The Early Years of Superconductivity

In the early years of superconductivity, progress to application was slow an intermittent. On the 10^th of July 1908, Heike Kamerlingh Onnes, Professor of Experimental Physics at the University of Leiden (Holland), was able to liquefy helium for the first time. Not only was he able to determine a boiling point for helium, at 4.3 K, but he was also able to further reduce the temperature to 1.7 K by reducing the pressure of the helium bath. He soon set about measuring the electrical resistance of metals in the new temperature regime. The resistance of metals is strongly dependent on temperature and once the dependency has been accurately measured, the resistance can be used a simple and convenient tool for low temperature thermometry. The low temperature behavior of metals was also seen as a tool to study electron theory (Einstein had applied to be assistant of Onnes in 1901 but had been rejected). A wonderfully detailed and authoritative account ("The Discovery and Early History of Superconductivity: The Real Story") of the events that led up to the discovery of superconductivity has been written by Rudolf de Bruyn Ouboter, Dirk van Delft and Peter Kes for "100 Years of Superconductivity", a book to be presented to attendees of EUCAS-ISEC-ICMC 2011. The Leiden group initially extended measurements on platinum wires, into the liquid helium range, and observed that their electrical resistance fell continuously with temperature to a minimum but finite value. The minimum resistance decreased as the impurity level of the metal decreased. Looking for a material with a higher available purity, they showed that the resistance of high purity gold fell to an even lower but measurable value. Seeking an even higher purity metal, considerable effort was then expended distilling pure mercury. When the resulting high purity mercury was tested, the electrical resistance fell steeply but continuously, as expected. At the boiling point of the helium (4.2 K) the resistance of the mercury wire had fallen to 500 times less than it had been at the melting point of the mercury. What happened next came as a complete surprise. As the mercury wires were slowly cooled below 4.2 K, Gilles Holst (who had been an assistant at Leiden for two years and would later become the first director of the Philips Research Laboratories) measured a sudden and massive drop in the electrical resistance. As best as they could measure, in just a few hundredths of a degree the resistance dropped to less than one millionth of the melting point value, and eventually to a thousand millionth of it; de Bruyn Ouboter, van Delft and Kes detail how Gilles Holst - whose thesis work was on quite a different topic - helped out Onnes by operating the Wheatstone bridge with the galvanometer in a room far away from the noise produced by the large pumps in the cryogenic laboratory). In 1912 Onnes termed the new electrical state that the mercury had entered below 4 K, the superconductive state. Having worked so hard to purify the mercury he was further surprised to find that adding gold and cadmium to the mercury did not stop it from entering the superconducting state. He also observed that very high currents could be passed though the mercury until a threshold current density was reached (as high as 1000 A/mm² at 2.45 K) at which point the mercury would return to the normal electrical state (ref. 1). This threshold value, that we now term the critical current, is perhaps the most important for practical application. Also significantly, in December 1912, he discovered that others metals, indeed metals that could be reasonably made into wires at room temperature, namely tin (3.8 K) and lead (6 K, later raised to 7.2 K) could be made superconducting (2). The first two superconducting solenoids were quickly manufactured by Gerrit Jan Flim (1875-190), the Chief of the Technical Department of the (Leiden) Cryogenic Department. At the Third International Congress of Refrigeration, held in Chicago in September 1913, Onnes predicted that superconductivity would enable the production of coils that could generate fields (100,000 gauss or 10 T) well in excess of that possible by conventional conductors (3). For comparison, the flux density between the poles of a "horseshoe" permanent magnet is 0.1 T. Less accurately, he predicted that such a development should not be far away. He was about to discover a roadblock to high field superconductivity. In a footnote to his Chicago address, he observed that a 0.05 T (500 gauss) field, developed in a simple superconducting solenoid, was sufficient to revert the superconductor to its normal state. By 1914 Leiden had produced curves of resistance as a function of applied field and had developed an empirical fit to the temperature dependence of the critical field (H_c):  H_c(T) ≈H_c0(1-(T/T_c)²). It seems surprising to us now that it was not until 1916, that the interdependence of critical current and critical field, was shown from an analysis of the Leiden data by Francis Silsbee of the National Bureau of Standards in America (4). For many years, it appeared that low critical current and the suppression in critical current very with small applied fields would make superconductors impractical for any application other than laboratory studies of solid sate physics.

