Magnetism piece fits no-resistance puzzle

Peter Weiss

Physicists hotly debate why certain copper oxide crystals can conduct electricity without resistance, or superconduct, at temperatures far higher than conventional superconductors can.

Now, a German-French-Russian team led by Bernhard Keimer of the Max Planck Institute for Solid State Research in Stuttgart, Germany, reports crucial evidence that a magnetic feature of the copper oxide plays a role.

Many atoms have a magnetic property called spin, which makes them behave as tiny bar magnets. Scientists noticed in experiments even 10 years ago that at a temperature just below the superconductivity threshold, the spins of many atoms in some copper oxide compounds fluctuated in a coordinated manner.

Many scientists dismissed those magnetic behaviors as irrelevant to the so-called high-temperature superconductors' loss of resistance. That's because the traits hadn't shown up in the first of those compounds, a lanthanum-containing material discovered in 1986 (SN: 11/18/00, p. 330: http://www.sciencenews.org/articles/20001118/bob9.asp). That material harbored only single layers of copper oxide, whereas the newer materials that show the spin phenomenon have double layers.

The lanthanum material was the only single-oxide-layer superconductor that could be grown large enough to test for the spin phenomenon. To get around that obstacle, Keimer and his colleagues spent months painstakingly hand-aligning small crystals of thallium barium copper oxide—a single-oxide-layer compound—and gluing them onto aluminum plates. The result was a system that models a single-oxide-layer crystal big enough for magnetic analysis using a neutron beam. The team describes its work in the Feb. 8 Science.

Just below the thallium compound's threshold superconducting temperature of 90 kelvins, the scientists detected the telltale sign of the fluctuating magnetic pattern. Now "it's pretty much been proven that the [spin coordination] is present for all high-temperature superconductors," comments Andrey V. Chubukov of the University of Wisconsin-Madison.

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References:

He, H., et al. 2002. Magnetic resonant mode in the single-layer high-temperature superconductor Tl₂Ba₂CuO_6+δ. Science 295(Feb. 8):1045–1047. Abstract available at http://dx.doi.org/10.1126/science.1067877.

Further Readings:

Weiss, P. 2000. Little big wire. Science News 158(Nov. 18):330–332. Available at http://www.sciencenews.org/articles/20001118/bob9.asp.

Sources:

Andrey V. Chubukov
University of Wisconsin-Madison
Physics Department
1150 University Avenue
Madison, WI 53706-1390

H. He
Max Planck Institute for Solid State Research
Heisenbergstrasse 1
70569 Stuttgart
Germany

Bernhard Keimer
Max Planck Institute for Solid State Research
Heisenbergstrasse 1
70569 Stuttgart
Germany

From Science News, Volume 161, No. 11, March 16, 2002, p. 173.