Neutron Probe Yields Break In Superconductor Mystery

The advance is the result of investigative work done at the National Institute of Standards and Technology's Center for Neutron Research (NCNR), and at the National High Magnetic Field Laboratory (NHMFL) at Florida State University (FSU).

Stray magnetic fields suppress superconductivity, the resistance-free passage of electric current. But the object of the team's scrutiny--a uranium-ruthenium-silicon compound (URu2Si2)--somehow accommodates the normal adversity between magnetism and superconductivity. At 17.5 degrees above absolute zero, once-nomadic electrons that had roamed freely about the compound's lattice-like atomic structure--and generated their own magnetic fields--behave in a more orderly and cooperative fashion. This coherence sets the stage for superconductivity.

URu2Si2 belongs to a class of materials called heavy fermions, known to be reluctant superconductors. This is because current-carrying electrons in the intermetallic material interact with surrounding particles and truly gain from the experience. The association adds mass--making the electrons behave as though they were a few hundred times more massive than "normal." The heavy electrons once were thought to make superconductivity impossible.

However, numerous heavy fermion superconductors now are known, and URu2Si2 ranks among the most curious of the lot.

Unexplained was how a "hidden order" suddenly arose in the wake of the magnetic instabilities caused by the roving electrons, each one spinning and producing its own miniature magnetic field. With neutron probes, researchers managed to track electron movements and determined that the wandering particles work out an unexpected accommodation in the spacing of their energy levels.

This research is supported by the National Science Foundation, NIST, the Natural Sciences and Engineering Research Council of Canada, the State of Florida and the Canadian Institute for Advanced Research.

*C.R. Wiebe, J.A. Janik, G.J. MacDougall, G.M.Luke, J.D. Garrett, H.D. Zhou, Y.J. Jo, L. Balicas, Y. Qiu, J.R.D. Copley, Z. Yamani and W.J.L. Buyers, Gapped itinerant spin excitations account for missing entropy in the hidden order state of URu2Si2, Nature Physics, Feb. 2007.