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Scientists Investigate the Mechanism Behind High-Temperature Superconductivity

Upton, NY - Using crystal samples prepared at the U.S. Department of Energy's Brookhaven National Laboratory, scientists from McMaster University in Ontario, Canada, have ruled out two proposed theories for the subatomic mechanisms of superconductivity, a phenomenon in which the electrical resistance of certain materials drops to zero. The results of the research appeared in the February 19, 2004 issue of Nature.

Scientists first observed superconductivity in materials chilled to certain "transition" temperatures very close to "absolute zero," or zero degrees Kelvin (K), the coldest temperature possible, which is equal to minus 452 degrees Fahrenheit (F). But more recent research has uncovered a class of so-called "high-temperature" superconductors that perform at temperatures as "warm" as 138 K. While still extremely cold by conventional standards, these higher temperatures are easier to achieve, making high-temperature superconductors more feasible for practical applications.

Within both low- and high-temperature superconductors, the electrons behave oddly: Instead of repelling one another, as like-charged objects should, they "stick" together in pairs, pulling each other along in a stream of unhindered current. While scientists know what causes this behavior in low-temperature superconductors, they do not have a clear picture of what happens in the high-temperature case.

There are several theories: One theory suggests that the electrons pair up by exchanging little "packets" of vibrational energy, called phonons. Another theory suggests that the electrons are held together as the result of a "magnetic resonance" effect, which occurs when a moving electron produces a tiny magnetic field that attracts a nearby electron.

In this study, the researchers ruled out both of these electron-pairing mechanisms, suggesting that another interaction must be responsible. Their test sample was a high-temperature superconducting compound containing the elements bismuth, strontium, calcium, copper, and oxygen, known as Bi-2212.

"In order to complete the project, we first needed to prepare a number of high-quality Bi-2212 superconductor crystals containing different amounts of oxygen, which caused the crystals to have different superconductor temperatures," said Brookhaven Lab physicist Genda Gu. "Using the state-of-the-art crystal growth facility in the Lab's Physics Department, I successfully prepared single crystals with very high levels of extra oxygen."

Gu prepared the Bi-2212 crystal with a special furnace that uses extreme heat to cause additional material to "grow" on a very small seed crystal, producing a large single crystal. Then, to increase the amount of oxygen in the crystal, he placed it into a device called a high-pressure cell, where it was surrounded by oxygen and subjected to the equivalent of nearly 22 tons per square inch of pressure. The high-pressure cell was heated to 823 K, or about 1,020 F, forcing the additional oxygen into the crystal.

The crystals were then handed off to McMaster University scientists Jungseek Hwang and Tom Timusk for analysis. The scientists took measurements from the crystals using infrared spectroscopy, a method in which bright rays of infrared light are shone at a sample to determine how it interacts with the light at the molecular level. Hwang and Timusk analyzed the measurements using two mathematical methods, and found that neither phonon exchange nor magnetic resonance could be responsible for electron pairing.

2004-150  |  Media & Communications Office

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