Excitations in heavy fermion systems

Thermodynamic, transport and neutron scattering experiments are reported on Ce1-xLaxRu2Si2 and CeNiSn heavy fermion compounds. The situation of the first system, where long range antiferromagnetism collapses for x < 0.08, is compared with recent theories based on antiferromagnetic spin fluctuations and on quantum criticality. The possibility of a tiny ordered moment in the assumed paramagnetic ground state x = 0.075 raises again the question on the real nature of the ground state. The description of the 4f electron at the metamagnetic transition of the pure CeRu2Si2 lattice is also discussed. Data on CeNiSn confirm a partial gap opening at low temperature and stress the necessity of theoretical models for the description of the complexity of the measured dynamical susceptibility.