(July 1998)

IBM Researchers reveal method for doubling superconductor Tc's

Rüschlikon/Switzerland, July 30, 1998 -- Scientists have succeeded in doubling the transition temperature at which a material becomes superconducting and loses all resistance to the transport of electrical current. In a paper published today in "Nature", a group of scientists from IBM's Zurich Research Laboratory, the Universities of Geneva and Neuchatel (Switzerland), as well as Antwerp (Belgium), reported the successful incorporation of strain into the atomic lattice of a superconducting oxide film, thereby raising the transition temperature of the oxide material from 25 to 49 Kelvin (0 Kelvin = -273 degree Celsius). Besides having practical significance and potential for new record transition temperatures, this finding also highlights the role played by atomic lattice parameters in the mechanism of superconductivity.

The key to the new method of obtaining a significantly higher transition temperature is to grow the oxide in the form of a thin epitaxial film on a substrate having a smaller interatomic spacing than the bulk oxide. For sufficiently thin films, this technique yields a perfectly crystalline material in which the interatomic distances are forced to exactly match those of the substrate. The net effect is a compression of the atomic lattice in the plane of the film, with a simultaneous expansion in the vertical or growth direction.

"For the materials we have investigated, we find that this arrangement has a much stronger impact on the transition temperature than an externally applied pressure which partly acts in unfavorable directions and prevents the effect to reach its full magnitude", says Jean-Pierre Locquet who leads the project at IBM's Zurich Research Laboratory.

The experimental results agree reasonably well with a simple theoretical model which also predicts a doubling of the transition temperature for some Bismuth compounds. This could lead to thin films which become superconducting at temperatures approaching 200 Kelvin (-73 degrees Celsius). Current record transition temperature is 133 Kelvin, found by a team of the Swiss Federal Institute of Technology in a mercury compound in 1993.

Oxides that become superconducting at high temperatures were first discovered at IBM's Zurich Research Laboratory by J. Georg Bednorz and K. Alex Mueller, a breakthrough for which they received the 1987 Nobel Prize in Physics. Oxide compounds and techniques for their high quality growth have potential for a variety of electronic and power current applications.

The authors of the scientific report published in Nature (Vol.394, no. 6692, 30 July 1998) are Jean-Pierre Locquet and Erich Mächler of IBM's Zurich Research Laboratory, Joel Perret and Jin Won Maria Seo of the University of Neuchatel, Jean Fompeyrine of the University of Geneva, and Gustaaf Van Tendeloo of the University of Antwerp. The project is partially supported by the Swiss National Research Program.

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