Electron-Tunneling Observation of Impurity Bands in Superconducting Manganese-Implanted Lead

Abstract

Dilute $\mathrm{Pb}\mathrm{Mn}$ alloys have been prepared in a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$/${}_{}{}^4{}_{}{}^{}\mathrm{He}$-dilution cryostat by low-temperature implantation of ${\mathrm{Mn}}^{+}$ ions into annealed Pb films forming the counter electrode of Mg-MgO-Pb tunnel junctions. The tunnel conductance as a function of the Mn concentration was measured at 200 mK. Two pronounced impurity bands are found within the superconducting gap at $\frac{\omega }{\Delta }=0.23\mathrm{} \mathrm{and} 0.79$, respectively. The results can be described by existing theories if scattering of different partial waves is taken into account.