J. J. Rieger

Transition from hexagonal to cubic structure and enhanced specific heat in CePt5−xAux

In our attempt to replace platinum successively by gold in CePt5−xAux, we have found that with increased Au content the initial hexagonal CaCu5 structure of CePt5 changes into the cubic AuBe5 structure for 1 < x ⩽ 3. Since this is the same lattice as in the heavy fermion system UPt4Au, with U replaced by Ce, we can now compare the role of the two different elements in this configuration. We have investigated the influence of the Au content on the low temperature specific heat and magnetic susceptibility, which both increase with increasing Au content, and discuss to what extent this enhancement might be created by the electronic density of states or by magnetic entropy. Furthermore, we have studied the transition from the U-based heavy fermion system into the Ce-based system in U1−yCeyPt4Au.

Formation of Kondo clouds in U(Sn1−xMx)3 compounds (M = Al, Ga, Ge)

Abstract USn3 has an unusually high γ value of 169 mJ mole−1 K−2 in comparison with its neighboring compounds UAl3, UGa3, USi3, and UGe3. To study the formation of this high γ value we prepared pseudobinary U(Sn1−xMx)3 compounds (M = Al, Ga, Si, Ge, and x = 0, 0.1, 0.25, 0.5). From X-ray diffraction, specific-heat and susceptibility measurements we conclude that the high γ value is not only due to f-ligand hybridization, but also to the formation of Kondo clouds (40%).

Hybridization effects in U2(Re,Os)B6 and U(Ru,Os)B4

Abstract U 2 (Re,Os)B 6 and U(Ru,Os)B 4 were studied by means of lattice constants, specific heat and d.c. magnetic susceptibility. M ( H ) measurements reveal completely paramagnetic alloys at 2 K and the χ ( T ) data show a tendency to a nearly temperature independent susceptibility below 20 K with values from 0.91 to 1.86 memu U-mol −1 at 2 K. From specific heat measurements the calculated Sommerfeld coefficient γ is in the range from 24 to 38 mJ U-mol −1 K −2 . Neither C T vs. T ( nor χ vs. T) plots revealed any sign of collective order in the observed temperature interval from 1.1 to 12 K (1.7–400 K). The observed properties are discussed as effects resulting mainly from the hybridization between the U 5f electrons with the transition metal d electrons.

Superconducting and magnetic properties of Y1−xUxRh4B4

Abstract The heat capacity, magnetic susceptibility and magnetization of Y 1− x U x Rh 4 B 4 were determined within the range from x = 0 to x = 0.65. The influence of the local magnetic moments of the U atoms (d U-U = 5.27 A ) in the CeCo 4 B 4 structure of the tetragonal RERh 4 B 4 was studied. A.c. susceptibility measurements showed a rather large value for the T c depression of d T c /d x = −1.88 K per at. % U in the lower concentration range x = 0.0 to x = 0.06. In comparison with the (Bardeen-Cooper-Schrieffer) BCS theory and the Abrikosov-Gorkov model for dilute magnetic impurities, the decrease in the jump in specific heat at T c vs. the depression of T c is rather strong in these samples, so the existence of a demagnetizing Kondo cloud around the U moments seems likely. In the samples containing more U, the magnetization and d.c. susceptibility showed a tendency towards magnetic behaviour, but no evidence for long-range magnetic order was found.

Moment dilution in the heavy fermion compound Ce2Bi

The heavy fermion compound Ce 2 Bi has two inequivalent Ce-sites. La was doped on these sites to study the influence of the antiferromagnetic order on the high electronic contribution of the specific heat (γ HT (T > T N ) 340 mJ/Ce-mol K 2 ). The slightly larger La atom replaces the Ce 2 site first and then subsequently the Ce 1 site in the La 2 Sb structure. Lattice parameters, magnetic susceptibility and specific heat are used to elucidate the magnetic nature of the two different cerium sites. We find a somewhat enhanced γ HT , a decrease in T N and furthermore a metamagnetic transition at 2.2 T (2 K), which is closely related to the long-range antiferromagnetic order