| The mechanism for superconductivity in the high temperature superconducting cuprates is currently unknown. Experiments that give information about the mechanism of superconductivity, therefore, are essential for the understanding of these materials. The energy dependence of the pairing mechanism for superconductivity is reflected in the superconducting gap function. One powerful probe of the energy dependence of the superconducting gap function is reflectance spectroscopy. Reflectance spectroscopy probes the optical conductivity, which is, in principle, a model independent probe of the superconducting gap function.; We present normal and superconducting state reflectance and thermal difference reflectance (TDR) spectra for {dollar}rm Tlsb2Basb2CaCusb2Osb8{dollar}, {dollar}rm Bisb2Srsb2CaCusb2Osb8{dollar}, {dollar}rm (BiPb)sb2Srsb2Casb2Cusb3Osb{lcub}10{rcub}{dollar}, and {dollar}rm YBasb2Cusb3Osb7{dollar} over the energy range 0.095 to 6.00 eV. The dielectric functions and the loss functions for each of the samples were obtained through a Kramers-Kronig analysis of the reflectance spectra. They indicate that all these compounds have an anomalous energy dependent optical scattering rate between 0 and 1 eV, and that each of the samples has a screened plasma response located near 1 eV and interband transitions in the neighborhood of 3.5 eV.; Combining the normal state reflectance spectra with TDR spectra allowed a calculation of the derivative dielectric functions for all the samples. The results validate the analysis of the reflectance spectra, but also revealed the presence of additional weak oscillators located between 1 and 2.5 eV for each of the samples which are not resolved in the reflectance spectra.; TDR spectra were collected for three of the samples at a series of temperatures above and ultimately below the critical temperature of the superconductors. The TDR spectra, in combination with the reflectance spectra, allow a measure of the ratio of the superconducting to normal state optical conductivity, which is a direct, model independent probe of the energy dependent, superconducting gap function of the superconductors. The data give strong evidence that the pairing interaction has two components: one located at phonon energies, and a second at energies near 2.0 eV. |
