The effect of 3$d$ paramagnetic impurities on superconductivity in quench-condensed amorphous Pb films

A modified dilution refrigerator equipped with Sb, Pb and NiCr sources is used to carry out an \textit{in situ} study of the effect of magnetic impurities (\textit{MI}) on the \underline {same} quench-condensed Pb films. Si substrate with pre-deposited Au contacts is mounted in dilution unit and cool down to 5K. To ensure the electrical and possibly structural homogeneity down to monolayer thickness, we deposit a thin layer of Sb prior to the Pb evaporation. At a thickness above 8 ? the film exhibits superconductivity with well-defined resistive transition and $T_{c}$ controlled by the film thickness. When a film of desired $T_{c}$ is obtained we incrementally evaporate \textit{MI} onto the film by heating a NiCr wire at constant current and perform \textit{in situ} measurements. We observe that $T_{c}$ is continuously suppressed with increasing \textit{MI} density while the resistive transitions remain sharp, although the \textit{MI} induces significant filling of states inside the gap. The $T_{c}$ as a function of \textit{MI} density is well described by the Abrikosov-Gorkov theory regardless of the starting $T_{c}$ and the pair-breaking strength of the \textit{MI} appears to be independent of the degree of disorder.