Superconducting metals research at University of Toronto breaks the law

December 12, 2001

Scientists who thought they were one step closer to solving the mysteries of superconducting metals may find themselves back at the theoretical drawing board thanks to new research by University of Toronto physicists.

"In our field of solid state physics, the big issue is whether the basic theory that governs how electrons behave in metals holds true with new materials like copper oxide-based high-temperature superconductors," says Robert Hill, a post-doctoral fellow in U of T's physics department and lead author on a paper appearing in the Dec. 13 issue of Nature. "Our research is the first hard evidence to show that it does not hold true."

Scientific understanding of metals has till now been governed by the Wiedemann-Franz law, one of the oldest and most well-established laws in solid state physics, Hill explains. Universally acknowledged as applicable to all metals, the law deals with two basic properties - the conduction of charge and heat by electrons. Fermi-liquid theory, which describes how electrons behave in matter, underpins the science behind the electronics and computer industry.

"Simply put, the rule generally was that a good conductor of heat was also a good conductor of electricity," says physics professor Louis Taillefer, co-author of the paper and Hill's supervisor. "But in these new copper oxide materials, the way the charge is conducted is completely unrelated to the way the heat is conducted, and that has a very large impact on how we understand these materials. We've basically broken a very fundamental law and we'll have to go back to the drawing board and think again about how electrons in this particular material are behaving."

To test the theory, the researchers - who conducted the work with Cyril Proust, a former post-doctoral fellow in Taillefer's research group, and Patrick Fournier and Rick Greene of the University of Maryland - destroyed the copper oxide's superconductivity to find out how it reacts at temperatures just above absolute zero (-273 C). They found that the material's charge and heat acted independently of each other, completely violating the Wiedemann-Franz law and Fermi-liquid theory.

This is something that no material in nature has been known to do. In the vast majority of materials measured thus far, there has been a very well-defined relationship between the heat and charge conductivity of an electron because both are carried by the same entity. In new materials like copper oxide, the researchers believe that the electrons split into different entities - one with a charge and one without.

The area of superconductors is a particularly hot area of research right now. At present, superconductors are mainly used in magnets, such as those in MRIs. The problem is that to ensure superconducting materials work efficiently, they must be cooled substantially (to around -150 C). The hope and goal of scientists around the world is to create a material that can be superconducting at room temperature.

"A real world example is electricity," says Hill. "Superconducting materials are now used in power lines to carry electricity. But their inability to work efficiently in high temperatures means that by the time electricity is carried from the power station to your house, 30 per cent is lost. If we had a material that was superconducting at high temperatures, these power lines could operate at 100 per cent efficiency."

However, this kind of scenario is still years away, caution Hill and Taillefer, who are also associated with the Canadian Institute for Advanced Research. "Once we gain a better understanding of how materials like these copper oxides work, it could open a window on a whole class of materials that have completely different sets of laws and properties," Taillefer says.
Their research was funded by the Natural Sciences and Engineering Research Council of Canada and supported by the Canadian Institute for Advanced Research, the Premier's Research Excellence Award and the NSF Division of Condensed Matter Physics in Maryland.


Janet Wong
U of T Public Affairs

Robert Hill
Department of Physics

Louis Taillefer
Department of Physics
819-821-8000, ext. 2051

University of Toronto

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