EXCITATIONS IN HEAVY-FERMION SYSTEMS

Thermodynamic, transport and neutron scattering experiments are report ed on Ce1-xLaxRu2Si2 and CeNiSn heavy fermion compounds. The situation of the first system, where long range antiferromagnetism collapses fo r x < 0.08, is compared with recent theories based on antiferromagneti c spin fluctuations and on quantum criticality. The possibility of a t iny 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 p ure CeRu2Si2 lattice is also discussed. Data on CeNiSn confirm a parti al gap opening at low temperature and stress the necessity of theoreti cal models for the description of the complexity of the measured dynam ical susceptibility.