Bednorz and Müller Win Nobel Prize for New Superconducting MaterialsAnil 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. Additional resources for Physics TodayHomepage: http://www.physicstoday.org/ Information: http://www.physicstoday.org/about_us Daily Edition: http://www.physicstoday.org/daily_edition

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SEARCH & DISCOVERY

BEDNORZ AND MULLER WIN NOBEL PRIZEFOR NEW SUPERCONDUCTING MATERIALS

Georg Bednorz and Alex Miiller (IBMZurich Research Laboratory atRiischlikon, Switzerland) share thisyear's Nobel Prize in Physics for"their discovery of new superconduct-ing materials." The announcement ofthe prize, worth $340 000 this year,caused little surprise among physi-cists. Few doubted that the discoveryby Bednorz and Miiller merited theNobel Prize; speculations on whatyear the prize would be awarded haveabounded since last January.

In the spring of 1986, Bednorz andMiiller reported the onset of super-conductivity in a mixed-phase oxide oflanthanum, barium and copper attemperatures about 10 K higher thanany previously known for supercon-ductivity. Since the early 1960s, thesuperconductors with the highestknown critical temperatures hadbeen found among intermetallic ma-terials with the so-called A15 struc-ture; the last increase in Tc hadoccurred in 1973 with successful syn-thesis of thin films of Nb3Ge. Thesearch for high-temperature super-conductors then lost direction whenefforts in the late 1970s to raise the Tceven higher in the A15 materialswere abandoned because the maxi-mum critical temperature in A15niobium-silicon compounds could notbe raised above 20 K.'

About three months after the dis-covery by Bednorz and Miiller wasconfirmed in November 1986, Paul(C. W.) Chu (University of Houston)reported superconductivity above90 K; Chu disclosed the composition ofthe 90-K material, an oxide of yt-trium, barium and copper, two weekslater. Obtaining superconductivityabove 77 K, the boiling point of liquidnitrogen, had been a psychologicalbarrier that many experts felt had tobe broken before large-scale applica-tions of superconductivity would beeconomically viable. Many proper-ties of the new oxide superconductorsmust be optimized before they can be

Alex Muller and Georg Bednorz (right) or rhe IBMZurich Research Laboratory in Ruschlikon, Switzerland.

turned into useful devices, but groupsled by Praveen Chaudhari at IBM(Yorktown Heights, New York) andby Malcolm Beasley and TheodoreGeballe at Stanford University dem-onstrated this summer that the criti-cal current, an important parameterfor many applications, is not inher-ently small in the new superconduc-tors. Because of the allure of the newmarkets that applications of super-conductivity might generate, theworlds of engineering and technology,business and finance, politics andscience policy have all felt the impactof the developments in superconduc-tivity since Bednorz and Muller madetheir discovery.

The Swedish Academy announce-ment says: "Bednorz and Mullerstand out clearly as the discoverers of

this specific superconductivity. Theyhave inspired other researchers tosynthesize substances which are su-perconducting at temperatures morethan four times higher (reckonedfrom the absolute zero at — 273 °C)than the earlier ones. The develop-ment is being followed with interestby workers in electrotechnology andmicroelectronics, and by physicistswho envisage exciting new applica-tions in measurement technology."The academy characterizes the break-through by Bednorz and Muller as a"result of systematic work, deep in-sight and experience of structuralproblems in the physics and chemis-try of the solid state . . . [and] theaudacity to concentrate on new pathsin their research."

Bednorz and Muller carried out a

© 1987 Americon Insriture of Physics PHYSICS TODAY DECEMBER 1967 17Downloaded 12 Feb 2013 to 142.1.166.207. Redistribution subject to AIP license or copyright; see http://www.physicstoday.org/about_us/terms

