Two-dimensional fluctuations at the quantum-critical point of CeCu_(6-x)Au_x

The heavy-fermion system CeCu_{6-x}Au_x exhibits a quantum critical point at x_c = 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 reveal rod-like features in the reciprocal ac plane translating to two-dimensional (2d) fluctuations in real space. We find 3d magnetic ordering peaks for x = 0.2 and 0.3 located on these rods which hence can be viewed as 2d precursors of the 3d order.