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03/21/95

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Heat test sheds new light on nature of novel superconductors

STANFORD -- Exceedingly delicate measurements of the rate at which high temperature superconductors absorb heat are providing new evidence that the ability of these materials to conduct electric current without resistance has a fundamentally different basis from that of conventional superconductors.

The heat capacity measurements, performed by Aharon Kapitulnik, professor of applied physics, and graduate student Kathryn Moler, strongly indicate that the force that binds electrons into pairs -- a coupling that makes superconductivity possible -- is strong in some directions but drops to zero in others.

Their observations are being presented by Moler in an invited lecture on March 21 at the annual meeting of the American Physical Society in San Jose. The results are consistent with, and significantly strengthen, the evidence that the electron-pair binding in high temperature superconductors takes a form that scientists call "d-wave." As a result, their work has received considerable interest within the scientific community since it was published last November in Physical Review Letters.

The electron-pair binding force in conventional superconductors -- which have been known since 1911 but which operate only at temperatures lower than minus 200 degrees Celsius -- is the same in all directions. Scientists call this an "s-wave" configuration. But many scientists believe that the lattice vibrations that link electrons at these low temperatures are too weak to explain superconductivity at higher temperatures. Since 1987, when materials that superconduct at temperatures as high as minus 150 degrees Celsius were discovered, condensed matter physicists have been trying to figure out what makes them work.

A number of different hypotheses have been advanced, but, until recently, there was no experimental evidence that allowed scientists to differentiate among them. In the last year or so, however, a series of experiments have begun to provide a growing, although not yet conclusive, body of support for the d-wave proposal first advanced by Douglas Scalapino, professor of physics at the University of California-Santa Barbara.

One of the early experiments that strongly suggested the accuracy of the d-wave model was performed by a research group headed by Zhi-Xun Shen, associate professor of applied physics at Stanford. In the experiment, the scientists used a method called photoemission spectroscopy to gain direct information about the status of the electrons in the superconductor by knocking them out of the material with a photon beam.

Next came an experiment performed by researchers at the University of British Columbia. They exposed the samples of high temperature superconductor to radio waves and measured how the outer layers of the material responded. One of the major achievements of the UBC group was the extreme purity of their material. Their research inspired the Stanford heat capacity measurements, which were done using UBC samples.

"Actually, heat capacity is one of the first ways that you would like to test these materials because it is sensitive to all of the low-level excitations associated with superconductivity," Kapitulnik said.

The two Stanford researchers took a tiny sample of one of the cuprate superconductors, commonly known as YBCO, weighing a few thousandths of a gram. They attached this to a layer of sapphire that was bonded to a large block of copper that was maintained at a constant temperature. Then they repeatedly heated up the superconducting sample by two hundredths of a degree Celsius and measured how long it took to cool down when exposed to an external magnetic field of varying strengths and orientations.

"We found two effects. One, that we expected, is present regardless of the orientation of the field relative to the direction of the layers of copper oxide that carry the superconducting current. But the second effect was new and appeared only when the magnetic field is perpendicular to the layers," Moler said.

This new measurement was larger than predicted by the s-wave model. If the electron-pair binding force is positive in all directions, as it is in the s-wave model, then it should not have this effect on heat capacity. To produce these variations, the binding force must go to zero in some directions, the Stanford researchers argued. D-wave theory is not the only model that fits their observations, but it is the one that not only fits but also has the most experimental support, the scientists said.

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