Optical Evidence of an Electronic Contribution to the Pairing Interaction in Superconducting Tl2Ba2Ca2Cu3O10

We report the results of a study of the temperature-dependent thermal difference reflectance spectra of Tl2Ba2Ca2Cu3O10. At temperatures below T c, a feature emerges in the spectra with an integrated amplitude that scales as Δ 0 2 (T), where Δ0(T) is the temperature-dependent superconducting gap. The temperature dependence and location of this feature can be described by an Eliashberg model with a coupling function that includes both an electron-phonon interaction and an interaction located at ∼1.6 eV. We find remarkably good agreement between theory and experiment based upon this description of the superconducting state.