F. Kromer

Evidence for the existence of two variants of UBe13

Abstract Low-temperature specific-heat and thermal-expansion measurements on high-quality single crystals of the heavy-fermion superconductor UBe 13 combined with literature results give evidence for the existence of two variants of this compound. They can be classified according to their superconducting transition temperatures into H-type (high T c : 0.85 ⩽ T c ⩽ 0.95K) and L-type (low T c : T c ≈ 0.75K) UBe 13 . A comparative investigation of both types reveals striking differences both above and below T c .

New results on the low-T phase diagram of U1-xThxBe13: A thermodynamic study

Abstract We report on low-temperature thermal-expansion and specific-heat measurements on single crystals of the heavy-fermion superconductor UBe13. A new “line of anomalies”, B*(T), in the superconducting state is discovered. The evolution of this feature is studied as a function of Th doping on several U1−xThxBe13 polycrystals (x⩽0.03), giving rise to a new “line of anomalies”, TL(x), in the T–x phase diagram. TL(x) hits the multi-critical point at xcr≈0.019 and merges smoothly into the Tc2 line that marks the lower of the two phase transitions at x>xcr. Our data strongly suggest that the anomaly at TL(x) represents the precursor of the transition at Tc2(x) for 0.019

Magnetic Field Response in (UTh)Be13

We report on recent magnetic field studies on U1−xThxBe13 with x≤0.1. These new results are related to previous data published by Kromer et al. [F. Kromer et al., Phys. Rev. B 62, 12477 (2000)]. The new magnetization measurements confirm the existence of an anomaly found earlier through calorimetric and dilatometric measurements at TL<Tc for x<xc1≈0.019. In the critical concentration range xc1<x<xc2≈0.0455, the critical-field curves for the mean field type of phase transition that occur at Tc1 and Tc2 (at zero field) appear to merge as T→0. This interesting fact which has been inferred already from the previous measurements could be confirmed for a sample with x=0.038. The normal state for Th concentrations x≈xc2 is rather unusual: A negative normal-state contribution, αn, is observed in the thermal expansion coefficient as well as the occurrence of a positive peak when superconductivity is suppressed by a magnetic field. Both effects are studied in detail and a comparison to specific heat results is given. Finally, we have investigated the non-linear contribution to the magnetization, χ(3), of UBe13 and U0.9Th0.1Be13. In contradiction to suggestions made previously by Aliev et al. [Aliev et al., Europhys. Lett. 32, 765 (1995)] for x=0.1, χ(3) remains negative down to T≈100 mK, the lowest temperature investigated.

Non-Fermi-Liquid Properties and Exotic Superconductivity in CeCu2Si2 and (UTh)Be13

Tetragonal CeCu2Si2 [1] and cubic UBe13 [2] belong to the class of strongly correlated (electron materials where a periodic lattice of partially filled f shells is embedded in a metallic environment. Below a characteristic “Kondo temperature” Tk, typically of the order of 10K, quasiparticles composed of both local f degrees of freedom and itinerant conduction-electron degrees of freedom form. As has been inferred from the giant Sommerfeld coefficient r of about 1 J/K2mole, the huge effective quasiparticle masses m· (100–1000 mel) are governed by the f degrees of freedom. The discontinuity of the specific heat at Tc which was found to scale with the large γ [1, 2] proved that superconductivity is, indeed, formed by those heavy fermions (HF).