Superconductivity is the phenomenon of certain materials exhibiting zeroelectrical resistance and the expulsion ofmagnetic fields below a characteristictemperature. Thehistory of superconductivity began with DutchphysicistHeike Kamerlingh Onnes's discovery of superconductivity inmercury 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 ofcondensed matter physics.
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 (seeTechnological applications of superconductivity).
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James Dewar initiated research into electrical resistance at low temperatures. Dewar andJohn Ambrose Fleming predicted that atabsolute 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 thethird law of thermodynamics and stated that absolute zero was unattainable.Carl von Linde andWilliam Hampson, both commercial researchers, nearly at the same time filed for patents on theJoule–Thomson effect for theliquefaction 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 theHampson–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 electricaloscillations 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 alow temperatureresonating 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 atLeiden 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.
Heike Kamerlingh Onnes and Jacob Clay reinvestigated Dewar's earlier experiments on the reduction of resistance at low temperatures. Onnes began the investigations withplatinum andgold, replacing these later withmercury (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 theNobel 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'sniobium at 10 K, and in 1941niobium nitride at 16 K.[citation needed]
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The next important step in understanding superconductivity occurred in 1933, whenWalther Meissner andRobert Ochsenfeld discovered that superconductors expelled applied magnetic fields, a phenomenon that has come to be known as theMeissner effect. In 1935, brothersFritz London andHeinz London showed that the Meissner effect was a consequence of the minimization of the electromagneticfree energy carried by superconducting current.
In 1937,Lev Shubnikov discovered a new type of superconductors (later calledtype-II superconductors), that presented a mixed phase between ordinary and superconductive properties.
In 1950, the phenomenologicalGinzburg–Landau theory of superconductivity was devised byLev Landau andVitaly Ginzburg. The Ginzburg–Landau theory, which combined Landau's theory of second-orderphase transitions with aSchrödinger-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 astype I andtype II supeconductivity. Abrikosov and Ginzburg were awarded the 2003Nobel Prize in Physics for their work (Landau having died in 1968). Also in 1950,Emanuel Maxwell and, almost simultaneously, C.A. Reynoldset al. found that the critical temperature of a superconductor depends on theisotopic mass of the constituentelement. This important discovery pointed to the electron-phonon interaction as the microscopic mechanism responsible for superconductivity.
On the experimental side, collaborations ofBernd T. Matthias in the 1950s withJohn Kenneth Hulm andTheodore 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 withMatthias' rules in 1954, a set of empirical guidelines on how to find these types of superconductors.[6]
The complete microscopic theory of superconductivity was finally proposed in 1957 byJohn Bardeen,Leon N. Cooper, andRobert Schrieffer. ThisBCS theory explained the superconducting current as a superfluid ofCooper pairs, pairs of electrons interacting through the exchange ofphonons. 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, whenNikolay Bogolyubov showed that the BCS wavefunction, which had originally been derived from a variational argument, could be obtained using a canonical transformation of the electronicHamiltonian. 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.
TheLittle–Parks effect was discovered in 1962 in experiments with empty and thin-walledsuperconducting cylinders subjected to a parallelmagnetic field. Theelectrical resistance of such cylinders shows a periodicoscillation with the magnetic flux through the cylinder, the period beingh/2e = 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 thefluxoid rather than the flux which is quantized in superconductors. The Little-Parks effect demonstrates that thevector potential couples to an observable physical quantity, namely the superconducting critical temperature.
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 atWestinghouse Electric Corporation and atWah 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 theJosephson effect, is exploited by superconducting devices such asSQUIDs. It is used in the most accurate available measurements of themagnetic flux quantumh/2e, and thus (coupled with thequantum Hall resistivity) for thePlanck constanth. Josephson was awarded the Nobel Prize in Physics for this work in 1973.[citation needed]
In 1973Nb
3Ge found to haveTc of 23 K, which remained the highest ambient-pressureTc until the discovery of the cuprate high-temperature superconductors in 1986 (see below).[citation needed]
In 1979, two new classes of superconductors where discovered that could not be explained by BCS theory:heavy fermion superconductors andorganic superconductors.[11]
The firstheavy fermion superconductor, CeCu2Si2, was discovered byFrank 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 andDenis Jérome synthesized the first organic superconductor (TMTSF)2PF6 (the corresponding material class was named after him later) with a transition temperature ofTC = 0.9 K, at an external pressure of 11 kbar.[14]
In 1986,J. Georg Bednorz andK. Alex Mueller discovered superconductivity in alanthanum-based cuprateperovskite material, which had a transition temperature of 35 K (Nobel Prize in Physics, 1987) and was the first of thehigh-temperature superconductors. It was shortly found (byChing-Wu Chu) that replacing the lanthanum withyttrium, i.e. makingYBCO, raised the critical temperature to 92 K, which was important becauseliquid 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 theoreticalcondensed-matter physics.[citation needed]
In March 2001, superconductivity ofmagnesium diboride (MgB
2) was found withTc = 39 K.[citation needed]
In 2008, theoxypnictide oriron-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]
Papers by H.K. Onnes
BCS theory
Other key papers
Patents
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