${}^{63,65}\mathrm{Cu}$ NQR-NMR relaxation measurements in ${\mathrm{CeCu}}_{6-x}{\mathrm{Au}}_x$ for $x=0,$ $x=0.1,$ and $x=0.8\mathrm{}$ are used to derive insights on temperature (T) and magnetic-field (H) dependences of the spin dynamics around the quantum critical point (QCP). The relaxation rate $2W$ is related to the $\mathbf{k}$ integrated, low energy, generalized susceptibility $\chi (\mathbf{k},\omega ,H,\overrightarrow{T}0).$ For $x=0\mathrm{}$ a Fermi-liquid behavior is detected, while for $x=0.8\mathrm{}$ the temperature dependence of $2W$ is the one expected for a nearly antiferromagnetic metal ordering at $T_N=2.2\mathrm{}\mathrm{K}.$ Instead, for $x=0.1,$ around the QCP, a response function of the form suggested by neutron scattering, namely of two-dimensional character with anomalous exponent and $(\omega /T)$ scaling, is found to explain the main experimental findings. An effect is observed in the low-temperature range for $H\gtrsim 1\mathrm{T},$ with a crossover to a gapped phase for the spin excitations at a field-dependent temperature.

• Received 16 September 2003

DOI:https://doi.org/10.1103/PhysRevB.68.220404