We present the thermopower S͑T͒ and the resistivity ͑T͒ of Lu 1−x Yb x Rh 2 Si 2 in the temperature range 3 Ͻ T Ͻ 300 K. S͑T͒ is found to change from two minima for dilute systems ͑x Ͻ 0.5͒ to a single large minimum in pure YbRh 2 Si 2 . A similar behavior has also been found for the magnetic contribution to the resistivity mag ͑T͒. The appearance of the low-T extrema in S͑T͒ and mag ͑T͒ is attributed to the lowering of the Kondo scale k B T K with decreasing x. The evolution of the characteristic energy scales for both the Kondo effect and the crystal electric field splitting ⌬ CEF are deduced. An extrapolation of T K to x = 1 allows us to estimate the Kondo temperature of YbRh 2 Si 2 to 29 K. For pure YbRh 2 Si 2 , T K and ⌬ CEF / k B lie within one order of magnitude and thus the corresponding extrema merge into one single feature.

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We present measurements of the thermal expansion coefficient α of polycrystalline LaFeAsO1−xFx (x ≤ 0.1). The magnetic and structural transitions of the samples with x ≤ 0.04 give rise to large anomalies in α(T ), while the onset of superconductivity in the crystals with x ≥ 0.05 is not resolved. Above the structural transition, the thermal expansion coefficient of LaFeAsO is significantly enhanced. This is attributed to fluctuations, which also affect the electrical transport properties of the compound. The complete absence of these fluctuations in the superconducting samples even for x = 0.05 is taken as evidence for an abrupt first-order type of suppression of the structural and magnetic transitions upon F doping.

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Low-temperature results of the thermodynamic and transport properties of polycrystalline Ce3Rh4Sn13 are presented. The study of the ground-state properties reveals that Kondo interactions become important below 2 K, as evidenced by a large Sommerfeld coefficient and high magnetic resistivity. At 1 K the specific heat exhibits a broad peak that is attributed to short-range magnetic order. Support is given by ac susceptibility measurements. The resistivity and the susceptibility around 80 K are dominated by effects due to the crystal electric field splitting of the Ce3+ multiplet. Additionally, clear indications for local vibrations of Sn(2) atoms within the cage-like structure are deduced from an Einstein term in the specific heat. The crystal structure is known to form two atomic cages: one in which the Ce atoms are located; the other containing the Sn(2) atoms. Although strong correlations at low T are present in this system, the thermopower is found to be surprisingly small.

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Abstract. We present measurements of the thermal expansion coefficient α of polycrystalline RFeAsO (R = La,Ce,Pr,Sm,Gd). Anomalies at the magnetic ordering transitions indicate a significant magneto-elastic coupling and a negative pressure dependence of TN. The structural transitions are associated by large anomalies in α. Rare earth magnetic ordering in CeFeAsO, PrFeAsO, and SmFeAsO yields large positive anomalies at low temperatures.

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