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New Insight on Superconductors

July 30, 2008

An important advance in understanding how the electrons in some materials become superconducting has been made by researchers from UC Davis, the Los Alamos National Laboratory and UC Irvine. The work, published July 31 in the journal Nature, could lead to a deeper understanding of superconductivity and to new materials that are superconducting at higher temperatures.

The team of researchers, led by Yi-feng Yang, a postdoctoral fellow at UC Davis, found a simple way to calculate the temperature at which a new state of matter, the Kondo liquid, emerges in the class of metal alloys called heavy-electron materials. At very low temperatures, these alloys can become superconductors that conduct electricity without resistance.

"We've found a framing concept for an important class of materials, which allows us to begin to understand how they relate to each other and perhaps to find new members of the group," said Yang's postdoctoral mentor and team member, David Pines, distinguished professor of physics at UC Davis and co-director of ICAM, the Institute for Complex Adaptive Matter.

Heavy electron materials are alloys of metals such as cerium, ytterbium and uranium. They contain both free-moving electrons that make them electrical conductors and a "Kondo" lattice of localized electrons. When the temperature of the material is lowered below a characteristic temperature, the localized electrons lose their magnetism as they become collectively "entangled" through quantum mechanical effects with the conduction electrons, which become heavy and form the Kondo liquid. At much lower temperatures these heavy electrons then become either magnetic or superconducting.

Yang received a fellowship from ICAM that enabled him to become "embedded" in an experimental group on heavy electron materials led by Joe D. Thompson at Los Alamos. With Thompson and Han-oh Lee at Los Alamos, and Zachary Fisk at UC Irvine, he reviewed 30 years of existing data on heavy-electron materials, plus new experimental data collected by Thompson and Lee, to establish a long-sought connection between single impurities and lattice behavior in these materials.

They found that the crucial temperature at which the Kondo liquid emerges depends in a remarkably simple way on the coupling of individual local spins to the conduction electrons, Pines said.

The discovery should help researchers find the organizing principles of heavy-electron superconductivity, because it clarifies the nature of the normal state out of which superconductivity emerges, Pines said.

The work was supported by the National Science Foundation and by the ICAM fellowship for Yang. ICAM is a multidisciplinary research program of the University of California that has 57 branches across the U.S. and globally, with its headquarters at UC Davis.

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