detailed and determined search forsuperconductors with high criticaltemperatures among oxides with me-tallic properties. They were motivat-ed to undertake this search by theproperties of the superconductingphase of BaPb^Bi^Oa and LiTi2O4.The critical temperature of BaPbj.j-Bix O3 varies with x. The highest Tc isonly 13.7 K, but the electronic densityof states is significantly smaller thanin superconductors with comparablecritical temperatures.1 Bednorz andMiiller argued that the lower densityof states in oxide superconductors wasprobably compensated for by the en-hanced electron-phonon interaction.In the theory of John Bardeen, LeonCooper and Robert Schrieffer, elec-trons in the superconducting state arepaired by their interaction with lat-tice vibrations, or phonons. The criti-cal temperature in the BCS theoryincreases with the strength of thepairing interaction, which in turnincreases with both the electron-phonon coupling and the electrondensity. But at none of the manylaboratories around the world thatstudied BaPb^^BijOa in great detailwas the possibility of examining otheroxides for superconductivity seriouslyconsidered. Bednorz and Miiller, incontrast, regarded the unusual prop-erties of BaPbi _x Bi, O3 seriously andsaw in them a possibility of findingsuperconductors with high criticaltemperatures. Increasing the elec-tron density in oxides to values com-parable to those in good metals, forexample, might allow a way to realizethis possibility, they reasoned. Basedon their knowledge of the structureand properties of oxides, Bednorz andMiiller concentrated on oxides con-taining copper or nickel in mixed-valence states, that is, oxides in whicha fraction of the transition metal ionsare in one valence state and anotherfraction are in a different valencestate.

"Alex reminded me recently thathe did not have to use very strongarguments to convince me to look forsuperconductors among metallic ox-ides," Bednorz told us at an interviewwe had with the two laureates on 28October in Yorktown Heights. Thesearch started in the summer of 1983;the breakthrough occured in January1986. Bednorz told us that hesearched the literature for knownoxides of copper and nickel and stud-ied carefully whatever was knownabout their high-temperature proper-ties. Metallic behavior for electricalconductivity, he said, was an impor-tant criterion in selecting the com-pounds they tried for superconductiv-ity. The search started with LaNiO3,

a metal in which nickel is in valencestate + 3. According to Bednorz,they tried to change the electronicbandwidth of the materials internal-ly, by substituting aluminum for nick-el, for example. But the substitutionwas not successful: La-Ni-Al-0 be-came insulating by cooling. "We thentried substitution on the lanthanumsites," Bednorz said. "We tried yt-trium; at that time we easily misseddiscovering the 90-K superconductor.But we got insulating material. Wehad the wrong combination—yttriumwith nickel instead of copper. Butthat's life! Our breakthrough camemuch later." The focus of the searchshifted to copper because partial sub-stitution of copper for nickel inLaNiO3 improved the metallic proper-ties. In the course of a literaturesearch, Bednorz learned about thework of Claude Michel, L. Er-Rakhoand Bernard Raveau (Universite deCaen) on Ba-La-Cu-O, soon after itwas published in 1985. "Havingworked already with a few coppersubstitutions, when I saw this materi-al, I realized one could do somethingwith copper alone and with replace-ment of lanthanum by two-valentbarium," Bednorz said. And he andMiiller did something quite wonderfulwith it. In their first paper, theyreported resistivity measurements onsamples of La5_IBa;tCu5O5(3_ y) forx = 1 and 0.75, and y positive, thatshowed onset of superconductivityabove 30 K.

One might feel tempted to regard asserendipitous the events that led Bed-norz and Miiller to the discovery ofsuperconductivity in Ba-La-Cu-O.But as experts around the worldfound out last winter, ternary oxidesare very complicated materials. Sev-eral stable chemical compositions arepossible for a given set of elements,and among oxides with the same set ofelements but different compositionssome are insulators and some goodconductors. Moreover, the ones thatare superconducting at low tempera-tures are extremely fragile chemical-ly and lose oxygen readily. But theirconductivity in the normal state aswell as the transition to the supercon-ducting phase depends sensitively onthe oxygen content. For example, inthe measurements Bednorz andMiiller reported in their first paper,the resistivity of the superconductingsample began to rise at temperaturesbelow about 100 K. Because the resis-tivity of good metals continues todecrease with decreasing tempera-ture, we asked Bednorz and Miillerwhy, in view of their search forsuperconductors among metallic ox-ides, they even bothered to measure

the resistivity to very low tempera-tures. "We did a systematic andcareful study, and even if the resistiv-ity went up to extremely high valuesat low temeperatures, we always wentdown to liquid helium temperature[4.2 K] in order to have a complete setof data," Bednorz replied. Further-more, both Bednorz and Miiller saidthey were aware that the resistivity ofthin films of BaPb, _ x Bix O3 also in-creases before the onset of supercon-ductivity, especially if the films do nothave the right amount of oxygen.

