Bednorz and Müller Win Nobel Prize for New Superconducting Materials Anil Khurana

Citation: Phys. Today 40(12), 17 (1987); doi: 10.1063/1.2820304 View online: http://dx.doi.org/10.1063/1.2820304 View Table of Contents: http://www.physicstoday.org/resource/1/PHTOAD/v40/i12 Published by the American Institute of Physics.

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BEDNORZ AND MULLER WIN NOBEL PRIZE FOR NEW SUPERCONDUCTING MATERIALS

Georg Bednorz and Alex Miiller (IBM Zurich Research Laboratory at Riischlikon, Switzerland) share this year's Nobel Prize in Physics for "their discovery of new superconduct- ing materials." The announcement of the prize, worth $340 000 this year, caused little surprise among physi- cists. Few doubted that the discovery by Bednorz and Miiller merited the Nobel Prize; speculations on what year the prize would be awarded have abounded since last January. In the spring of 1986, Bednorz and Miiller reported the onset of super- conductivity in a mixed-phase oxide of lanthanum, barium and copper at temperatures about 10 K higher than any previously known for supercon- ductivity. Since the early 1960s, the superconductors with the highest known critical temperatures had been found among intermetallic ma- terials with the so-called A15 struc- ture; the last increase in Tc had occurred in 1973 with successful syn- thesis of thin films of Nb3Ge. The search for high-temperature super- Alex Muller and Georg Bednorz (right) or rhe IBM conductors then lost direction when Zurich Research Laboratory in Ruschlikon, Switzerland. efforts in the late 1970s to raise the Tc even higher in the A15 materials were abandoned because the maxi- turned into useful devices, but groups this specific superconductivity. They mum critical temperature in A15 led by Praveen Chaudhari at IBM have inspired other researchers to niobium-silicon compounds could not (Yorktown Heights, New York) and synthesize substances which are su- be raised above 20 K.' by Malcolm Beasley and Theodore perconducting at temperatures more About three months after the dis- Geballe at Stanford University dem- than four times higher (reckoned covery by Bednorz and Miiller was onstrated this summer that the criti- from the absolute zero at — 273 °C) confirmed in November 1986, Paul cal current, an important parameter than the earlier ones. The develop- (C. W.) Chu (University of Houston) for many applications, is not inher- ment is being followed with interest reported superconductivity above ently small in the new superconduc- by workers in electrotechnology and 90 K; Chu disclosed the composition of tors. Because of the allure of the new microelectronics, and by physicists the 90-K material, an oxide of yt- markets that applications of super- who envisage exciting new applica- trium, barium and copper, two weeks conductivity might generate, the tions in measurement technology." later. Obtaining superconductivity worlds of engineering and technology, The academy characterizes the break- above 77 K, the boiling point of liquid business and finance, politics and through by Bednorz and Muller as a nitrogen, had been a psychological science policy have all felt the impact "result of systematic work, deep in- barrier that many experts felt had to of the developments in superconduc- sight and experience of structural be broken before large-scale applica- tivity since Bednorz and Muller made problems in the physics and chemis- tions of superconductivity would be their discovery. try of the solid state ... [and] the economically viable. Many proper- The Swedish Academy announce- audacity to concentrate on new paths ties of the new oxide superconductors ment says: "Bednorz and Muller in their research." must be optimized before they can be stand out clearly as the discoverers of Bednorz and Muller carried out a

© 1987 Americon InDownloadedsriture of Phy s12ics Feb 2013 to 142.1.166.207. Redistribution subject to AIP license or copyright; see http://www.physicstoday.org/about_us/termsPHYSICS TODAY DECEMBER 1967 17 detailed and determined search for a metal in which nickel is in valence the resistivity to very low tempera- superconductors with high critical state + 3. According to Bednorz, tures. "We did a systematic and temperatures among oxides with me- they tried to change the electronic careful study, and even if the resistiv- tallic properties. They were motivat- bandwidth of the materials internal- ity went up to extremely high values ed to undertake this search by the ly, by substituting aluminum for nick- at low temeperatures, we always went properties of the superconducting el, for example. But the substitution down to liquid