Tunneling in Dilute Al-Mn Alloys: Observation of Resonant Scattering and Implications for High-Temperature Superconductors*

We report on the observation of superconducting energy gap suppression by resonant scattering. Tunneling measurements of dilute Al-Mn alloys demonstrate the absence of density-of-states smearing that accompanies pair breaking and verify the detailed predictions of the Kaiser resonant scattering theory. These materials represent model systems for quasi-particle scattering and interference phenomena in the high- temperature superconductors.