The existence of a new electronic state in superconductors, materials that can carry an electric current without resistance, has been confirmed experimentally according to research to be published in the 12 November 2004 issue of the journal Science by a team led by Ying Liu, associate professor of physics at Penn State. "We have established direct evidence for the existence of an odd-parity superconductor, which previously had been theorized but never demonstrated in an unambiguous experiment," says Liu. The results culminate six years of experimentation by Liu in collaboration with a former graduate student, Karl Nelson, and a former postdoctoral associate, Zhiqiang Mao at Penn State; and Yoshiteru Maeno, a professor of physics at Kyoto University.
Picture of a strontium ruthenate Sr2RuO4 SQUID used to demonstrate that strontium ruthenate is an odd-parity superconductor. A thin layer of a conventional superconductor is deposited on the front and two sides of a black Sr2RuO4 crystal. Au wires of 25 microns in diameter are attached to the SQUID for current measurements.
Crystal structure of strontium ruthenate Sr2RuO4: layers of RuO6 octahedrons separated by SrO2 layers.
In addition to their scientific interest, superconductors have a number of practical applications. These include superconducting magnets, which have enabled the development of high-resolution magnetic-resonance imaging in medicine, and superconducting wires, which transport electrical power without loss due to heating of the cable by electrical resistance.
A material becomes superconducting because electrons in the material form pairs, known as Cooper pairs. Liu likens the pairing process to dancers on a dance floor: "The electrons, crowded together, form pairs and move to the music of phase coherence, a quantummechanical property that synchronizes the steps of all the dancing pairs." These pairs, described mathematically by a quantummechanical wave function, move tightly together despite tendencies that would force them apart.
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