THERMAL-DIFFERENCE REFLECTANCE SPECTROSCOPY OF THE HIGH-TEMPERATURE CUPRATE SUPERCONDUCTORS

The temperature-dependent thermal-difference reflectance (TDR) spectra of thin-film samples of Tl2Ba2Ca2Cu3O10, (BiPb)(2)Sr2Ca2Cu3O10, Tl2Ba 2CaCu2O8, and YBa2Cu3O7 have been measured for photon energies between 0.3 and 4.5 eV at temperatures above and below each material's superc onducting critical temperature. The amplitude of the characteristic op tical structure near the screened plasma frequency of each sample in t he normal-state TDR spectrum varies approximately linearly with temper ature, T, indicating that the temperature-dependent optical scattering rate in these materials scales with temperature as T-2. From the TDR spectra collected above and below the critical temperature of each sam ple, the superconducting to normal-state reflectance ratio, R(S)/R(N), has been obtained. In all of these spectra, there are significant dev iations from unity in R(S)/R(N) at photon energies on the order of 2.0 eV. This optical structure cannot be accounted for using the conventi onal Mattis-Bardeen description of the optical properties of a superco nductor or its strong-coupling extension where electron-pairing intera ctions are limited to energies less than 0.1 eV. However, both the tem perature and energy dependence of the structure in the R(S)/R(N) spect ra may be adequately described within Eliashberg theory with an electr on-boson coupling function which consists of both a low-energy compone nt (<0.1 eV) and a high-energy component located between 1.6 and 2.1 e V.