Magnetic and Structural Quantum Phase Transitions in CeCu6−xAux are Independent

The heavy-fermion compound ${\mathrm{CeCu}}_{6-x}{\mathrm{Au}}_x$ has become a model system for unconventional magnetic quantum criticality. For small Au concentrations $0\le x<0.16$, the compound undergoes a structural transition from orthorhombic to monoclinic crystal symmetry at a temperature $T_s$ with $T_s\to 0$ for $x\approx 0.15$. Antiferromagnetic order sets in close to $x\approx 0.1$. To shed light on the interplay between quantum-critical magnetic and structural fluctuations we performed neutron-scattering and thermodynamic measurements on samples with $0\le x\le 0.3$. The resulting phase diagram shows that the antiferromagnetic and monoclinic phase coexist in a tiny Au concentration range between $x\approx 0.1$ and 0.15. The application of hydrostatic and chemical pressure allows us to clearly separate the transitions from each other and to explore a possible effect of the structural transition on the magnetic quantum-critical behavior. Our measurements demonstrate that at low temperatures the unconventional quantum criticality exclusively arises from magnetic fluctuations and is not affected by the monoclinic distortion.