Antiferromagnetic criticality at a heavy-fermion quantum phase transition

The interpretation of the magnetic phase diagrams of strongly correlated electron systems remains controversial. In particular, the physics of quantum phase transitions, which occur at zero temperature, is still enigmatic. Heavy-fermion compounds are textbook examples of quantum criticality, as doping, or the application of pressure or a magnetic field can lead to a quantum phase transition between a magnetically ordered state and a paramagnetic regime. A central question concerns the microscopic nature of the critical quantum fluctuations. Are they antiferromagnetic or of local origin? Here we demonstrate, using inelastic neutron scattering experiments, that the quantum phase transition in the heavy-fermion system Ce 1− x La x Ru 2 Si 2 is controlled by fluctuations of the antiferromagnetic order parameter. At least for this heavy-fermion family, the Hertz–Millis–Moriya spin fluctuation approach seems to be a sound basis for describing the quantum antiferromagnetic–paramagnetic instability. The identification of the magnetic-fluctuation mode at a quantum phase transition of the archetypical heavy-fermion compound Ce 1− x La x Ru 2 Si 2 indicates that quantum criticality in this system is governed by collective antiferromagnetic behaviour, rather than by local magnetic moments as has been suggested.