NO FAR-INFRARED-SPECTROSCOPIC GAP IN CLEAN AND DIRTY HIGH-TC SUPERCONDUCTORS

We report far-infrared transmission measurements on single crystal sam ples derived from Bi2Sr2CaCu2O8. The impurity scattering rate of the s amples was varied by electron-beam irradiation, 50 MeV O-16(+6) ion ir radiation, heat treatment in vacuum, and Y doping. Although substantia l changes in the infrared spectra were produced, in no case was a feat ure observed that could be associated with the superconducting energy gap. These results all but rule out ''clean limit'' explanations for t he absence of the spectroscopic gap in this material, and provide evid ence that the superconductivity in Bi2Sr2CaCu2O8 is gapless.