Higher critical magnetic fields and critical temperatures

Leiden enjoyed a monopoly on liquid helium research until after the first world war. In 1923 a helium liquefier, based on the Leiden design, started operation at the University of Toronto. Four years later a helium liquefier capable of 10 liters per hour was started at the Physikalisch-Technische Reichsanstalt (PTR) near Berlin under the direction of Walther Meissner. In successive year, from 1928-1930 the PTR identified three important new superconductors; Ta (T_c of 4.4 K ), Thorium (T_c of 1.4 K) and Niobium (T_c of 9.2 K)(5). An alloy of niobium, Nb-47wt.%Ti, is now by far the most important commercial superconductor with it's widespread use in the magnets for magnetic resonance imaging (MRI) systems in hospitals as well many other applications (see section: Ductile Superconductors). Nb based technology is also the current standard for digital superconducting circuits (see Section: Superconducting Electronics). Meissner's group would go on to find that most of the transition elements in group IV and V were superconducting. In figure 1 we show a listing of the elemental superconductors with their locations in the periodic table. In the same period a further important discovery came from the group of Wander Johannes de Haas, who became co-director of the Leiden laboratory 1924 (with Willem Hendrik Keesom). It was found that a solid solution of 4 % bismuth in gold was found to be superconducting at 1.9 K (6) despite neither of the components being superconducting at ambient pressure. A similar result was found later the same year when copper sulfide (T_c of 1.1 K) was examined by Meissner(7). This time an insulator (sulfur) had been combined with a very good normal conductor (Cu), to produce a superconductor. The Meissner group went on to find a large number of carbides and nitrides with high transition temperatures, in particular NbC (T_c >10 K ).

The Meissner Effect and Type II Superconductivity

In 1933 an important discovery was made by Meissner and his student Robert Ochsenfeld, using cylinders of single crystal and polycrystalline lead. They showed that in cooling a superconductor below T_c in the presence of an applied field (lower that H_c), the existing flux inside the superconductor is suddenly expelled. This behavior is now known as the Meissner effect(9). Furthermore, when the external field is removed there is no trapped flux or induced dipole. If field is applied when the superconductor is is in the superconducting state, currents must be produced near the surface in order to maintain constant flux. The currents near the surface, counteracting the external field, must then be a stable. Just two years later, brothers Fritz and Heinz London developed a set of electrodynamic equations, now known as the London equations, which described the Meissner effect by supplementing Maxwell's equations.(10) A consequence of these equations in the London penetration depth, λ_L, which is the maximum depth that magnetic field can penetrate into a type I superconductor. See article: High-T_c Superconductors, Physical Structures, and Role Of Constituents, Section: Common Features Of High-T_c Superconductors And Magnetic Noise, And Barkhausen Effect, Section: Flux Pinning And Losses In Superconductors.

 

The same year as the London's paper was published, the Kharkov group of L. V. Shubnikov (who had worked with W. J. De Haas at the Kammerlingh Onnes Laboratory from 1926-30 and was familiar recent discoveries of alloy superconductivity at Leiden), showed that single crystals of PbTl₂ had two distinct critical fields which they named H_c1 and H_c2 (11). Up to a lower critical field (H_c1), the flux is excluded, above that field the flux begins to penetrate and increases in its penetration until an upper critical field (H_c2) is reached, when the flux completely penetrates and superconductivity is extinguished. The superconductors that show this characteristic would come to be know as Type II superconductors. This class of superconductor includes all the technically useful superconductors including all alloy and compound superconductors as well as the elements niobium, vanadium and technetium. See article: Superconductors, Type I And II and article: Superconducting Critical Current Unfortunately the importance of the work at Kharkov was not fully appreciated in the outside the Soviet Union as Shubnikov's group was victimized in one of Stalin's purges (with Shubnikov eventually dying in prison in 1945). Only with his posthumous exoneration was it possible for his Soviet colleagues to openly acknowledge his contributions to their work.

Although the period of the Second World War resulted in a hiatus in superconductor activity for most groups there was one significant advance. In Germany, Ascherman and coworkers found that niobium-nitride had a T_c 15 K (12), the first time the liquid helium region had been surpassed. The boiling temperature for a cryogen can be fixed at any temperature between the triple point and the critical point by maintaining the corresponding system pressure. Above helium (which does not have a true triple point) the next cryogen is liquid hydrogen, which has a triple point of 13.80 K and a critical point of 32.98 K.