The discovery by Bednorz andMiiller has engendered unprecedent-ed, worldwide research activity insuperconductivity. The story of someof the major developments their workhas spawned and the rapid pace atwhich these occurred has becomesomewhat of a legend. Their firstpaper was received at the editorialoffice of Zeitschrift fur Physik on 17April 1986; it was published in theSeptember issue of the journal. Thepaper received little attention. ButShoji Tanaka's group at the Universi-ty of Tokyo (see the article on page 53)and Chu's at the University of Hous-ton independently confirmed super-conductivity in Ba-La-Cu-O after theysaw the paper by Bednorz and Miillerin Zeitschrift fur Physik. The groupspresented their evidence at an im-promptu session of the Materials Re-search Society meeting in Boston on 5December 1986. The Tokyo grouphad by this time independently deter-mined La2_IBa;cCuO4_v to be thesuperconducting phase, as had Bed-norz, Miiller and their collaborator M.Takashige, and both groups had ob-tained further evidence of supercon-ductivity by measuring the Meissnereffect, or magnetic flux exclusion.But Bednorz, Miiller and Takashigedid not circulate any preprints report-ing their work on the Meissner effect,although they had completed thework in the beginning of October.

"When in October I saw the resultsfrom Zurich on the Meissner effect,"Richard Greene (IBM YorktownHeights) said, "I believed that Bed-norz and Mtiller had indeed discov-ered a new, higher-temperature su-perconductor." Chaudhari told usthat he brought back a few supercon-ducting samples from Zurich, andGreene started in late October experi-ments on specific heat and on deter-mining the strengths of the criticalmagnetic fields that destroy super-conductivity. "During 1986, we atIBM proceeded cautiously," Chaud-hari said. "Even though we wereconvinced that Bednorz and Miillerhad discovered a new high-tempera-ture superconductor, Alex and I dis-

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SEARCH b DISCOVERYcussed studying time-dependent prop-erties such as persistent currents tomake sure we understood why theMeissner effect was so small." Butthe news from Boston spread fast. Inthe last week of December, severalgroups from around the world report-ed that the critical temperature forsuperconductivity in the oxide stud-ied by Bednorz and Miiller could beraised to about 40 K by replacingbarium with strontium. And Chustunned the world of physical scienceswith his announcement on 16 Febru-ary that he and his collaborators hadobtained superconductivity above90 K in an oxide material. Chu'spaper reporting the discovery wasreceived at the editorial office ofPhysical Review Letters on 6 Febru-ary; he announced the discovery, hetold us, only after the paper wasaccepted for publication. Chinese andJapanese physicists announced in thelast week of February that they hadindependently discovered supercon-ductivity above 90 K. As a result ofthese developments, a hastily ar-ranged special session on the newsuperconductors at the annual Marchmeeting of The American PhysicalSociety in New York turned into ahistoric occasion and was dubbed the"Woodstock of physics." The themesong at this Woodstock was one-two-three, after the chemical compositioni?Ba2Cu307_y, where R is a rare earthelement, of the materials with Tcabove 90 K. (See PHYSICS TODAY, April1987, page 17.)

Several chapters in standard text-books in solid-state physics may haveto be rewritten when the properties ofthe new superconducting oxides andthe mechanism of superconductivityin them are properly understood.Many theorists have proposed novelmechanisms for superconductivity inthe oxides because they believe thatphonon-mediated pairing of electronscannot give critical temperatures ashigh as 90 K. The critical tempera-ture of the 90-K materials changesnegligibly with the isotopic mass ofthe various elements. (See PHYSICSTODAY, July, page 17.) Theorists re-gard this lack of the isotope effect asevidence that pairing of electrons inthe oxide superconductors is mediat-ed by an electronic or magnetic exci-tation and not by phonons. We askedBednorz and Miiller how high theythought they could raise the criticaltemperature when they set out ontheir search and whether any of thelimits that theorists have discussed onhow high the Tc can be in the phononmechanism in any way discouragedthem in their research. Miiller relat-ed an anecdote in reply. During his