helium temperature phase of BaPb^Bi^Oa and LiTi2O4. was not successful: La-Ni-Al-0 be- [4.2 K] in order to have a complete set The critical temperature of BaPbj.j- came insulating by cooling. "We then of data," Bednorz replied. Further- Bix O3 varies with x. The highest Tc is tried substitution on the lanthanum more, both Bednorz and Miiller said only 13.7 K, but the electronic density sites," Bednorz said. "We tried yt- they were aware that the resistivity of of states is significantly smaller than trium; at that time we easily missed thin films of BaPb, _ x Bix O3 also in- in superconductors with comparable discovering the 90-K superconductor. creases before the onset of supercon- critical temperatures.1 Bednorz and But we got insulating material. We ductivity, especially if the films do not Miiller argued that the lower density had the wrong combination—yttrium have the right amount of oxygen. of states in oxide superconductors was with nickel instead of copper. But The discovery by Bednorz and probably compensated for by the en- that's life! Our breakthrough came Miiller has engendered unprecedent- hanced electron-phonon interaction. much later." The focus of the search ed, worldwide research activity in In the theory of John Bardeen, Leon shifted to copper because partial sub- superconductivity. The story of some Cooper and Robert Schrieffer, elec- stitution of copper for nickel in of the major developments their work trons in the superconducting state are LaNiO3 improved the metallic proper- has spawned and the rapid pace at paired by their interaction with lat- ties. In the course of a literature which these occurred has become tice vibrations, or phonons. The criti- search, Bednorz learned about the somewhat of a legend. Their first cal temperature in the BCS theory work of Claude Michel, L. Er-Rakho paper was received at the editorial increases with the strength of the and Bernard Raveau (Universite de office of Zeitschrift fur Physik on 17 pairing interaction, which in turn Caen) on Ba-La-Cu-O, soon after it April 1986; it was published in the increases with both the electron- was published in 1985. "Having September issue of the journal. The phonon coupling and the electron worked already with a few copper paper received little attention. But density. But at none of the many substitutions, when I saw this materi- Shoji Tanaka's group at the Universi- laboratories around the world that al, I realized one could do something with copper alone and with replace- ty of Tokyo (see the article on page 53) studied BaPb^^BijOa in great detail and Chu's at the University of Hous- was the possibility of examining other ment of lanthanum by two-valent oxides for superconductivity seriously barium," Bednorz said. And he and ton independently confirmed super- considered. Bednorz and Miiller, in Miiller did something quite wonderful conductivity in Ba-La-Cu-O after they contrast, regarded the unusual prop- with it. In their first paper, they saw the paper by Bednorz and Miiller erties of BaPbi _ Bi, O seriously and reported resistivity measurements on in Zeitschrift fur Physik. The groups x 3 presented their evidence at an im- saw in them a possibility of finding samples of La5_IBa;tCu5O5(3_ y) for superconductors with high critical x = 1 and 0.75, and y positive, that promptu session of the Materials Re- temperatures. Increasing the elec- showed onset of superconductivity search Society meeting in Boston on 5 tron density in oxides to values com- above 30 K. December 1986. The Tokyo group parable to those in good metals, for had by this time independently deter- example, might allow a way to realize One might feel tempted to regard as mined La2_IBa;cCuO4_v to be the this possibility, they reasoned. Based serendipitous the events that led Bed- superconducting phase, as had Bed- on their knowledge of the structure norz and Miiller to the discovery of norz, Miiller and their collaborator M. and properties of oxides, Bednorz and superconductivity in Ba-La-Cu-O. Takashige, and both groups had ob- Miiller concentrated on oxides con- But as experts around the world tained further evidence of supercon- taining copper or nickel in mixed- found out last winter, ternary oxides ductivity by measuring the Meissner valence states, that is, oxides in which are very complicated materials. Sev- effect, or magnetic flux exclusion. a fraction of the transition metal ions eral stable chemical compositions are But Bednorz, Miiller and Takashige are in one valence state and another possible for a given set of elements, did not circulate any preprints report- fraction are in a different valence and among oxides with the same