Experimental evidence of low frequency defect vibrations in the fcc metal lead observed by superconducting tunneling

Superconducting tenneling measurements of Al-Al_xO_y-Pb junctions taken after low temperature irradiation with 10 MeV oxygen ions are reported. The normalized differential conductance $\text{σ}{}_{\mathrm{norm}}\text{= (}\text{d}\text{I}\text{d}\text{U}\text{)}{}_{\mathrm{s}}\text{/ (}\text{d}\text{I}\text{d}\text{U}\text{)}{}_{\mathrm{n}}$ shows additional low frequency structures to an enhancement of the Eliashberg function α²F(ω) in the range from 1.5 to 3.2 meV. The position in energy and the annealing behaviour corroborate that vibrational modes or irradiation induced self-interstitials are observed.