We report the first observation of non-Fermi-liquid (NFL) effects in a clean Yb compound at ambient pressure and zero magnetic field. The electrical resistivity and the specific-heat coefficient of high-quality single crystals of YbRh(2)Si(2) present a linear and a logarithmic temperature dependence, respectively, in more than a decade in temperature. We ascribe this NFL behavior to the presence of (presumably) quasi-2D antiferromagnetic spin fluctuations related to a very weak magnetic phase transition at T(N) approximately 65 mK. Application of hydrostatic pressure induces anomalies in the electrical resistivity, indicating the stabilization of magnetic order.

We report the presence of two disconnected superconducting domes in the pressure-temperature phase diagram of partially germanium-substituted CeCu2Si2. The lower density superconducting dome lies on the threshold of antiferromagnetic order, indicating magnetically mediated pairing, whereas the higher density superconducting regime straddles a weakly first-order volume collapse, suggesting a pairing interaction based on spatially extended density fluctuations. Two distinct pairing mechanisms thus appear to operate in the single, wide, superconducting range of stoichiometric CeCu2Si2, both of which might apply more generally to other classes of correlated electron systems.

We present results of low-temperature calorimetric and resistive measurements on the isostructural heavy-fermion compounds CeCu 2 Si 2 and CeNi 2 Ge 2 . 'Non-Fermi-liquid' effects are established which suggest the nearness of an antiferromagnetic quantum critical point (QCP) in both systems. The observed deviations from the properties of a Landau Fermi liquid (FL) may be related to anomalous energy dependences of both the quasiparticle mass and the quasiparticlequasiparticle scattering cross section. For CeNi 2 Ge 2 , a moderately heavy FL can be recovered by application of moderate values of either magnetic field or hydrostatic pressure. For p = 1.7 GPa a novel, non-superconducting, phase transition has been discovered at T 1 1 K.