Breaking the Law

Abstract

Physics Conduction of heat in solids is often achieved through the same means as electrical conduction—via charge carriers—and materials that are good electrical conductors (such as metals) also tend to conduct heat efficiently. For metals, the ratio between the thermal and electrical conductivities is determined by the Wiedemann-Franz (WF) law. However, in one-dimensional systems, a distinct state known as the Tomonaga-Luttinger (TL) liquid is predicted to occur, wherein the spin and charge degrees of freedom of an electron separate into spinons and holons; because heat is conducted by both but electricity only by holons, a violation of the WF law may occur. Wakeham et al. report such a violation in the purple bronze Li0.9Mo6O17, which is a conductor consisting of weakly interacting one-dimensional chains. The violation they find is as large as a factor of 100,000 at the lowest temperatures accessed and consistent with the scenario of spin-charge separation in a TL liquid, even though the material is only approximately one-dimensional. If, as the authors speculate, its TL liquid nature is a result of strong correlations within the chains, it may be possible to tune the dimensionality of the system, and thus effectively recombine spinons and holons, by appropriate chemical substitution. Nat. Commun. 2 , 10.1038/ncomms1406 (2011).