NEWSFOCUS on July 21, 2011 After 2 decades of monumental effort, physicists still cannot explain 100,000 papers on the materials. Several theo- rists claim they have deciphered them— high-temperature superconductivity. But they may have identified the although their explanations clash. Still, high- puzzles they have yet to solve temperature superconductivity has refused to submit to some of the world’s best minds. “The theoretical problem is so hard that there isn’t an obvious criterion for right,” says High T : The Mystery Steven Kivelson, a theorist at Stanford Univer- www.sciencemag.org c sity in Palo Alto, California. Experimenters are producing a flood of highly detailed data, but physicists struggle to piece the results together, That Defies Solution says Joseph Orenstein, an experimenter at the University of California, Berkeley, and TWENTY YEARS AGO, A FIRESTORM OF giddy enthusiasm. “We had prominent people Lawrence Berkeley National Laboratory. “It discovery swept through the world of physics. saying it would all be explained quickly and must be close to unique to have so much infor- Downloaded from German experimenter J. Georg Bednorz and that we would have superconducting power mation and so little consensus on what the his Swiss colleague Karl Alexander Müller lines and levitating trains,” Ashcroft says. questions should be,” Orenstein says. kindled the flames in September 1986 when Ashcroft himself had doubts, however, as The problem is more than a sliver under they reported that an odd ceramic called he told a class of graduate students a few the nail. High-temperature superconductiv- lanthanum barium copper oxide carried elec- months later. (I was a member of the class.) ity has shown that physicists’ conceptual tricity without any resistance at a temperature The materials comprised four and five ele- tools can’t handle materials in which elec- of 35 kelvin—12 degrees above the previous ments and possessed elaborate layer-cake trons shove one another so intensely that it’s record for a superconductor. The blaze ran wild structures. They broke the rules about what impossible to disentangle the motion of one a few months later when Paul Chu of the Uni- should make a good superconductor. In short, from that of the others. Such “strongly cor- versity of Houston, Texas, and colleagues syn- Ashcroft predicted, high-temperature super- related” electrons pop up in nanodevices thesized yttrium barium copper oxide, a com- conductivity would remain the outstanding and novel magnets, organic conductors and pound that lost resistance at an unthinkable problem in condensed matter physics for the other exotic superconductors. “High- 93 K—conveniently warmer than liquefied air. next 25 years. temperature superconductivity is the A frenzy of slapdash experimenting and That prognostication is coming true. Two stumbling block of the whole discipline of sensational claims ensued, says Neil Ashcroft, decades after high-temperature super- condensed matter physics,” says Peter a theorist at Cornell University. He organized a conductors were discovered, physicists still do Abbamonte, an experimenter at the Univer- session on the new high-temperature super- not agree on how electrons within them pair sity of Illinois, Urbana-Champaign. conductors at the meeting of the American to glide through the materials effortlessly at In spite of the difficulty of the puzzle, many Physical Society in New York City the follow- temperatures as high as 138 K. Researchers physicists say they are closing in on a solution. ing March. The “Woodstock of physics” haven’t failed for lack of trying. According to Most now agree on certain key properties of stretched until 4 a.m. and bubbled over with some estimates, they have published more than the materials. Precision experiments are AND M. O. JONES/UNIVERSITY OF OXFORD EDWARDS CREDITS (TOP TO BOTTOM): P. 1072 17 NOVEMBER 2006 VOL 314 SCIENCE www.sciencemag.org Published by AAAS HIGH-TEMPERATURE SUPERCONDUCTIVITY TURNS 20 NEWSFOCUS Hot stuff. The structure of mercury barium calcium conductors shove one another so mightily High Tc Superconductivity copper oxide, a superconductor at 138 K. that they tend to jam up with one electron on each copper ion, like gridlocked commuters. revealing surprising details of the compounds. That impasse can be broken only by tweaking And computer simulations—and perhaps even the material’s chemical composition to siphon mockups fashioned of ultracold atoms and away some of the electrons to create positively laser light—could soon show physicists charged “holes,” a process called doping. whether their basic model of the problem is cor- The challenge then is to explain how elec- rect. “If I had to make a prediction,” Kivelson trons that fiercely repel each other manage to says, “I would say that in 10 years time the pair anyway. Some researchers argue that problem will be solved.” waves of magnetism play a similar role to the Conventional Superconductivity one phonons play in conventional supercon- The ultimate chess game