Ginzburg-Landau-Abrikosov

Despite the huge potential of superconductivity, the difficulty in obtaining liquefied helium had limited research on superconductivity to a handful of laboratories worldwide. However, just as the advances in cryogenics at the turn of the 20^th century had made the discovery of superconductivity possible, so had advances in cryogenic technology, particularly the commercialization of the Collins liquefier, broadened the availability of the superconducting state after the second world war. 1950 saw the publication of Vitalii Ginzburg and Lev Landau's landmark modifications to the London" equations (13). The resulting Ginzburg-Landau equation is described in the "Superconductors, Type I And II, Superconducting Critical Current High-T_c Superconductors, Physical Structures, And Role Of Constituents section: Common Features Of High-T_c Superconductors of "Engineering Superconductivity". An important new material parameter, the coherence length, x, was defined as the distance over which the density of the superconducting electrons decreases at a superconducting-normal interface. The Ginzburg-Landau formulations were able to predict the conditions under which Type I and Type II behavior would occur using the parameter κ= λ/x. From this work came the now familiar flux line lattice description of Type II superconductors by Landau's student Aleksei Abrikosov eventually published in 1957(14). See Superconducting Critical Current. In type II superconductors, the flux tube has a radius λ with an internal normal core of radius x.

In 1953 Bern Matthias at the Bell laboratories raised the T_c ceiling or superconductors to 17.86 K with NbN-NbC. That discovery was followed the same year by John Hulm's group at the University of Chicago with another 17 K superconductor V₃Si but of a new crystal structure, A15, that would eventually supply a series of important superconductors. Another A15, Nb₃Sn, would be added the following year at Bell Labs, with a further increase in T_c at 18 K .

The Beginning of Engineering Superconductivity

It was not until 1954 that the first successful superconducting magnet was made (by George Yntema at the University of Illinois), thereby ushering in the age of engineering superconductivity. Yntema used Nb wire, which had been shown (by D. Shoenberg at Cambridge University) to have a markedly better critical field than any of the other known superconductors. The resulting magnet produced a field of 0.71 T at 4.2 K. He also discovered that increasing cold work in the strands markedly increased the current density that they could carry. It was beginning to be clear that critical current was, to a major extent, a property that could be increased independently of the intrinsic bulk properties of H_c2 and T_c. By August 1960 Stan Autler (at MIT Lincoln Laboratory) had produced a 2.5 T field at 4.2 K, again using Nb. Even more significantly, had applied the persistent current in a solenoid to provide the magnetic field for a solid state maser, perhaps the first application of superconductivity. A flurry of activity followed focused on the high T_c, high H_c2 A15 compound Nb₃Sn. Nb₃Sn, at that time however, was difficult to fabricate into magnets because of the brittle nature of Nb₃Sn and because the simple elemental powder in tube process required a 1000 °C heat treatment. It was soon usurped by two ductile alloy superconductors, first Nb-Zr (T_c ~ 12 K ), which was being sold in long lengths for magnet application as early as 1961 by Wah Chang and then Nb-Ti (T_c = 7-10 K ), which became the dominant commercial superconductor. Whereas the Ginzburg-Landau theory coupled with Abrikosov's work provided and enduring phenomenological description of superconductivity, it did not provide microscopic description. That was to be supplied in 1957 when John Bardeen, Leon Cooper, and John Schrieffer of the University of Illinois at Urbana, published their Nobel prize winning theory of superconductivity (15). The superconducting electrons of the phenomenological description proved to be two electrons (Cooper pairs (16)) with opposite directions for both spin and momentum. The coherence length was the size of the Cooper pair and the order parameter was proportional to the electron energy gap, which itself was proportional to the T_c. The BCS (Bardeen Cooper Schreiffer) theory as it has come to be know, provided a theory adequate for low temperature and field, three years later, however, Lev Gorkov would provide one that would be useful at high fields (17). The key to Gorkov description was that implied a variation in the energy gap parameter with position.The next major theoretical advance came in 1962, from a graduate student at Cambridge University, Brian D. Josephson. He predicted that superconducting current would tunnel through a thin insulating layer or weak link separating two superconducting electrodes and that a phase difference is produced between the superconducting electrons in the two electrodes. The phase difference generates a voltage difference between the two electrodes. The Josephson effect, as it is now known, is the basis for superconducting electronic devices such as the SQUID, and very high precision voltage stands, it would also earn Josephson a Nobel prize.

