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 .

a b s t r a c tLa 3 Co 4 Sn 13 and La 3 Ru 4 Sn 13 were categorized as BCS superconductors. In a plot of the critical field H c2 vs T, La 3 Ru 4 Sn 13 displays a second superconducting phase at the higher critical temperature T + c , characteristic of inhomogeneous superconductors, while La 3 Co 4 Sn 13 shows bulk superconductivity below T c . We observe a decrease in critical temperatures with external pressure and magnetic field for both compounds. Additionally, for La 3 Ru 4 Sn 13 we find that dT + c =dP > dT c =dP. The pressure dependences of T c are interpreted according to the McMillan theory and understood to be a consequence of lattice stiffening. The investigation of the superconducting state of La 3 Co x Ru 4Àx Sn 13 shows a T + c that is larger then T c for x < 4. This unique and unexpected observation is discussed as a result of the local disorder and/or the effect of chemical pressure when Ru atoms are partially replaced by smaller Co atoms.

Measurements of magnetization, specific heat, electrical resistivity, Hall effect, and magnetoresistance on single crystalline samples of LaFeAsO grown in a NaAs flux are reported. While this material is known to be a semimetal, the temperature dependence of the electrical resistivity data presented herein is reminiscent of semiconducting behavior and exhibits distinct features associated with a structural transition and spin density wave (SDW) order. Magnetoresistance and Hall coefficient measurements were performed in magnetic fields up to 9 T applied perpendicular to the basal plane using a van der Pauw configuration. The charge carrier density and mobility indicate that electrons are the majority charge carriers and exhibit features indicative of the structural transition and SDW formation. Low temperature X-ray diffraction measurements have confirmed that the structural transition in these samples occurs near 140 K, compared to a transition temperature of 156 K observed in polycrystalline samples. Isotherms of magnetoresistivity measured as a function of magnetic field can be scaled onto a single curve in which the scaling field is a linear function of temperature between 2.2 K and 180 K.

We investigated the vortex dynamics in the non-centrosymmetric superconductor Li2Pt3B in the temperature range 0.1 K -2.8 K. Two different logarithmic creep regimes in the decay of the remanent magnetization from the Bean critical state have been observed. In the first regime, the creep rate is extraordinarily small, indicating the existence of a new, very effective pinning mechanism. At a certain time a vortex avalanche occurs that increases the logarithmic creep rate by a factor of about 5 to 10 depending on the temperature. This may indicate that certain barriers against flux motion are present and they can be opened under increased pressure exerted by the vortices. A possible mechanism based on the barrier effect of twin boundaries is briefly discussed.The occurrence of superconductivity in compounds with non-centrosymmetric crystal structures has attracted considerable attention recently. Besides various other systems, superconductivity has also been reported in the ternary boride compounds Li 2 Pd 3 B and Li 2 Pt 3 B which have superconducting critical temperatures of 7-8 K and 2.4 K, respectively [1,2]. These two isostructural compounds crystallize in a structure consisting of distorted boron centered octahedra of BPd 6 or BPt 6 in an approximately cubic arrangement with an interpenetrating lithium formation [3]. Both substructures, and hence the composite crystal structure, lack inversion symmetry. Several unusual properties appear in noncentrosymmetric superconductors depending upon various factors, in particular the specific form of the spinorbit coupling in such systems as well as the pairing symmetry [4,5,6]. In contrast to the strongly correlated noncentrosymmetric heavy fermion superconductors CePt 3 Si [7], CeRhSi 3 [8], and UIr [9] for which superconductivity is associated with a magnetic quantum phase transition, there is no evidence of magnetic order or strong electron correlations in either Li 2 Pd 3 B or Li 2 Pt 3 B. Measurements of the London penetration depth suggest that Li 2 Pd 3 B has a full quasiparticle gap in the superconducting phase, while for Li 2 Pt 3 B, the data indicate line nodes in the energy gap [10]. NMR measurements [11] suggest that Li 2 Pd 3 B is a spin singlet, s-wave superconductor. In contrast, in Li 2 Pt 3 B, the spin susceptibility measured by the Knight shift remains unchanged across the superconducting transition temperature, and the spin-lattice relaxation rate 1/T 1 shows no coherence peak below T c , decreasing as T 3 with decreasing temperature, consistent with gap line nodes. In this letter, we investigate a further intriguing property of the unconventional superconductor Li 2 Pt 3 B, observed in the vortex dynamics. We demonstrate that the behavior of the flux creep is very unusual, displaying at short times extremely small creep rates followed by a faster avalanche-like escape of magnetic flux.Samples of Li 2 Pt 3 B used in this experiment were synthesized in an arc furnace utilizing a two-step process similar to that outlined in the work of Badica et al. [2...