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Superconductivity: a physics rush.

Superconductivity: A Physics Rush

In science, "recent' usually means "inthe last few years.' In the search for high-temperature superconductivity it has come to mean "yesterday.'

High-temperature superconductivityis the physicist's dream that now seems to be coming true--in a tremendous rush. The discovery of substances that lose all electrical resistance at manageable temperatures has been a goal of researchers for 75 years. The discovery last year of a new class of substances that become superconductors at around 30 kelvins, made by J. Georg Bednorz and K. Alex Mueller of the IBM Zurich Research Laboratory in Switzerland, triggered an avalanche that continues to intensify. The news has run to Japan, to China, to the United States, and the superconducting transition temperatures of new substances keep going up--52.5 K, 94 K, 98 K (SN: 3/14/87, p. 164).

Last week in New York City, one of themost extraordinary sessions of the American Physical Society ever held--it ran from 7:30 p.m. to 3:15 a.m. and played to an audience of thousands--addressed the latest progress in the field. This includes a claim to superconductivity at 125 K (still unconfirmed by others), from C. Politis of the University of Karlsruhe, West Germany, and a report of indications of superconductivity at 234 K by a group from the University of California at Berkeley and the Lawrence Berkeley Laboratory (Alex Zettl, Angelica Stacy and Marvin Cohen). The next day, at a press conference, Koichi Kitazawa of the University of Tokyo related that he had learned just in the last few days, from Japanese newspaper reports, that a group at Kagoshima University had found room-temperature (300 K) superconductivity, or at least they seem to have found the Josephson effect--a superconducting phenomenon--at room temperature.

The meeting also displayed superconductingtapes and rings made of the new materials, a first step toward technological application.

The pace has been breathless, and itseems to be still accelerating. The first materials found by Bendorz and Mueller, whose superconducting transition temperatures around 30 K are 10 K higher than any previously known, are now called "low-temperature superconductors,' sic transit gloria mundi. Announcements are dated by the hour: A summary put out by AT&T Bell Labs carried the overline "UPDATE--noon, 3/19/87.'

Superconductivity, which was discoveredin 1911 by Heike Kamerlingth Onnes of the University of Leiden, in the Netherlands, first appeared in solid mercury at a temperature of 4.2 K, which happens to be the liquefaction temperature of helium. Over the years, slow and painstaking work gradually pushed up the maximum temperature of superconductivity. The late Bernd Matthias, one of the great people in the field, used to show a graph of this progress. According to Robert Dynes of AT&T Bell Labs, if physicists had continued to proceed at the rate shown on that graph it should have taken until 2190 to reach the 94 K temperature that is the highest well-confirmed result as of today.

Refrigeration is one of the great hindreancesto practical use of superconductors. Those in technological use now require liquid helium for refrigeration. Helium is rare, requires refrigeration to 4.2 K and is not a particularly effective coolant. Liquid nitrogen, essentially liquid air, is cheap, easy to obtain, a much better coolant than helium and needs cooling to only 77 K, a much easier refrigeration technology. Several of the new materials are superconducting at liquid nitrogen temperature, and neither experiments nor theoreticians have been predicting any ceilings.

Some of the physicists seem to think itwon't be too long before the appearance of superconductors that work at temperatures accessible to household refrigerators. The "unusual drops in resistance'-- as Marvin Cohen of the University of California at Berkeley described what his group found in a compound of yttrium, barium, copper and oxygen at 234 K-- occur at a temperature equal to -38| F. If that should prove to be superconductivity, a cold winter day in Bemidji or International Falls could give outdoor superconductivity. If that sounds like a joke, Paul Chu of the University of Houston notes that 80 K is already ambient temperature on the shadow sides of both natural and artificial objects in space. Some of these substances could work without refrigeration in those environments.

Difficulty of fabrication is another hindranceto using superconductors. The ones in current use are all metals or metal alloys, and some of them are hard to work or have inconvenient mechanical properties. The new materials reported at the meeting are copper oxides containing rare-earth elements. P.M. Grant of the IBM Almaden Research Center in San Jose, Calif., whose group has deciphered the crystal structure of the yttrium-barium-copper oxygen compounds, reports that it is a perovskite--a crystal of a basically octahedral shape--that seems to be built of layers that are alternately conducting and insulating.

Theorists who spoke at the meetingseemed to agree that the presence of copper and oxygen and the chemical relationship between them is important for superconductivity, but whether the relationship involves planes of atoms or simply lines of them is not agreed. Again, the presence of rare earths seems important, and that is slightly paradoxical, as rare earths are magnetic, and magnetism and superconductivity have usually been incompatible.

Particularly striking is a compounddiscovered at Los Alamos (N.M.) National Laboratory (LANL) that includes gadolinium, a particularly magnetic rare earth. As James Smith of LANL pointed out, gadolinium is something physicists used to introduce into a superconducting material to see how much of it was necessary to destroy the superconductivity. Here gadolinium seems to enhance it.

Incidentially, rare earths are rare chemicallybut not geologically. As pointed out by Zhongxian Zhao of the Academica Sinica group in Beijing, which first found a 94 K superconductor, China has rich deposits of rare earths, and so technological development of these compounds would be good for the Chinese economy as well as a benefit for all humanity.

Most theorists who spoke at the meetingagreed that a new mechanism was necessary to explain these high-temperature superconductors. The one holdout was D.A. Papaconstantopoulos of the Naval Research Laboratory, who insisted that slight modifications of the traditional Bardeen-Cooper-Schrieffer (BCS) theory would work.

To get superconductivity, the conductionelectrons in the material must form pairs. However, electrons normally repel each other, and some intermediary is required to induce them to pair. In the BCS theory the intermediary is a phonon, a vibration or ripple in the lattice of the crystal.

For the new superconductors, Philip W.Anderson of Princeton (N.J.) University reported a computer simulation of the crystal that suggests, rather than a phonon, a phenomenon called "resonating valence.' In that case, valence electrons, the electrons that bind the atoms together in the crystal (which are not the same as the conduction electrons that form electric currents) jump from one atom to another, causing force distortions that bind the conduction electrons in pairs. Other theorists view interactions between the bound electrons known as excitons as the intermediaries in pairing.

These new materials are fairly easy forchemists to make, and so any two or three physicists and/or chemists can get together and get into the act. As Neil W. Ashcroft of Cornell University put it, this is "truly tabletop physics,' a line that drew thunderous applause from the more than 1,100 people in the room. Many of these substances are ceramics, and technology has long experience in dealing with ceramics. In spite of their brittleness, they have already been fabricated into potentially useful forms. Mueller showed a film that is fully super-conducting, as did R. J. Cava of AT&T Bell Labs. Groups at Stanford University led by Malcolm Beasley, chairman of the Applied Physics Department there, and at Energy Conversion Devices, Inc., of Troy, Mich., led by Stanford Ovshinsky, also claim production of superconducting films. Bertram Batlogg of AT&T Bell Labs showed a ring, the first step to a solenoid, a common shape for a magnet.

In spite of talk of superconductingelectric transmission lines and other large-scale applications, observers, particularly Brian Schwartz of Brooklyn (N.Y.) College and Alex P. Malozemoff of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., think the first applications are likely to be in microcircuits, particularly in the interconnections between microcircuits, and possibly in memory elements. There, superconductivity could do a great deal for speed and miniaturization. The Stanford group claims already to have made thin film interconnects of this sort, but they refuse to tell how they did it, patent pending.

Photo: Bednorz (left) and Mueller.
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Author:Thomsen, Dietrick E.
Publication:Science News
Date:Mar 28, 1987
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