Search for localized phonon modes inlead-based alloys by superconducting tunneling

To investigate the appearance of localized phonon modes, superconductive tunneling was performed into films of Pb_0·9M_0·1 (M = Ag, Cd, Ga, Ge, In, Mg, Mg, Sb, Sn, Te, Zn) and Pb_0·97Sn_0·03. The films were quench-condensed and later annealed at 30, 100 and 300°K. Structure in tunneling curves resulting from localized phonons was observed only for a well annealed In alloy film. For most of the impurities an absence of observable local phonon modes could be explained as due to a lack of solid solubility or of sufficient structural order in the film. For Na and Sn alloys, however, these arguments cannot be used. No modes could be seen for such alloys even in films condensed at room temperature, heat-treated slightly, and quenched directly into liquid helium.