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Abstract:
Cu-63,Cu-65 NQR-NMR relaxation measurements in CeCu6-xAux for x=0,
x=0.1, and x=0.8 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 k
integrated, low energy, generalized susceptibility
chi(k,omega,H,T-->0). For x=0 a Fermi-liquid behavior is detected,
while for x=0.8 the temperature dependence of 2W is the one expected
for a nearly antiferromagnetic metal ordering at T-N=2.2 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 Hgreater than or similar to1 T, with a crossover to a gapped
phase for the spin excitations at a field-dependent temperature.
