College Park, Maryland      June 6 - 10 , 2004

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TP57: A Quantum Multicritical point in CeCu_6-xAu_x

R. A. Robinson (Bragg Institute, Australian Nuclear Science and Technology Organisation, Australia), D. Goossens (Bragg Institute, Australian Nuclear Science and Technology Organisation, Australia; Research School of Chemistry, Australian National University, Canberra, ACT 0200, AUSTRALIA), M. S. Torikachvili (Department of Physics, San Diego State University, San Diego, CA 92182, USA), K. Kakurai (Advanced Science Research Center, JAERI, Tokai-mura, Ibaraki-ken 319-1195, JAPAN)

CeCu_6-xAu_x is a well-known heavy-fermion system in which the ground state is antiferromagnetically ordered for x > 0.1 and temperatures below 1K. Non-Fermi-liquid behaviour occurs around this critical concentration. The parent compound, CeCu₆, exhibits a structural phase transition near 230K, where it changes from the Pnma orthorhombic room-temperature structure to the P21/c monoclinic structure. The monoclinicity increases as temperature falls, with β reaching 91.44° at 10K. In the work presented here, powder neutron diffraction is used to explore the monoclinicity at 10K as a function of composition for 0.0 < x < 0.08, just short of the critical concentration. Extrapolation of the square of the monoclinic strain, (abcos(β))², suggests that the distortion vanishes by x = 0.14. A reanalysis of single-crystal diffraction data on the magnetically ordered side of the phase diagram indicates that long-range magnetic order disappears at the same critical concentration. At a minimum, the structural distortion and antiferromagnetism seem to be competing with each other, and this raises the intriguing possibility that lattice degrees of freedom are important in the non-Fermi-liquid regime.

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