Impurity Effects on the Superconductive Critical Temperature of Indium and Aluminum

We find that the effects of 0.01 to 1.0 atomic percent of various impurities on the superconductive critical temperature of indium and aluminum are strikingly similar to those previously reported for tin. For sufficiently small amounts of all solutes there is an electronic mean free path effect on the critical temperature such that $T_c$ decreases linearly with increasing reciprocal free path. For larger impurity concentrations the curves of critical temperature versus concentration fall into groups according to the valence difference between solvent and solute. $T_c$ has a sharp upward trend for higher valence solutes; for those with lower valence the curves of $T_c$ tend to flatten out. The similarity of these impurity effects for three such different superconductors suggests that they are a fundamental property independent of the detailed nature of the superconductor.