Electron-tunneling observation of localized excited states in superconducting manganese-doped lead

We have made electron-tunneling measurements on a dilute, superconducting leadmanganese alloy. A well-defined structure was observed in the ac-conductance—voltage curves, indicating excited states within the BCS energy gap. These states were partially accounted for by Shiba theory when spin-dependent $s$-, $p$-, and $d$-wave scattering were included. The phase shifts used in doing that were the results of band calculations. The experimental data also show the existence of a broad background density of states in the energy gap, which cannot be accounted for by the theory.