X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce 1Àx Yb x CoIn 5 (0 x 1) reveal that many of the characteristic features of the x ¼ 0 correlated electron state are stable for x 0:775 and that phase separation occurs for x > 0:775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T c decreases linearly with x towards 0 K as x ! 1, in contrast with other HF superconductors where T c scales with T coh .

One of the most notorious non-Fermi-liquid properties of both archetypal heavy-fermion systems 1-4 and the high-T c copper oxide superconductors 5 is an electrical resistivity that evolves linearly (rather than quadratically) with temperature, T . In the heavy-fermion superconductor CeCoIn 5 (ref. 6), this linear behaviour was one of the first indications of the presence of a zero-temperature instability, or quantum critical point. Here, we report the observation of a unique control parameter of T -linear scattering in CeCoIn 5 , found through systematic chemical substitutions of both magnetic and non-magnetic rareearth, R, ions into the Ce sublattice. We find that the evolution of inelastic scattering in Ce 1−x R x CoIn 5 is strongly dependent on the f -electron configuration of the R ion, whereas two other key properties-Cooper-pair breaking and Kondo-lattice coherence-are not. Thus, T -linear resistivity in CeCoIn 5 is intimately related to the nature of incoherent scattering centres in the Kondo lattice, which provides insight into the anomalous scattering rate synonymous with quantum criticality 7 . Although recent theories 4,[8][9][10] provide possible routes to an explanation of T-linear resistivity-found in both f -electron systems (for example, Y 1−x U x Pd 3 (ref. [1][2][3][4]6 , its coexistence with conventional (T 2 ) Hall-angle scattering 11,12 and its inconsistency with oneparameter scaling 13 . Most recently, its observation over three decades of T at the field-tuned quantum critical point (QCP) of CeCoIn 5 has been linked to a violation of the Wiedemann-Franz law 14 , an indication that this scattering rate is associated with the failure of Fermi-liquid theory in its most basic form.Here, we present a rigorous study of the effects of rare-earth substitution on three closely related features of the exotic metal CeCoIn 5 : unconventional superconductivity, Kondo-lattice coherence and anomalous charge-carrier scattering. By diluting the Ce lattice within high-quality single-crystal specimens of Ce 1−x R x CoIn 5 with both non-magnetic (full or empty 4f -shell) and stable-4f -moment substituent ions of varying size and electronic configuration, we are able to inject both 'Kondo holes' (isoelectronic ions without magnetic moments) and strongly localized magnetic moments into the coherent Kondo lattice. This has allowed us to probe the spin exchange between the Ce 3+ localized magnetic moments and the spins of the conduction electrons involved in Cooper pairing, Kondo screening and anomalous transport in a controlled way, revealing a surprising contrast between the response of coherent phenomena and non-Fermi-liquid behaviour to this perturbation. Figure 1 shows the evolution of both the superconducting transition temperature T c (identified by the transition in resistivity, ρ) and Kondo-lattice coherence temperature T coh (identified by the maximum in ρ(T )) for all rare-earth substitutions made in Ce 1−x R x CoIn 5 through the complete range of concentrations where both features exist. As shown, the...

Abstract. Single crystals of the compound LaFePO were prepared using a flux growth technique at high temperatures. Electrical resistivity measurements reveal metallic behavior and a resistive transition to the superconducting state at a critical temperature T c ∼ 6.6 K. Magnetization measurements also show the onset of superconductivity near 6 K. In contrast, specific heat measurements manifest no discontinuity at T c . These results lend support to the conclusion that the superconductivity is associated with oxygen vacancies that alter the carrier concentration in a small fraction of the sample, although superconductivity characterized by an unusually small gap value can not be ruled-out. Under applied magnetic fields, T c is suppressed anisotropically for fields perpendicular and parallel to the ab-plane, suggesting that the crystalline anisotropy strongly influences the superconducting state. Preliminary highpressure measurements show that T c passes through a maximum of nearly 14 K at ∼ 110 kbar, demonstrating that significantly higher T c values may be achieved in the phosphorus-based oxypnictides.

