Pair breaking in the heavy-fermion superconductors Ce1−xMxCu2.2Si2 and U1−xMxBe13 (M: Th, La, Y and Gd)

We report measurements of Tc and ΔC for Ce1−xMxCu2.22Si2 (M = La, Y, Gd) and U1−xMxBe13 (M: Th, La, Y). “Diamagnetic” pair breaking by nonmagnetic dopants is observed which implies an anisotropic nature of both the superconducting order parameter and the impurity-scattering mechanism. Gd impurities in CeCu2.2Si2 give rise to additional “paramagnetic” pair breaking.