Doping Kondo lattice system CeRu2Si2 with Rh-8 % (Ce(Ru0.92Rh0.08)2Si2) leads to drastic consequences due to the mismatch of the lattice parameters between CeRu2Si2 and CeRh2Si2. A large variety of experiments clarifies the unusual properties of the ground state induced by the magnetic field from longitudinal antiferromagnetic (AF) mode at H = 0 to polarized paramagnetic phase in very high magnetic field. The separation between AF phase, paramagnetic phase and polarized paramagnetic phase varying with temperature, magnetic field and pressure is discussed on the basis of the experiments down to very low temperature. Similarities and differences between Rh and La substituted alloys are discussed with emphasis on the competition between transverse and longitudinal AF modes, and ferromagnetic fluctuations.

We report field-and temperature-dependent measurements of the thermoelectric power (TEP) across the pseudometamagnetic transition (MMT) in CeRu 2 Si 2 . We applied the thermoelectric gradient parallel and perpendicular to the field along the c axis of the tetragonal crystal which is the easy magnetization axis. At the MMT at H m = 7.8 T, a strong anomaly in the TEP is observed for both configurations with opposite signs. The anomaly at H m becomes a cascade of anomalies at very low temperature which seems to be a generic feature of the TEP at a change in the topology of the Fermi surface (FS) in heavy Fermion multiband systems. Simultaneously, quantum oscillations in the magnetic field dependence of the TEP are observed for both configurations below and above the MMT.During two decades, magnetic quantum phase transitions (QPT) from antiferromagnetic (AF) or ferromagnetic (FM) phases to paramagnetic (PM) ground states have been discussed mainly in the Doniach frame [1] or in the picture of the itinerant spin fluctuations [2]. These approaches neglect the possibility of a Fermi surface (FS) reconstruction at the QPT. However, in some heavy-fermion compounds a FS reconstruction has been shown directly by quantum oscillations experiments. Prominent examples are the AF compounds CeRh 2 Si 2 [3] and CeRhIn 5 [4], where the evolution of the FS has been studied as a function of pressure through their magnetic quantum critical points. The rapid change of the Hall coefficient in the heavy-fermion compound YbRh 2 Si 2 as a function of field through the critical field H c , where the antiferromagnetic order is suppressed, has been also interpreted as a signature of FS reconstruction [5,6]. These observations led to the development of unconventional models such as the breakdown of Kondo effect at the QPT [7]. The emerging picture is that a variation from a small FS to a large FS through the critical pressure P c or the critical magnetic field H c in AF systems can be observed near the magnetic quantum criticality [8].However, an unambiguous proof of a FS change and furthermore its complete determination by quantum oscillation experiments or angle-resolved-photo-emission spectroscopy is often very difficult. Thus, a confirmation requires a convergence of various macroscopic and microscopic measurements. Among them the thermoelectric power (TEP) is a very powerful probe as it is linked to the energy ( ) derivative of the electrical conductivity σ ( ) at low temperature [9]:Thus, the TEP is directly related to the derivative of the density of states N ( ). The strength of the TEP to detect * Email address: alexandre.pourret@cea.fr FS singularities has been demonstrated clearly three decades ago on simple metals notably in the study of Lifshitz transitions [10] which are topological transitions of the FS. They do not break any symmetry and appear as a crossover at finite temperature, but will be quantum phase transitions at T = 0. In multiband systems like heavy fermion compounds the TEP response is complex. The thermoelectric re...