Influence of La and Gd Impurities on the Two Phase Transitions in U0.97Th0.03Be13

The phase diagram of U _1–x Th _x Be ₁₃ exhibits two irregular points at x _C1=1.9 at.% and x _C2=4.55 at.% which mark the endpoints of the concentration range where two phase transitions in specific heat measurements are observed. While it is clear that the upper one belongs to a superconducting phase transition, there are different interpretations for the lower one. It has been suggested that the lower transition involves magnetic correlations which coexist with a single superconducting state or that the lower transition separates two different superconducting states (one or both are probably non-s-wave like). In this scenario the onset of local magnetic order is discussed as being due to an accompanied antiferromagnetic transition or as a product of broken time reversal symmetry. To get more information about the nature of the two superconducting phases, substitution experiments with non-magnetic La and magnetic Gd on the U sites in U _0.97 Th _0.03 Be ₁₃ were performed. From specific heat measurements we argue that the upper transition behaves with La/Gd doping like a conventional s-wave superconductor, whereas the lower transition cannot be brought in line with common pictures of superconducting transitions. Also at the