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

TL;DR

This study uses far infrared transmission measurements on Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ single crystals with varied impurity levels to investigate the presence of a superconducting energy gap, finding evidence for gapless superconductivity.

Contribution

It provides experimental evidence that Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ exhibits gapless superconductivity, challenging previous clean limit theories.

Findings

No spectroscopic gap observed despite spectral changes

Results suggest superconductivity is gapless in this material

Impurity variations did not reveal a gap in infrared spectra

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.