  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 aretextbook 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 Ce1-xLaxRu2Si2 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.
