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 TK lower than the critical temperature Tc, we observe an impurity band inside the quasi-BCS gap, in agreement with theoretical predictions. Our results on Cu: Fe (TKTc~5) do not show such structure and disagree with theory.