Big Magnets

Much of the theoretical development in the 1960s and 1970s was in the area of flux-pinning. Increasing the critical current density in superconductors reduces costs because less superconductor is required, it also makes it possible to operate magnets at higher magnetic fields. When electricity flows through a superconductor it produces a Lorentz force between the current and the flux lines. If the Lorentz force is allowed to move the flux line lattice freely within the superconductor then power is dissipated eventually the resulting heating drives the superconductor normal. Introducing microstructural features, such as non-superconducting precipitates and grain boundaries that pin the flux lines in place can, however, reTransitional movement of the flux line lattice. Understanding the nature of flux pinning is key to understanding the way critical current density can be improved in superconductors. The mechanisms and theory of flux-pinning is comprehensively reviewed in article: Superconductivity and Magnetism.From the late 1960s, onwards the needs of the high energy physics community propelled considerable advances in superconducting strand technology, initially for bubble-chamber and then accelerator magnets. In March 1983, the first superconducting accelerator ring was completed at Fermi National Accelerator Laboratory. With 774 6 m long dipole magnets and 210 quadrupole magnets covering a four mile circle it exemplifies the progress that had been made. The key features were now in place, the superconducting strand was now in the required form of a composite of fine (>30 µm in diameter) filaments in a high purity, high normal conductivity Cu (or Al) matrix for stability (see Superconductors, Cryogenic Stabilization) and the strand was twisted in order to reduce eddy currents. The strand itself was cabled with other superconducting strands to form a thick ribbon-like conductor. Increased understanding of the microstructural development of the superconductor and its role in flux-pinning would further increase the critical current density making possible the next generation of accelerators, see Article: Ductile Superconductors. Four years later the largest superconducting magnet yet was fabricated for the DELPHI project at the CERN particle accelerator laboratory. The 7.4 m long, 6.2 m diameter, 84 tonnes magnet survived a 1600 km trip to CERN by road, ship, and barge. In addition to magnet technology the high energy physics community also benefited from superconducting cavity technology. When the Large Electron Positron (LEP) collider was initially run in 1989, with 128 conventional copper accelerating cavities, they provided enough energy to take the energy of each beam to 50 GeV. By upgrading the ring with 272 superconducting cavities from the LEP ring was eventual able to reach 104 GeV per beam in April 2000. In 1986, Alex Müller and Georg Bednorz, at the IBM Research Laboratory in Rüschlikon, Switzerland, made a ceramic perovskite of lanthanum, barium, copper, and oxygen that superconducted at 35 K (18). In fact, small amounts of this material were later found to be superconducting at 58 K due to lead impurities. The impact of this discovery can be gauged by the almost immediate awarding of the Nobel Prize to the two discoverers.

We have come to expect continuous advances in the properties of both HTS and LTS superconductors and have yet to be disappointed. In the years since the discovery of the HTS superconductors we have seen steady improvements in their current densities, on both the laboratory and industrial scale, as well as in the production piece lengths.

Recommended Reading

A book entitled "100 Years of Superconductivity", edited by Horst Rogalla and Peter Kes, will be presented to attendees of EUCAS-ISEC-ICMC 2011, and contains many interesting articles covering the history of superconductivity in much more detail than is presented here including the excellent article on the Leiden discoveries mentioned above.

An article by Anatoly Shepelev (Kharkov Institute of Physics and Technology) and David Larbalestier (NHMFL) published in the CERN Courier on the events surrounding the discovery of type II superconductors is vailable here:
https://cerncourier.com/cws/article/cern/47503

Paul Grant has assembled a collection of classic superconductivity papers at:
https://www.w2agz.com/BD_woodstock07.htm#Background_Bibliography

In 1986 the Applied Superconductivity Conference celebrated the 75^th Anniversary of the discovery of superconductivity with a symposium on the history of superconductivity. The symposium is published in full in IEEE Trans. Magn., 23, pp. 354-415, 1986.

An important text that is both informing and entertaining is that of Per Fridtjof Dahl:Per Fridtjof Dahl, Superconductivity: Its historical roots and development from mercury to the ceramic oxides. Although out of print there are some pretty good prices out there for used copies.

Recently V. L. Ginzberg has written a review:V L Ginzburg, "Superconductivity: the day before yesterday - yesterday - today - tomorrow ," Physics - Uspekhi 43 (6) 573 ± 583 (2000)https://ufn.ioc.ac.ru/ufn2000/ufn00_6/ufn006b.pdfThere is now a $7 charge for this document although the Russian language version is still available for free at:https://data.ufn.ru//ufn2000/ufn00_6/Russian/r006b.pdfA brief introduction to the BCS theory of superconductivity and the development of the history behind it by David Pines,

John Bardeen's first postdoctoral researcher (research assistant professor) at the University of Illinois during the period, 1952-1955, can be found at: https://cnls.lanl.gov/Highlights/1997-06/html/node2.htmlThe internet continues to grow as a remarkable repository of knowledge, often a quick internet search will yield fascinating material. For instance Ted Geballe and John Hulm wrote a biographical memoirs of Bernd Matthias that can be found at:https://books.nap.edu/html/biomems/bmatthias.html

A classical and still excellent introductions to the phenomenon of superconductivity are:A. C. Rose-Innes, F. H. Rhoderick, Introduction to Superconductivity, Oxford, UK: Pergamon, 1969.