The evolution of the Fermi surface of CeRh1−xCoxIn5 was studied as a function of Co concentration x via measurements of the de Haas-van Alphen effect. By measuring the angular dependence of quantum oscillation frequencies, we identify a Fermi surface sheet with f -electron character which undergoes an abrupt change in topology as x is varied. Surprisingly, this reconstruction does not occur at the quantum critical concentration xc, where antiferromagnetism is suppressed to T = 0. Instead we establish that this sudden change occurs well below xc, at the concentration x ≃ 0.4 where long range magnetic order alters its character and superconductivity appears. Across all concentrations, the cyclotron effective mass of this sheet does not diverge, suggesting that critical behavior is not exhibited equally on all parts of the Fermi surface.

Specific heat C(T ) measurements were made on single crystals of the superconducting filled skutterudite series Pr(Os 1−x Ru x ) 4 Sb 12 down to 0.6 K. Crystalline electric field fits in the normal state produced parameters which were in agreement with previous measurements. Bulk superconductivity was observed for all values of the Ru concentration x with transition temperatures consistent with previous experiments, confirming a minimum in T c at x = 0.6. The C(T ) data below T c appear to be more consistent with power law behavior for x = 0 (PrOs 4 Sb 12 ), and with exponential behavior for 0.05 ≤ x ≤ 0.2. An enhanced electronic specific heat coefficient γ was observed for x ≤ 0.4, further supporting x ≃ 0.6 as a critical concentration where the physical properties abruptly change. Significant enhancement of ∆C/T c above the weak coupling value was only observed for x = 0 and x = 0.05.

The filled skutterudite compound SmOs 4 Sb 12 was prepared in single crystal form and characterized using x-ray diffraction, specific heat, electrical resistivity, and magnetization measurements.The SmOs 4 Sb 12 crystals have the LaFe 4 P 12 -type structure with lattice parameter a = 9.3085Å.Specific heat measurements indicate a large electronic specific heat coefficient of ≈ 880 mJ/mol K 2 , from which an enhanced effective mass m * ≈ 170 m e is estimated. The specific heat data also suggest crystalline electric field (CEF) splitting of the Sm 3+ J = 5/2 multiplet into a Γ 7 doublet ground state and a Γ 8 quartet excited state separated by ∼ 37 K. Electrical resistivity ρ(T) measurements reveal a decrease in ρ(T) below ∼ 50 K that is consistent with CEF splitting of ∼ 33 K between a Γ 7 doublet ground state and Γ 8 quartet excited state. Specific heat and magnetic susceptibility measurements display a possible weak ferromagnetic transition at ∼ 2.6 K, which could be an intrinsic property of SmOs 4 Sb 12 or possibly due to an unknown impurity phase.

Magnetization, specific heat, and electrical resistivity measurements were made on single crystals of the filled skutterudite compound PrOs4As12. Specific heat measurements indicate an electronic specific heat coefficient γ ∼ 50−200 mJ/mol K 2 at temperatures 10 K ≤ T ≤ 18 K, and ∼ 1 J/mol K 2 for T ≤ 1.6 K. Magnetization, specific heat, and electrical resistivity measurements reveal the presence of two, or possibly three, ordered phases at temperatures below ∼ 2.3 K and in fields below ∼ 3 T. The low temperature phase displays antiferromagnetic characteristics, while the nature of the ordering in the other phase(s) has yet to be determined.

Single crystals of the filled-skutterudite compound NdOs4Sb12 have been investigated by means of electrical resistivity, magnetization, and specific heat measurements. The NdOs4Sb12 crystals have the LaFe4P12-type cubic structure with a lattice parameter of 9.3Å. Possible heavy-fermion behavior is inferred from specific heat measurements, which reveal a large electronic specific heat coefficient γ ≈ 520 mJ/mol-K 2 , corresponding to an effective mass m * ≈ 98 me. Features related to a ferromagnetic transition at ∼ 0.9 K can be observed in electrical resistivity, magnetization and specific heat. Conventional Arrott-plot analysis indicates that NdOs4Sb12 conforms to mean-field ferromagnetism.