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A new study suggests that one kind of high-temperature superconductor violates a 150-year-old law of solid state physics. The Wiedemann-Franz law predicts that at low temperatures, metals should conduct both heat and electricity well. But now physicists have found that the heat and charge in a copper oxide-based superconductor each follow different paths near absolute zero. Continuing down this avenue of research could lead to a fuller understanding of superconducting materials and how to find better ones. A report describing these results appears today in the journal Nature.
The Wiedemann-Franz law is a consequence of the hugely successful Fermi liquid theory, which holds that in large groups, electrons can be thought of as particles with a higher mass but the same charge and spin as usual. Yet physicists have seen hints that copper-oxide-based superconductors violate the Fermi liquid theory when in their non-superconducting state. In explanation, some scientists have proposed that groups of electrons sometimes behave as if each one consisted of a pair of entities that transmit charge and spin separately. Because both entities would carry heat, a material full of such electrons would not have the same tight correlation of heat and charge. To test this possibility, the study authors erased the superconductivity of the copper oxide compound PCCO and observed its heat and charge transmission below one degree Kelvin. "Our measurement is perhaps the most direct evidence of this spin-charge separation," co-author Louis Taillefer of the University of Toronto explains. "It's exciting because it reveals an instance where some new physics is going on."
Taillefer says the next step will be to examine the behavior of PCCO at a wide range of electron concentrations to see where the Fermi liquid theory holds and folds, in an attempt to develop a better theory of the behavior of electrons in these compounds. Such an understanding could ultimately make it easier to find materials that make the transition to superconductivity at even higher temperatures.