set of ing their work on the Meissner effect, state. elements but different compositions although they had completed the some are insulators and some good work in the beginning of October. "Alex reminded me recently that conductors. Moreover, the ones that "When in October I saw the results he did not have to use very strong are superconducting at low tempera- from Zurich on the Meissner effect," arguments to convince me to look for tures are extremely fragile chemical- Richard Greene (IBM Yorktown superconductors among metallic ox- ly and lose oxygen readily. But their Heights) said, "I believed that Bed- ides," Bednorz told us at an interview conductivity in the normal state as norz and Mtiller had indeed discov- we had with the two laureates on 28 well as the transition to the supercon- ered a new, higher-temperature su- October in Yorktown Heights. The ducting phase depends sensitively on perconductor." Chaudhari told us search started in the summer of 1983; the oxygen content. For example, in that he brought back a few supercon- the breakthrough occured in January the measurements Bednorz and ducting samples from Zurich, and 1986. Bednorz told us that he Miiller reported in their first paper, Greene started in late October experi- searched the literature for known the resistivity of the superconducting ments on specific heat and on deter- oxides of copper and nickel and stud- sample began to rise at temperatures mining the strengths of the critical ied carefully whatever was known below about 100 K. Because the resis- magnetic fields that destroy super- about their high-temperature proper- tivity of good metals continues to decrease with decreasing tempera- conductivity. "During 1986, we at ties. Metallic behavior for electrical IBM proceeded cautiously," Chaud- conductivity, he said, was an impor- ture, we asked Bednorz and Miiller why, in view of their search for hari said. "Even though we were tant criterion in selecting the com- convinced that Bednorz and Miiller pounds they tried for superconductiv- superconductors among metallic ox- ides, they even bothered to measure had discovered a new high-tempera- ity. The search started with LaNiO3, ture superconductor, Alex and I dis-

1 8 PHYSICS TODAY DECEMBER 1987 Downloaded 12 Feb 2013 to 142.1.166.207. Redistribution subject to AIP license or copyright; see http://www.physicstoday.org/about_us/terms SEARCH b DISCOVERY cussed studying time-dependent prop- sabbatical at IBM Yorktown Heights (ETH) in Zurich in 1952 and 1958. He erties such as persistent currents to (1978-80), he said, he collaborated was a staff member at the physics make sure we understood why the with Melvin Pomerantz on an experi- department of ETH from 1952 to Meissner effect was so small." But ment on microwave absorption in 1958. He was next a project manager the news from Boston spread fast. In thin films of granular aluminum. at the Batelle Institute, Geneva, from the last week of December, several The critical temperature of the films, 1959 to 1963. Miiller was appointed groups from around the world report- composed of small, oxide-coated as a lecturer at the University of ed that the critical temperature for grains of aluminum, was about two Zurich in 1962 and was made Titular superconductivity in the oxide stud- times that of pure aluminum which Professor in 1970. He joined the IBM ied by Bednorz and Miiller could be has a Tc of 1.1 K. Miiller found that Zurich Research Laboratory as a re- raised to about 40 K by replacing very interesting and wondered search staff member in 1963, man- barium with strontium. And Chu whether a similar enhancement in Tc aged its physics department from stunned the world of physical sciences could be obtained in metallic super- 1972 to 1985, and has been an IBM with his announcement on 16 Febru- conductors whose critical tempera- Fellow since 1982. "It is very satisfy- ary that he and his collaborators had tures were already in the 10-15-K ing that our work has generated so obtained superconductivity above range. But the theorists told him that much interest and has been recog- 90 K in an oxide material. Chu's because of its low Tc, aluminum is nized by the Swedish committee," paper reporting the discovery was described by the weak-coupling BCS Miiller said about winning the Nobel received at the editorial office of theory, but that the enhancement was Prize less than a year after his work Physical Review Letters on 6 Febru- not possible in superconductors with with Bednorz was accepted by their ary; he announced the discovery, he Tc of 10-15 K, which are better colleagues. "We never thought about told us, only after the paper was described