SUBSTITUTION STUDIES AND THE NATURE OF SUPERCONDUCTIVITY IN UPT3

The influence of substitutions on the superconductivity in the systems (U1-xYx)Pt3, (U1-xThx)Pt3, and U(Pt1-xPdx)3 have been studied by resi stance and specific-heat measurements for T < 1 K. The results of yttr ium and thorium substitution are discussed in terms of the models deve loped by Hirschfeld et al. and Schmitt-Rink et al. in which the power laws in the low-temperature behavior of several properties are calcula ted under the assumption of resonant impurity scattering. Both yttrium and palladium are concluded to have a pair-breaking effect in UPt3. F or palladium substitution an increase of DELTAT(c) = T(c1) - T(c2) wit h concentration is reported. The importance of these results for the q uestion of the nature of the order parameter and for the hypothesis of a coupling between the superconducting and magnetic order parameter i s discussed.