Signatures of gap inhomogeneities in optical properties of superconductors
The high temperature superconductor Bi2Sr2CaCu 2O8+[delta] displays nanoscale spatial inhomogeneity in its superconducting energy gap value at all superconducting doping ranges. These regions of nanoscale gap inhomogeneity are seen in scanning tunneling microscopy (STM) experiments. This work focuses on how these inhomogeneities influence bulk optical properties such as the optical conductivity and the optical self-energy. We apply Bruggeman's effective medium approximation as a phenomenological model of this material which is viewed as a composite system. With nanoscale gap variation on the order of that found in STM images, the conductivity can be well approximated by that due to a single gap value with d-wave symmetry. Further, we show that this conclusion does not hold for the case of s-wave gap symmetry, where the optical properties are predicted to display signatures of a distribution of gaps.