No Far-Infrared-Spectroscopic Gap in Clean and Dirty High-T${}_C$ Superconductors

Abstract

We report far infrared transmission measurements on single crystal samples derived from Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$. The impurity scattering rate of the samples was varied by electron-beam irradiation, 50MeV ${}^{16}$O${}^{+6}$ ion irradiation, heat treatment in vacuum, and Y doping. Although substantial changes in the infrared spectra were produced, in no case was a feature observed that could be associated with the superconducting energy gap. These results all but rule out ``clean limit'' explanations for the absence of the spectroscopic gap in this material, and provide evidence that the superconductivity in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$ is gapless.

4 pages and 3 postscript figures attached. REVTEX v3.0. Accepted for publication in Phys. Rev. Lett. IRDIRTY

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