Electronic and Phonon Contributions to the Pairing in the Cuprates

Thermal Difference Reflectance (TDR) Spectroscopy has been used to determinethe superconducting gap parameter for several of the superconducting cupratesover a wide range of energies, extending from the infrared (0.3 eV)to the ultraviolet (5.3 eV). A contribution to the pairing is found in eachcase from the phonons, and from an electronic excitation with energy thatranges from 1.6 eV and 2.3 eV for the different compounds attributed to thed⁹–d¹⁰L charge-transfer excitation between Cu and O. In every case thereflectance ratio between the superconducting and normal state, Rs/Rnplotted as a function of photon energy can be well described using theEliashberg theory. The theory also predicted a characteristic shape for thelow energy part of such spectra due to the phonons. We report theobservation of this feature in measurements on films of Tl₂Ba₂CaCu₂O₈. Wediscuss the significance of the success of the Eliashberg theory inexplaining these results and successfully predicting new effects in thelight of the correlations that had been thought to invalidate such atheoretical approach.