Quantum Criticality and Scaling of the Magnetic Response in CeCuAu.

The heavy-fermion system CeCuAu which exhibits antiferromagnetic order for Au concentrations x>0.1, can be easily tuned to a quantum phase transition, either by chemical composition, i.e., Au concentration, or by an external magnetic field. We present extensive inelastic neutron scattering measurements in order to study the momentum and energy dependence of the critical spin fluctuations for concentration- and magnetic field-tuned CeCuAu. The quasi-2D fluctuations, observed for x=0.1 at the magnetic instability, are already present in pure CeCu and persist to x=0.2, i.e., well into the magnetically ordered regime. This is a strong hint that disorder does not play a major role in this system. The magnetic response for quantum critical CeCuAu exhibits critical slowing down and obeys scaling relations that depend on the tuning parameter. While the E/ T scaling of the dynamic susceptibility in concentration-tuned CeCuAu is an indication for local quantum criticality, the E/ T scaling in field-tuned CeCuAu points to a conventional spin-density-wave instability. [ABSTRACT FROM AUTHOR]