Two-Dimensional Fluctuations at the Quantum-Critical Point ofCeCu6−xAux

The heavy-fermion system ${\mathrm{CeCu}}_{6-x}{\mathrm{Au}}_x$ exhibits a quantum-critical point at $x_c\approx 0.1$ separating nonmagnetic and magnetically ordered ground states. The pronounced non-Fermi-liquid behavior at $x_c$ calls for a search for the relevant quantum critical fluctuations. Systematic measurements of the inelastic neutron scattering cross section $S(q,\omega )$ for $x=0.1\mathrm{}$ reveal rodlike features in the reciprocal $\mathrm{ac}$ plane translating to two-dimensional (2D) fluctuations in real space. We find 3D magnetic ordering peaks for $x=0.2\mathrm{}$ and $0.3$ located on these rods which hence can be viewed as 2D precursors of 3D order.