[en] The Meccano of heavy fermion systems is shown with special focus on the ideal case of the CeRu₂Si₂ series. The nice tools given by the temperature, the pressure, and the magnetic field allow to explore different ground states. Experimentally, the Grueneisen parameter, i.e. the ratio of the thermal expansion by the specific heat is a colored magic number; its temperature, pressure, and magnetic field dependence is a deep disclosure of competing hierarchies and the conversion of this adaptive matter to external responses

[en] The pressure-temperature phase diagram of CeRhIn5 has been studied under high magnetic field by resistivity measurements. Clear signatures of a quantum critical point has been found at a critical pressure of p_c ≅ 2.5 GPa. The field induced magnetic state in the superconducting state is stable up to the highest measured field. At p_c the anti ferromagnetic ground-state under high magnetic field collapses very rapidly. Clear signatures of p_c are the strong enhancement of the resistivity in the normal state and of the inelastic scattering term. No clear T² temperature dependence could be found for pressures above T_c. Front the analysis of the upper critical field within a strong coupling model we present the pressure dependence of the coupling parameter λ and the gyromagnetic ratio g. No signatures of a spatially modulated order parameter could be evidenced. A detailed comparison with the magnetic field-temperature phase diagram of CeCoIn5 is given. The comparison between CeRhIn5 and CeCoIn5 points out the importance to take into account the field dependence of the effective mass in the calculation of the superconducting upper critical field H_c2. It suggests also that when the magnetic critical field H_M(O) becomes lower than H_c2(0) the persistence of a superconducting pseudo-gap may stick the antiferromagnetism to H_c2(0). (authors)

[en] Recently, coexistence of ferromagnetism (T_Curie = 2.8 K) and superconductivity (T_sc = 0.8 K) has been reported in UCoGe, a compound close to a ferromagnetic instability at ambient pressure P. Here we present resistivity measurements under pressure on a UCoGe polycrystal. The phase diagram obtained from resistivity measurements on a polycrystalline sample is found to be qualitatively different to those of all other ferromagnetic superconductors. By applying high pressure, ferromagnetism is suppressed at a rate of 1.4 K/GPa. No indication of ferromagnetic order has been observed above P approximate to 1 GPa. The resistive superconducting transition is, however, quite stable in temperature and persists up to the highest measured pressure of about 2.4 GPa. Superconductivity would therefore appear also in the paramagnetic phase. However, the appearance of superconductivity seems to change at a characteristic pressure P^* ∼ 0.8 GPa. Close to a ferromagnetic instability, the homogeneity of the sample can influence strongly the electronic and magnetic properties and therefore bulk phase transitions may differ from the determination by resistivity measurements. (authors)

[en] The magnetically ordered ground state of CeRhIn₅ at ambient pressure and zero magnetic field is an incommensurate helicoidal phase with the propagation vector k = (1/2,1/2,0.298) and the magnetic moment in the basal plane of the tetragonal structure. We determined by neutron diffraction the two different magnetically ordered phases of CeRhIn₅ evidenced by bulk measurements under applied magnetic field in the basal plane. The low temperature high magnetic phase corresponds to a commensurate sine-wave structure of the magnetization with k = (1/2,1/2,1/4). At high temperature, the phase is incommensurate with k = (1/2,1/2,0.298) and a possible small ellipticity. The propagation vector of this phase is the same as that of the zero-field structure. (fast track communication)

[en] The heavy-fermion compounds Ce(Cu_1-xNi_x)₂Ge₂ and CeCu₂(Si_1-yGe_y)₂ were explored using electrical transport, magnetic susceptibility, and neutron scattering measurements. The hybridization of CeCu₂Ge₂ can be changed by substituting either Cu by Ni or Ge by Si. From the local type of antiferromagnetism in CeCu₂Ge₂ to the heavy-fermion superconductor CeCu₂Si₂, the systems reveal a rich magnetic phase diagram with up to three distinct phase transitions below 5 K. In the case of substituting Cu by Ni, the system shows a transition from a local-type of ordering to a band-like magnetism of heavy-fermions. For the concentration x ∼ 0.8 the magnetic order is supressed and systematic deviations from the Fermi-liquid behavior are observed in the temperature dependence of the resistivity. (orig.)

[en] UCoGe is one of the few compounds showing the coexistence of ferromagnetism and superconductivity at ambient pressure. With T_Curie = 3K and T_SC = 0.6 K it is near a quantum phase transition; the pressure needed to suppress the magnetism is slightly higher than 1 GPa. We report simultaneous resistivity and ac-susceptibility measurements under pressure on a polycrystal with very large single-crystalline domains and a resistivity ratio of about 6. Both methods confirm the phase diagram established before by resistivity measurements on a polycrystal. The ferromagnetic phase is suppressed for P ∼ 1.2 GPa. Astonishingly, the superconductivity persists at pressures up to at least 2.4 GPa. In other superconducting and ferromagnetic heavy fermion compounds like UGe₂ and URhGe, the superconducting state is situated only inside the larger ferromagnetic region. Therefore, UCoGe seems to be the first example where superconductivity extends from the ferromagnetic to the paramagnetic region.

No abstract available

No abstract available

[en] We present detailed measurements of the electrical resistivity and microcalorimetry under magnetic field and high pressure which confirm the proximity of YbRh₂Si₂ to a quantum critical point at ambient pressure. Our data seems to rule out the divergence of the heavy quasiparticle mass at the magnetic field tuned quantum critical point. Further Moessbauer spectroscopy gives evidence that the observed ESR resonance cannot be assigned directly to the trivalent Yb local spin dynamics. We report for the first time on quantum oscillations in YbRh₂Si₂. The obtained angular dependence is compared with band-structure calculation. A significant change of the resistivity and specific heat under high pressure and magnetic field is observed. Above 5GPa ferromagnetic coupling plays the dominant role. The unusual high pressure phase diagram of YbRh₂Si₂ cannot be explained in term of the Doniach phase diagram. In Yb heavy fermion case, slow valence fluctuations may occur due to an occupation number of the trivalent state which differs notably from unity. The differences between the Ce and Yb heavy fermion systems are discussed. In particular a comparison is made between CeRh₂Si₂ and YbRh₂Si₂. The observed field and pressure variations of the Hall voltage in CeRh₂Si₂ are not straightforwardly linked to the change in the Fermi surface topology. A tiny change in the 4f electron occupation number will have a huge effect on the effective mass of different orbits of the complex band structure and thus on the Hall effect in this multiband electronic structure. A similar scenario must be certainly considered for YbRh₂Si₂. (author)

[en] The magnetic phase diagram of the system CeCu₂(Si_1-xGe_x)₂ with 0≤x≤1 is studied by electrical transport, magnetic susceptibility, and neutron scattering measurements. The transition from heavy-fermion superconductivity in CeCu₂Si₂ to the local moment type of antiferromagnetism in CeCu₂Ge₂ is explored. The system reveals a rich phase diagram with up to three distinct transitions below 5 K. The nature of the phases is not fully resolved yet. copyright 1996 The American Physical Society