Evidence for a Surface-Phonon Contribution to Thin-Film Superconductivity: Depression ofTcby Noble-Gas Overlayers

When care is taken to prevent contamination, the superconducting transition temperatures of thin Al, Sn, Tl, and amorphous-Bi films are depressed by overlayers of Ne or Ar. For a given film, the ratio of the shift in $T_c$ produced by Ar to that produced by Ne is observed to be 1.8. The results can be explained in terms of a modification of the phonon spectrum by the presence of the noble gas. A simple model predicts the observed ratio of the effect of Ar to that of Ne and the relative magnitudes of the shifts for the four different metals.