The heavy-fermion system CeCu6−xAux exhibits a quantum critical point at xc ≈ 0.1 separating nonmagnetic and magnetically ordered ground states. The pronounced non-Fermi-liquid behavior at xc calls for a search for the relevant quantum critical fluctuations. Systematic measurements of the inelastic neutron scattering cross section S(q, ω) 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. 75.30.Mb, 71.27.+a, 75.20.Hr Continuous quantum phase transitions which occur in a strict sense only at temperature T = 0 are driven by quantum fluctuations instead of thermal fluctuations as for ordinary classical phase transitions [1,2]. This leads to unusual and rich behavior even at finite temperatures in the neighborhood of the critical point. Because of the uncertainty principle the energy scale of fluctuations introduces a time scale which leads to an intricate coupling of static and dynamic critical behavior. For instance, the critical behavior of the specific heat will depend on the dynamical critical exponent z relating the typical lifetime ξ τ and correlation length ξ of critical fluctuations, ξ τ ∝ ξ z . Such a quantum phase transition can be achieved by changing a coupling parameter which plays a role analogous to temperature in ordinary phase transitions. In recent years, many physical realizations of quantum phase transitions have been found. The case of a magnetic-nonmagnetic transition in heavy-fermion metals is particularly interesting because of the involvement of itinerant electrons. Excitations of a system of interacting itinerant electrons in a metal, i.e. quasiparticles, are usually described within the Fermi-liquid theory, with the specific heat C ∝ T , a Pauli susceptibility independent of T , and an electrical resistivity contribution ∆ρ ∝ T 2 due to quasiparticle-quasiparticle scattering. Interactions renormalize the quasiparticle masses with respect to the free-electron mass m 0 . Even in heavy-fermion systems with quasiparticle masses as high as several 100 m 0 , Fermi-liquid behavior is the rule rather than the exception [3]. In heavy-fermion systems, the coupling parameter tuning the magnetic-nonmagnetic transition is the (antiferromagnetic) exchange interaction J between 4f or 5f magnetic moments and conduction electrons [3]. If it is strong, a local singlet state is formed via the Kondo effect around each 4f or 5f site, leading to a nonmagnetic ground state. On the other hand, a weak (but non-zero) exchange interaction leads to a Rudermann-Kittel-Kasuya-Yosida coupling between moments and hence to magnetic order. In the exemplary system CeCu 6−x Au x doping of CeCu 6 with the larger Au atom leads -via lattice expansion -to a weakening of the Kondo effect and hence to long-range antiferromagnetic order for x > x c ≈ 0.1, with a linear increase of the 1 [4].At x c where T N vanishes, i...

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