ductors. Others focus solely on how the elec- Even “conventional” superconductivity, which trons shuffle past one another in a quantum- was discovered in 1911, is mind-bending. Elec- mechanical game of chess. Still others say that trons in a metal move in quantum waves of dis- patterns of charge or current, or even phonons, tinct energies. Quantum mechanics prohibits play a crucial role. Pairing might even require two electrons from occupying the same wave or all of these things in combination, which “state,” so they stack into the states from the would be many physicists’nightmare scenario. lowest energy on up. But when metals such as lead and niobium are cooled to near absolute Familiar solutions zero, the electrons in them can lower their total Some of the theories being refined today Copper Oxygen Electron Niobium energy by pairing like ballroom dancers. That emerged soon after Bednorz and Müller’s partnership produces superconductivity, as discovery, and the dividing lines that run Shall we dance? Instead of the motion of ions, the on July 21, 2011 explained in 1957 by theorists John Bardeen, through the field were drawn in those heady subtle waltz of electrons along atomic planes may Leon Cooper, and John Robert Schrieffer. days. For example, as early as 1987 some cause pairing in high-temperature materials. The pairing alters the spacing of the rungs theorists argued that high-temperature super- on the energy ladder, creating a gap near the top conductivity arose not from phonons but tions, creating an up-down-up-down pattern of the stack. To break from its partner, an elec- from the interaction of the electrons alone. known as antiferromagnetism. The electrons tron must jump the gap to an empty state. There But even those who agree on that principle can tilt and flip, and waves of wobble coursing isn’t enough energy around to allow that, so the often disagree on the details. through this arrangement can provide the glue pairs glide along unperturbed. Something must The idea that waves of magnetism drive for pairing, says David Pines, a theorist at Los glue the pairs together, and according to the the superconductivity is based on the fact that Alamos National Laboratory in New Mexico www.sciencemag.org Bardeen-Cooper-Schrieffer (BCS) theory, the electrons act like little magnets. Those on and the University of California, Davis. adhesive is quantized vibrations of the crys- adjacent copper ions point in opposite direc- But Philip Anderson, a theorist at Princeton talline material, or “phonons.” A University, says that no glue is necessary. Just passing electron attracts the slower- “The theoretical problem is so months after the discovery of high-temperature moving ions in the crystal lattice, hard that there isn’t an obvious superconductors, he proposed a scheme known which squeeze together to produce a as the resonating valence bond (RVB) theory, knot of positive charge that attracts criterion for right.” which focuses on subtle quantum connections Downloaded from another electron (see diagram). —Steven Kivelson, Stanford University between electrons on neighboring copper ions. High-temperature materials lit- In the theory, no waves of any kind pass erally take superconductivity to a between electrons, Anderson says. new plane. The compounds con- Thanks to the weird rules of quantum tain planes of copper and oxygen mechanics, each electron can point both up ions that resemble chess boards, and down simultaneously. Moreover, neigh- with a copper ion at every corner of boring electrons can join in an odd quantum a square and an oxygen ion along state called a singlet in which both electrons each side. Electrons hop from cop- point both up and down at once, but the two per ion to copper ion. Between the electrons always point in opposite direc- planes lie elements such as lan- tions—either down-up or up-down. When ; LINDA A. CICERO/STANFORD NEWS SERVICE ; LINDA A. CICERO/STANFORD thanum, strontium, yttrium, bis- enough holes open in the plane, singlets form SCIENCE muth, and thalium. But it is along and begin to slide freely past one another, the copper-and-oxygen planes that eventually producing superconductivity. the electrons pair and glide. Others contend that both the magnetic Just how that happens is any- fluctuation and RVB theories leave out some thing but clear. The electrons in an essential piece of physics. Stanford’s Kivelson ordinary metal hardly notice one believes stripes of electric charge on the planes, another and interact mainly with which have been seen in some materials, may phonons. In contrast, the electrons be necessary to trigger the pairing. Chandra in high-temperature super- Varma, a theorist at the University of California, CREDITS (TOP TO BOTTOM): P. HUEY/ CREDITS (TOP TO BOTTOM): P. www.sciencemag.org SCIENCE VOL 314 17 NOVEMBER 2006 1073 Published by AAAS NEWSFOCUS Riverside, proposes that loops of leagues present data online in Science this week current flowing inside each copper- (www.sciencemag.org/cgi/content/abstract/ and-oxygen square are key.