Tunneling Study of Impurity Bands in Kondo Superconducting Systems: Cu Alloys Backed by Pb

We present tunneling experiments on Cu: $X$ ($X$ stands for Cr, Fe, or Mn) thin alloy films in superconducting proximity with Pb. For cases of Cr and Mn, with Kondo temperatures $T_{\mathrm{K}}$ lower than the critical temperature $T_c$, we observe an impurity band inside the quasi-BCS gap, in agreement with theoretical predictions. Our results on Cu: Fe ($\frac{T_{\mathrm{K}}}{T_c}\sim 5$) do not show such structure and disagree with theory.