A type of superconductor seems to be breaking a fundamental theory of physics, according to a study in the 13 December issue of Nature. Researchers found that the substance conducts heat and electrical charges in a way that defies all theory. If correct, the results will force physicists to reconsider what they thought they knew about a range of materials, including the special kind of superconductor used in the experiment.
For almost half a century, physicists have described conducting materials with the so-called Fermi-liquid theory of electrons. The theory allows researchers to model how electrons flow through a material by considering it a liquid of electronlike particles. These imaginary particles have the characteristics of true electrons, but they aren't repulsed by one another or attracted to positively charged nuclei.
That lack of interaction vastly simplifies calculations and has allowed theorists to create Fermi-liquid models of complex materials, which lays the groundwork for building everything from better computer chips to metal insulators. "The Fermi-liquid model is one of the cornerstones of condensed matter physics," says Louis Taillefer of the Canadian Institute for Advanced Research (CIAR) in Toronto. "Essentially all materials can be explained with this theory."
But scientists have struggled for more than a decade to apply the principles of this theory to a special kind of superconductor. Now, researchers at the CIAR and the Center for Superconductivity Research in College Park, Maryland, have found the first solid evidence that this superconductor, called a "cuprate," does not obey the Fermi-liquid model. The group studied how a cuprate conducts heat and electrical charge. Because the imaginary particles of Fermi-liquid theory carry heat and charge in equal amounts, the researchers expected to see equal rates of conductance from both. Instead, they measured significantly more heat than charge flowing through the material at certain temperatures.
"This is a really striking and fundamental result," says Steve Kivelson of the University of California, Los Angeles. Though researchers suspected that the Fermi-liquid model breaks down in cuprates, these results provide the first hints about how the model falters. It should help theorists to entertain new ways of describing certain materials.