Quantum criticality and Lifshitz transition in the Ising system CeRu$_2$Si$_2$ : comparison with YbRh$_2$Si$_2$
Résumé
New thermoelectric power (TEP) measurements on prototype heavy-fermion compounds close to magnetic quantum criticality are presented. The highly sensitive technique of TEP is an unique tool to reveal Fermi surface instabilities, referred here as Lifshitz transitions. The first focus is on the Ising CeRu$_2$Si$_2$ series. Doping CeRu$_2$Si$_2$ with Rh produces a decoupling between the first order metamagnetic transition and the pseudo-metamagnetism observed in the pure compound. Comparison is made with the case of YbRh$_2$Si$_2$ which is often considered as the archetype of local quantum criticality by contrast to CeRu$_2$Si$_2$, taken as an example of spin-density wave criticality. Up to now for ferromagnetic materials showing ferromagnetic wings, no simple case appears where the Fermi surface is preserved between the ferromagnetic and paramagnetic phases. An open issue is the consequence of Lifshitz transitions on superconductivity in these multiband systems.