No far-infrared-spectroscopic gap in clean and dirty high-Tcsuperconductors

Abstract

We report far-infrared transmission measurements on single crystal samples derived from ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. The impurity scattering rate of the samples was varied by electron-beam irradiation, 50 MeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}_{}^{+6\mathrm{}}{}_{}{}^{}$ 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 ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ is gapless.