A quantum multi-critical point in CeCu6−xAux

Abstract

CeCu6−xAux is a well-known heavy-fermion system in which the ground state is antiferromagnetically ordered for x>0.1 and temperatures below 1 K. Non-Fermi liquid behaviour occurs around this critical concentration. The parent compound, CeCu6, exhibits a structural phase transition near 230 K, 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 10 K. In the work presented here, powder neutron diffraction is used to explore the monoclinicity at 8 K 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, (ac cosβ)2, suggests that the distortion vanishes at x=0.13. A reanalysis of single-crystal diffraction data on the magnetically ordered side of the phase diagram, x>0.1, indicates that long-range order disappears at exactly 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.