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Science 1 June 2007:
Vol. 316. no. 5829, pp. 1320 - 1322
DOI: 10.1126/science.1140762
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Reports

Anisotropic Violation of the Wiedemann-Franz Law at a Quantum Critical Point

Makariy A. Tanatar,1,2*{dagger} Johnpierre Paglione,2,3* Cedomir Petrovic,4 Louis Taillefer1,5{ddagger}

A quantum critical point transforms the behavior of electrons so strongly that new phases of matter can emerge. The interactions at play are known to fall outside the scope of the standard model of metals, but a fundamental question remains: Is the basic concept of a quasiparticle—a fermion with renormalized mass—still valid in such systems? The Wiedemann-Franz law, which states that the ratio of heat and charge conductivities in a metal is a universal constant in the limit of zero temperature, is a robust consequence of Fermi-Dirac statistics. We report a violation of this law in the heavy-fermion metal CeCoIn5 when tuned to its quantum critical point, depending on the direction of electron motion relative to the crystal lattice, which points to an anisotropic destruction of the Fermi surface.

1 Département de Physique et RQMP, Université de Sherbrooke, Sherbrooke, Canada.
2 Department of Physics, University of Toronto, Toronto, Canada.
3 Department of Physics, University of California, San Diego, CA 92093, USA.
4 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
5 Canadian Institute for Advanced Research, Toronto, Canada.

* These authors contributed equally to this work.

{dagger} Present address: Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kyiv, Ukraine.

{ddagger} To whom correspondence should be addressed. E-mail: louis.taillefer{at}physique.usherbrooke.ca

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