thin-walled superconducting cylinders subjected to a parallel magnetic field. The electrical resistance of such cylinders shows a periodic oscillation with the magnetic flux through the cylinder, the period being h /2 e  = 2.07×10−15 V·s. The explanation provided by William Little and Ronald Parks is that the resistance oscillation reflects a more fundamental phenomenon, i.e. periodic oscillation of the superconducting critical temperature (Tc). This is the temperature at which the sample becomes superconducting. The Little-Parks effect is a result of collective quantum behavior of superconducting electrons. It reflects the general fact that it is the fluxoid rather than the flux which is quantized in superconductors. The Little-Parks effect demonstrates that the vector potential couples to an observable physical quantity, namely the superconducting critical temperature.

Commercial activity

Soon after discovering superconductivity in 1911, 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 1955, George Yntema succeeded in constructing a small 0.7-tesla iron-core electromagnet with superconducting niobium wire windings. [7] Then, in 1961, 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. [8] Despite being brittle and difficult to fabricate, niobium-tin has since proved extremely useful in supermagnets generating magnetic fields as high as 20 teslas. In 1962, Ted Berlincourt and Richard Hake discovered that less brittle alloys of niobium and titanium are suitable for applications up to 10 teslas. [9] [10] Promptly thereafter, commercial production of niobium-titanium supermagnet wire commenced at Westinghouse Electric Corporation and at Wah Chang Corporation.[ citation needed ] Although niobium-titanium boasts less-impressive superconducting properties than those of niobium-tin, niobium-titanium has, nevertheless, become the most widely used “workhorse” supermagnet material, in large measure a consequence of its very high ductility and ease of fabrication. 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. Conectus, a European consortium for superconductivity, 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.[ citation needed ]

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 the Planck constant h. Josephson was awarded the Nobel Prize in Physics for this work in 1973.[ citation needed ]

In 1973 Nb
3
Ge
found to have Tc of 23 K, which remained the highest ambient-pressure Tc until the discovery of the cuprate high-temperature superconductors in 1986 (see below).[ citation needed ]

Unconventional supercconductivity.

First unconventional superconductors

In 1979, two new classes of superconductors where discovered that could not be explained by BCS theory: heavy fermion superconductors and organic superconductors. [11]

The first heavy fermion superconductor, CeCu2Si2, was discovered by Frank Steglich. [12] Since then over 30 heavy fermion superconductors were found (in materials based on Ce, U), with a critical temperature up to 2.3 K (in CeCoIn5). [13]

Klaus Bechgaard and Denis Jérome synthesized the first organic superconductor (TMTSF)2PF6 (the corresponding material class was named after him later) with a transition temperature of TC = 0.9 K, at an external pressure of 11 kbar. [14]

High-temperature superconductors

Superconductor timeline Sc history.gif
Superconductor timeline

In 1986, J. Georg Bednorz and K. Alex Mueller discovered superconductivity in a lanthanum-based cuprate perovskite material, which had a transition temperature of 35 K (Nobel Prize in Physics, 1987) and was the first of the high-temperature superconductors. It was shortly found (by Ching-Wu Chu) that replacing the lanthanum with yttrium, i.e. making YBCO, raised the critical temperature to 92 K, which was important because liquid nitrogen could then be used as a refrigerant (at atmospheric pressure, the boiling point of nitrogen is 77 K). This is important commercially because 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. 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.[ citation needed ]

In March 2001, superconductivity of magnesium diboride (MgB
2
) was found with Tc = 39 K.[ citation needed ]

In 2008, the oxypnictide or iron-based superconductors were discovered, which led to a flurry of work in the hope that studying them would provide a theory of the cuprate superconductors.[ citation needed ]

In 2013, room-temperature superconductivity was attained in YBCO for picoseconds, using short pulses of infrared laser light to deform the material's crystal structure. [15]

