Comparison between Ce and Yb heavy fermion compounds: CeRh₂ Si₂ versus YbRh₂ Si₂

The similarities and differences of the pressure and magnetic field phase diagrams of the heavy fermion systems CeRh₂ Si₂ and YbRh₂ Si₂ are reviewed. In CeRh₂ Si₂ the transition from antiferromagnetic order to a paramagnetic state seems to be first order with a clear volume discontinuity. Drastic differences appear not only in the pressure dependence of their Néel temperature but also in their magnetic field response as a comparison of the CeRu₂ Si₂ case with YbRh₂ Si₂ shows which reflects the position of their quantum critical field with respect to the magnetic field Kondo scale. However, a new generation of experiments in extreme conditions is required: very high magnetic field for CeRh₂ Si₂, very low temperature for YbRh₂ Si₂.