sabbatical at IBM Yorktown Heights(1978-80), he said, he collaboratedwith Melvin Pomerantz on an experi-ment on microwave absorption inthin films of granular aluminum.The critical temperature of the films,composed of small, oxide-coatedgrains of aluminum, was about twotimes that of pure aluminum whichhas a Tc of 1.1 K. Miiller found thatvery interesting and wonderedwhether a similar enhancement in Tccould be obtained in metallic super-conductors whose critical tempera-tures were already in the 10-15-Krange. But the theorists told him thatbecause of its low Tc, aluminum isdescribed by the weak-coupling BCStheory, but that the enhancement wasnot possible in superconductors withTc of 10-15 K, which are betterdescribed by the so-called strong-cou-pling theory. "After I heard this,"Miiller said, "I decided not to listen totheorists anymore."

How much longer would Bednorzand Miiller have continued theirsearch if they had not discoveredsuperconductivity in January 1986?"We were quite persistent," both thelaureates smiled and said. What wasthe next combination on their list?Did they have a list? "We had a list,and we still have one in the lab.Maybe we will meet each other againin a few years and I will then tell youmore about our list," Bednorz said.Details of the three-year search atIBM Zurich, it seems, will providegrist for the mill of historians andsociologists of science for many years.

Miiller got his master's and doctor-al degrees in physics at the SwissFederal Institute of Technology

(ETH) in Zurich in 1952 and 1958. Hewas a staff member at the physicsdepartment of ETH from 1952 to1958. He was next a project managerat the Batelle Institute, Geneva, from1959 to 1963. Miiller was appointedas a lecturer at the University ofZurich in 1962 and was made TitularProfessor in 1970. He joined the IBMZurich Research Laboratory as a re-search staff member in 1963, man-aged its physics department from1972 to 1985, and has been an IBMFellow since 1982. "It is very satisfy-ing that our work has generated somuch interest and has been recog-nized by the Swedish committee,"Miiller said about winning the NobelPrize less than a year after his workwith Bednorz was accepted by theircolleagues. "We never thought aboutthe prize," he added. "We only want-ed to go beyond the intermetallic A15compounds."

Bednorz got his undergraduate de-gree at the University of Miinster in1976 and his doctorate at ETH in1982. He did research for his PhDthesis at IBM Zurich under Miiller'ssupervision. He has been a researchstaff member at IBM Zurich since1982. "It is hard to describe," he saidwhen we asked him about what itfeels like to win the Nobel Prize. "Ihave to look at myself.... I have tolearn," he added.

—ANIL KHURANA

Reference1. For a summary of major developments

in superconducting materials in thepast two decades, see, for example,M. R. Beasley, T. H. Geballe, PHYSICSTODAY, October 1984, p. 60.

TWO-NEUTRINO DOUBLE p-DECAYSEEN; NEUTRINOLESS DECAY SOUGHTMany nuclei with even numbers ofprotons and neutrons can undergodouble beta decay, emitting two elec-trons and two neutrinos, but thehalf life for this process is so long thatit had been deduced until recentlyonly by measuring the abundance ofdaughter nuclei from double betadecay of elements in geologic materi-als. Now Steven Elliott, Alan Hahnand Michael Moe of the University ofCalifornia at Irvine have observed thedouble beta decay of selenium-82 intheir laboratory and determined itshalf life to be 1.1 + gJxlO20 years,1 atime interval that is orders of magni-tude longer than any previously de-tected in a laboratory. Besides beinga feat in its own right, the Irvine

measurement is a milestone en routeto a more elusive goal—observation ofa double beta decay in which twoelectrons but no neutrinos emerge.This neutrinoless decay, which pro-ceeds by the exchange of a virtualneutrino between the two neutrons,can occur only if neutrinos have mass.It is strictly forbidden by the standardtheory of electroweak interactionsbut is predicted as a manifestation ofthe small symmetry breaking thatarises in some grand unified fieldtheories.

The Irvine measurement of a twoneutrino double beta decay rate giveshope that future experiments may besensitive even to very low rates for theneutrinoless double beta decay. It

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