In 2017 it was suggested that undiscovered superhard materials (e.g. critically doped beta-titanium Au) might be a candidate for a new superconductor with Tc, substantially higher than HgBaCuO (138 K), possibly up to 233 K, which would be higher even than H2S. A lot of research suggests that additionally nickel could replace copper in some perovskites, offering another route to room temperature. Li+ doped materials can also be used, i.e. the spinel battery material LiTi2Ox and the lattice pressure can increase Tc to over 13.8 K. Also LiHx has been theorized to metallise at a substantially lower pressure than H and could be a candidate for a Type 1 superconductor. [16] [17] [18] [19]

Historical publications

Papers by H.K. Onnes

BCS theory

Other key papers

Patents

See also

  1. Shachtman, Tom: Absolute Zero and the Conquest of Cold. (Boston: Houghton Mifflin, 1999)
  2. Sengers, Johanna Levelt: How Fluids Unmix: Discoveries by the School of Van der Waals and Kamerlingh Onnes. (Amsterdam : Koninklijke Nerlandse Akademie van Wetenschappen, 2002)
  3. Van Delft, Dirk: Freezing Physics: Heike Kamerlingh Onnes and the Quest for Cold. (Amsterdam: Koninklijke Nerlandse Akademie van Wetenschappen, 2008)
  4. The Discovery of Superconductivity
  5. V.L. Ginzburg, E.A. Andryushin (2004). Superconductivity. World Scientific. ISBN   978-981-238-913-8.
  6. Geballe, T. H.; Hulm, J. K. (1996). Bernd Theodor Matthias 1918–1990 (PDF). National Academy of Science.
  7. G. B. Yntema, “Superconducting Winding for Electromagnet”, Phys. Rev. 98, 1197 (1955).
  8. J. E. Kunzler, E. Buehler, F. S. L. Hsu, and J. H. Wernick, “Superconductivity in Nb3Sn at High Current Density in a Magnetic Field of 88 kgauss”, Phys. Rev. Lett. 6, 89 (1961).
  9. T. G. Berlincourt and R. R. Hake, “Pulsed-Magnetic-Field Studies of Superconducting Transition Metal Alloys at High and Low Current Densities”, Bull. Am. Phys. Soc. II 7, 408 (1962).
  10. T. G. Berlincourt, “Emergence of Nb-Ti as Supermagnet Material”, Cryogenics 27, 283 (1987).
  11. Sigrist, Manfred (2005). "Introduction to Unconventional Superconductivity". AIP Conference Proceedings. 789. AIP: 165–243. doi:10.1063/1.2080350.
  12. Steglich, F.; Aarts, J.; Bredl, C.D.; Lieke, W.; Meschede, D.; Franz, W.; Schäfer, H. (1979). "Superconductivity in the Presence of Strong Pauli Paramagnetism: CeCu2Si2". Physical Review Letters. 43 (25): 1892–1896. Bibcode:1979PhRvL..43.1892S. doi:10.1103/PhysRevLett.43.1892. hdl: 1887/81461 . S2CID   123497750.
  13. Petrovic, C.; Pagliuso, P.G.; Hundley, M.F.; Movshovich, R.; Sarrao, J.L.; Thompson, J.D.; Fisk, Z.; Monthoux, P. (2001). "Heavy-fermion superconductivity in CeCoIn5 at 2.3 K". Journal of Physics: Condensed Matter. 13 (17): L337. arXiv: cond-mat/0103168 . Bibcode:2001JPCM...13L.337P. doi:10.1088/0953-8984/13/17/103. S2CID   59148857.
  14. Jérome, D.; Mazaud, A.; Ribault, M.; Bechgaard, K. (1980). "Superconductivity in a synthetic organic conductor (TMTSF)2PF 6". Journal de Physique Lettres. 41 (4): 95–98. doi:10.1051/jphyslet:0198000410409500.
  15. Mankowsky, R.; Subedi, A.; Först, M.; et al. (December 3, 2014). "Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5". Nature. 516 (1): 71–73. arXiv: 1405.2266 . Bibcode:2014Natur.516...71M. doi:10.1038/nature13875. PMID   25471882. S2CID   3127527.
  16. "Nickel for thought: Compound shows potential for high-temperature superconductivity". phys.org. June 16, 2017. Retrieved August 2, 2017.
  17. "Scanning the surface of lithium titanate". Tohoku University. July 4, 2017. Retrieved August 2, 2017.
  18. "Lab discovers titanium-gold alloy that is four times harder than most steels". phys.org. July 20, 2016. Retrieved August 2, 2017.
  19. Overhauser, A.W. (1987). "Light-Metal Hydrides as Possible High-Temperature Superconductors". International Journal of Modern Physics B. 01 (3n04): 927–930. Bibcode:1987IJMPB...1..927O. doi:10.1142/S0217979287001328.