by the so-called strong-cou- the prize," he added. "We only want- accepted for publication. Chinese and pling theory. "After I heard this," ed to go beyond the intermetallic A15 Japanese physicists announced in the Miiller said, "I decided not to listen to compounds." last week of February that they had theorists anymore." Bednorz got his undergraduate de- independently discovered supercon- How much longer would Bednorz gree at the University of Miinster in ductivity above 90 K. As a result of and Miiller have continued their 1976 and his doctorate at ETH in these developments, a hastily ar- search if they had not discovered 1982. He did research for his PhD ranged special session on the new superconductivity in January 1986? thesis at IBM Zurich under Miiller's superconductors at the annual March "We were quite persistent," both the supervision. He has been a research meeting of The American Physical laureates smiled and said. What was staff member at IBM Zurich since Society in New York turned into a the next combination on their list? 1982. "It is hard to describe," he said historic occasion and was dubbed the Did they have a list? "We had a list, when we asked him about what it "Woodstock of physics." The theme and we still have one in the lab. feels like to win the Nobel Prize. "I song at this Woodstock was one-two- Maybe we will meet each other again have to look at myself.... I have to three, after the chemical composition in a few years and I will then tell you learn," he added. i?Ba2Cu307_y, where R is a rare earth more about our list," Bednorz said. —ANIL KHURANA element, of the materials with Tc Details of the three-year search at above 90 K. (See PHYSICS TODAY, April IBM Zurich, it seems, will provide Reference 1987, page 17.) grist for the mill of historians and 1. For a summary of major developments Several chapters in standard text- sociologists of science for many years. in superconducting materials in the books in solid-state physics may have Miiller got his master's and doctor- past two decades, see, for example, to be rewritten when the properties of al degrees in physics at the Swiss M. R. Beasley, T. H. Geballe, PHYSICS the new superconducting oxides and Federal Institute of Technology TODAY, October 1984, p. 60. the mechanism of superconductivity in them are properly understood. Many theorists have proposed novel mechanisms for superconductivity in TWO-NEUTRINO DOUBLE p-DECAY the oxides because they believe that phonon-mediated pairing of electrons SEEN; NEUTRINOLESS DECAY SOUGHT cannot give critical temperatures as high as 90 K. The critical tempera- Many nuclei with even numbers of measurement is a milestone en route ture of the 90-K materials changes protons and neutrons can undergo to a more elusive goal—observation of negligibly with the isotopic mass of double beta decay, emitting two elec- a double beta decay in which two the various elements. (See PHYSICS trons and two neutrinos, but the electrons but no neutrinos emerge. TODAY, July, page 17.) Theorists re- half life for this process is so long that This neutrinoless decay, which pro- gard this lack of the isotope effect as it had been deduced until recently ceeds by the exchange of a virtual evidence that pairing of electrons in only by measuring the abundance of neutrino between the two neutrons, the oxide superconductors is mediat- daughter nuclei from double beta can occur only if neutrinos have mass. ed by an electronic or magnetic exci- decay of elements in geologic materi- It is strictly forbidden by the standard tation and not by phonons. We asked als. Now Steven Elliott, Alan Hahn theory of electroweak interactions Bednorz and Miiller how high they and Michael Moe of the University of but is predicted as a manifestation of thought they could raise the critical California at Irvine have observed the the small symmetry breaking that temperature when they set out on double beta decay of selenium-82 in arises in some grand unified field their search and whether any of the their laboratory and determined its theories. limits that theorists have discussed on half life to be 1.1 + gJxlO20 years,1 a The Irvine measurement of a two how high the Tc can be in the phonon time interval that is orders of magni- neutrino double beta decay rate gives mechanism in any way discouraged tude longer than any previously de- hope that future experiments may be them in their research. Miiller relat- tected in a laboratory. Besides being sensitive even to very low rates for the ed an anecdote in reply. During his a feat in its own right, the Irvine neutrinoless double beta decay. It

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