We report measurements of electrical resistivity, magnetic susceptibility, specific heat, and thermoelectric power on the system Pr1-xCexPt4Ge12. Superconductivity is suppressed with increasing Ce concentration up to x = 0.5, above which there is no
evidence for superconductivity down to 1.1 K. The Sommerfeld coefficient {gamma} increases with increasing x from 48 mJ/mol K^2 up to 120 mJ/mol K^2 at x = 0.5, indicating an increase in strength of electronic correlations. The temperature dependence of the specific heat at low temperatures evolves from roughly T^3 for x = 0 to e^(-Delta /T) behavior for x = 0.05 and above, suggesting a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity. Fermi-liquid behavior is observed throughout the series in low-temperature magnetization, specific heat, and electrical resistivity measurements.
The electronic structure of (Ce,Yb)CoIn5 has been studied by a combination of photoemission, x-ray absorption and bulk property measurements. Previous findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for x>0.3 were confirmed.
One new result of this study is that the Yb valence for x<0.2 increases rapidly with decreasing x from 2.3+ toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around x=0.2. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of about 50% cross sectional area reductions of the alpha- and beta-sheets for x=1 compared to x=0, and a smaller, essentially proportionate, size change of the alpha-sheet for x=0.2. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing x in this alloy system are discussed.
One of the greatest challenges to Landaus Fermi liquid theory - the standard theory of metals - is presented by complex materials with strong electronic correlations. In these materials, non-Fermi liquid transport and thermodynamic properties are oft
en explained by the presence of a continuous quantum phase transition which happens at a quantum critical point (QCP). A QCP can be revealed by applying pressure, magnetic field, or changing the chemical composition. In the heavy-fermion compound CeCoIn$_5$, the QCP is assumed to play a decisive role in defining the microscopic structure of both normal and superconducting states. However, the question of whether QCP must be present in the materials phase diagram to induce non-Fermi liquid behavior and trigger superconductivity remains open. Here we show that the full suppression of the field-induced QCP in CeCoIn$_5$ by doping with Yb has surprisingly little impact on both unconventional superconductivity and non-Fermi liquid behavior. This implies that the non-Fermi liquid metallic behavior could be a new state of matter in its own right rather then a consequence of the underlying quantum phase transition.