The most detailed accound of the events surrounding the discovery of Type II Superconductivity can be found in A. G. Shepelev's the "The Discovery of Type II Superconductors (Shubnikov Phase)" in "Superconductor," Edited by: Adir Moyses Luiz, ISBN 978-953-307-107-7, Publisher: Sciyo, Publication date: August 2010 https://www.intechopen.com/books/show/title/superconductor

M. Tinkham, Introduction to Superconductivity, New York: McGraw-Hill, 1975.T. P. Orlando, K . A. Delin, Foundations of Applied Superconductivity, Reading, MA: Addison-Wesley, 1991.A. M. Campbell, J. E. Evetts, Adv. Phys., 21: 199-428, 1972.

Bibliography

1. H. Kamerlingh Onnes, Further Experiments with liquid helium. H. On the electrical resistance of pure metals etc. VII The potential difference necessary for the electric current through mercury below 4.19 K (continuation), Comm. Physical Lab. Leiden, 133b, 29, 1913. [Leiden references use systematic naming from Per Fridtjof Dahl - see recommended reading]
2. H. Kamerlingh Onnes, Further experiments with liquid helium. H. On the electrical resistance of pure metals etc. (continued). VIII. The sudden disappearance of the ordinary resistance of tin, and the super-conductive state of lead, Comm. Physical Lab. Leiden, 133d, 51, 1913.
3. H. Kamerlingh Onnes, Report on the researches made in the Leiden cryogenics laboratory between the second and third international congress of refrigeration: Superconductivity, Comm. Physical Lab. Leiden Suppl., 34b: 55-70, 1913.
4. F. B. Silsbee, A note on electrical conduction in metals at low temperatures, Washington Academy of Sciences, Journal, 6:597-602, 1916.
5. W. Meissner and H. Franz, Messungen mit Hilfe von flüssigen Helium. VIII. Supraleitfähigkeit von Niobium, Physikalisch-Technische Reichsanstalt, Mitteilung: 558-559, 1930.
6. W. J. De Haas, E. van Aubel, and J. Voogd, A superconductor consisting of two non-superconductors, Akademie der Wetenschappen, Amsterdam, Proceedings, 32: 730, 1929.
7. W. Meissner, Messungen mit Hilfe vo flüssigem Helium. V. Suprleitfähigkeit von Kupfersulfid, Physikalisch-Technische Reichsanstalt, Mitteilung, 571, 1929
8. W. J. de Haas and J. Voogd, The influence of magnetic fields on supracondcutors, Akademie der Wetenschappen, Amsterdam, Proceedings, 33: 262-270, 1930.
9. W. Meissner and R. Oschenfeld, Ein neuer Effect bei Eintritt der Supraleitfähigkeit, Naturwiss., 21: 787-788, 1933.
10. F. London, H. London, The electromagnetic equations of the supraconductor, Proc. R. Soc. London, Ser, A., 149: 71-88, 1935.
11. J. N. Rjabinin and L. V. Schubnikov, Magnetic properties and critical currents of supercondcuting alloys, Physikalische Zeitschrift der Sowjetunion, 6: 605-607, 1935. A much more detailed history of the discoveries of this period is available in the A. G. Shepelev article listed in the "recommended reading" section.
12. G. Aschermann, E. Freiderich, E. Justi and J. Kramer, Supraleitfähige Verbindenungen mit extrem hohen Sprungtemperaturen (NbH und NbN), Physik. Zeit., 42: 349-60, 1941.
13. V. L. Ginzburg and L. D. Landau, On the theory of superconductivity, Zhurnal Eksperimental'noi I Teoreticheskoi Fiziki, 20: 1064-1082, 1950.
14. A. A. Abrikosov, On the magnetic properties of superconductors of the second group, Sov. Phys. JETP, 5: 1174-1182, 1957.
15. J. Bardeen, L. N. Cooper and J. R. Schreiffer, "Theory of superconductivity, Phys. Rev., 108: 1175-1204, 1957.
16. L. N. Cooper, bound electron pairs in a degenerate Fermi gas, Phys. Rev., 104: 1189-1190, 1956.
17. L. P. Gorkov, Theory of superconducting alloys in a strong magnetic field near the critical temperature, Soviet Physics JETP, 10: 998-1004, 1960.
18. G. Bednorz, K . A. Müller, Possible high T_c superconductivity in the Ba-La-Cu system, Z. Phys. B, 64: 189-197, 1986.

Excerpted from "Engineering Superconductivity," ed. Peter J. Lee, Wiley-Interscience, New York, 2001
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