Related Research Articles

<span class="mw-page-title-main">BCS theory</span> Microscopic theory of superconductivity

In physics, theBardeen–Cooper–Schrieffer (BCS) theory is the first microscopic theory of superconductivity since Heike Kamerlingh Onnes's 1911 discovery. The theory describes superconductivity as a microscopic effect caused by a condensation of Cooper pairs. The theory is also used in nuclear physics to describe the pairing interaction between nucleons in an atomic nucleus.

<span class="mw-page-title-main">Superconductivity</span> Electrical conductivity with exactly zero resistance

Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike an ordinary metallic conductor, whose resistance decreases gradually as its temperature is lowered, even down to near absolute zero, a superconductor has a characteristic critical temperature below which the resistance drops abruptly to zero. An electric current through a loop of superconducting wire can persist indefinitely with no power source.

Conventional superconductors are materials that display superconductivity as described by BCS theory or its extensions. This is in contrast to unconventional superconductors, which do not. Conventional superconductors can be either type-I or type-II.

<span class="mw-page-title-main">Meissner effect</span> Expulsion of a magnetic field from a superconductor

The Meissner 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. This expulsion will repel a nearby magnet.

<span class="mw-page-title-main">Heike Kamerlingh Onnes</span> Dutch physicist, Nobel prize winner (1853–1926)

Heike Kamerlingh Onnes was a Dutch physicist and Nobel laureate. He exploited the Hampson–Linde cycle to investigate how materials behave when cooled to nearly absolute zero and later to liquefy helium for the first time, in 1908. He also discovered superconductivity in 1911.

<span class="mw-page-title-main">High-temperature superconductivity</span> Superconductive behavior at temperatures much higher than absolute zero

High-temperature superconductors are defined as materials with critical temperature above 77 K, the boiling point of liquid nitrogen. They are only "high-temperature" relative to previously known superconductors, which function at even colder temperatures, close to absolute zero. The "high temperatures" are still far below ambient, and therefore require cooling. The first breakthrough of high-temperature superconductor was discovered in 1986 by IBM researchers Georg Bednorz and K. Alex Müller. Although the critical temperature is around 35.1 K, this new type of superconductor was readily modified by Ching-Wu Chu to make the first high-temperature superconductor with critical temperature 93 K. Bednorz and Müller were awarded the Nobel Prize in Physics in 1987 "for their important break-through in the discovery of superconductivity in ceramic materials". Most high-Tc materials are type-II superconductors.

<span class="mw-page-title-main">Superconducting magnet</span> Electromagnet made from coils of superconducting wire

A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire has no electrical resistance and therefore can conduct much larger electric currents than ordinary wire, creating intense magnetic fields. Superconducting magnets can produce stronger magnetic fields than all but the strongest non-superconducting electromagnets, and large superconducting magnets can be cheaper to operate because no energy is dissipated as heat in the windings. They are used in MRI instruments in hospitals, and in scientific equipment such as NMR spectrometers, mass spectrometers, fusion reactors and particle accelerators. They are also used for levitation, guidance and propulsion in a magnetic levitation (maglev) railway system being constructed in Japan.

The Little–Parks effect was discovered in 1962 by William A. Little and Ronald D. Parks in experiments with empty and thin-walled superconducting cylinders subjected to a parallel magnetic field. It was one of the first experiments to indicate the importance of Cooper-pairing principle in BCS theory.

<span class="mw-page-title-main">Indium nitride</span> Chemical compound

Indium nitride is a small bandgap semiconductor material which has potential application in solar cells and high speed electronics.

Niobium–titanium (Nb-Ti) is an alloy of niobium and titanium, used industrially as a type II superconductor wire for superconducting magnets, normally as Nb-Ti fibres in an aluminium or copper matrix.

