OBSERVATION OF THE HOLSTEIN SHIFT IN HIGH-T(C) SUPERCONDUCTORS WITH THERMAL-MODULATION REFLECTOMETRY

We use the experimental technique of thermal-modulation reflectometry to study the relatively small temperature dependence of the optical conductivity of superconductors. Due to a large cancellation of systematic errors, this technique is shown to be a very sensitive probe of small changes in reflectivity. We analyze thermal-modulation reflection spectra of single crystals and epitaxially grown thin films of YBa2Cu3O7-delta and obtain the alpha(t)r2F(omega) function in the normal state, as well as the superconductivity-induced changes in reflectivity. We present detailed model calculations, based on the Eliashberg-Migdal extension of the BCS model, which show good qualitative and quantitative agreement with the experimental spectra.