<span class="mw-page-title-main">Type-II superconductor</span> Superconductor characterized by the formation of magnetic vortices in an applied magnetic field

In superconductivity, a type-II superconductor is a superconductor that exhibits an intermediate phase of mixed ordinary and superconducting properties at intermediate temperature and fields above the superconducting phases. It also features the formation of magnetic field vortices with an applied external magnetic field. This occurs above a certain critical field strength Hc1. The vortex density increases with increasing field strength. At a higher critical field Hc2, superconductivity is destroyed. Type-II superconductors do not exhibit a complete Meissner effect.

Superconductors can be classified in accordance with several criteria that depend on physical properties, current understanding, and the expense of cooling them or their material.

For a given temperature, the critical field refers to the maximum magnetic field strength below which a material remains superconducting. Superconductivity is characterized both by perfect conductivity and by the complete expulsion of magnetic fields. Changes in either temperature or magnetic flux density can cause the phase transition between normal and superconducting states. The highest temperature under which the superconducting state is seen is known as the critical temperature. At that temperature even the weakest external magnetic field will destroy the superconducting state, so the strength of the critical field is zero. As temperature decreases, the critical field increases generally to a maximum at absolute zero.

Niobium-germanium (Nb3Ge) is an intermetallic chemical compound of niobium (Nb) and germanium (Ge). It has A15 phase structure.

<span class="mw-page-title-main">122 iron arsenide</span>

The 122 iron arsenide unconventional superconductors are part of a new class of iron-based superconductors. They form in the tetragonal I4/mmm, ThCr2Si2 type, crystal structure. The shorthand name "122" comes from their stoichiometry; the 122s have the chemical formula AEFe2Pn2, where AE stands for alkaline earth metal (Ca, Ba Sr or Eu) and Pn is pnictide (As, P, etc.). These materials become superconducting under pressure and also upon doping. The maximum superconducting transition temperature found to date is 38 K in the Ba0.6K0.4Fe2As2. The microscopic description of superconductivity in the 122s is yet unclear.

<span class="mw-page-title-main">Quantum oscillations</span> Experiments used to map the Fermi surface

In condensed matter physics, quantum oscillations describes a series of related experimental techniques used to map the Fermi surface of a metal in the presence of a strong magnetic field. These techniques are based on the principle of Landau quantization of Fermions moving in a magnetic field. For a gas of free fermions in a strong magnetic field, the energy levels are quantized into bands, called the Landau levels, whose separation is proportional to the strength of the magnetic field. In a quantum oscillation experiment, the external magnetic field is varied, which causes the Landau levels to pass over the Fermi surface, which in turn results in oscillations of the electronic density of states at the Fermi level; this produces oscillations in the many material properties which depend on this, including resistance, Hall resistance, and magnetic susceptibility. Observation of quantum oscillations in a material is considered a signature of Fermi liquid behaviour.

Macroscopic quantum phenomena are processes showing quantum behavior at the macroscopic scale, rather than at the atomic scale where quantum effects are prevalent. The best-known examples of macroscopic quantum phenomena are superfluidity and superconductivity; other examples include the quantum Hall effect, Josephson effect and topological order. Since 2000 there has been extensive experimental work on quantum gases, particularly Bose–Einstein condensates.

Several hundred metals, compounds, alloys and ceramics possess the property of superconductivity at low temperatures. The SU(2) color quark matter adjoins the list of superconducting systems. Although it is a mathematical abstraction, its properties are believed to be closely related to the SU(3) color quark matter, which exists in nature when ordinary matter is compressed at supranuclear densities above ~ 0.5 1039 nucleon/cm3.

John Kenneth Hulm was a British-American physicist and engineer, known for the development of superconducting materials with applications to high-field superconducting magnets. In 1953 with George F. Hardy he discovered the first A-15 superconducting alloy.

In physics, the Matthias rules refers to a historical set of empirical guidelines on how to find superconductors. These rules were authored Bernd T. Matthias who discovered hundreds of superconductors using these principles in the 1950s and 1960s. Deviations from these rules have been found since the end of the 1970s with the discovery of unconventional superconductors.