Electron-Phonon Coupling Deduced from Phonon Line Shapes

Abstract

We investigate the Fano-type line shape of the Ba mode of Y1—xCaxBa2Cu3O6+y films observed in Raman spectra with A1g symmetry. The line shape is described with an extended Fano formula that allows us to obtain the bare phonon parameters and the self-energy effects based on a phenomenological description of the real and imaginary part of the low-energy electronic response. It turns out that the phonon intensity originates almost entirely from a coupling to this electronic response with negligible contributions from interband (high-energy) electronic excitations. In the normal state we obtain a measure of the electron–phonon coupling via the mass-enhancement factor λ. We find λ- = 6.8 ± 0.5% around optimum doping and only weak changes of the self-energy in the superconducting state. With increased disorder at the Ba site we find a decreased intensity of the Ba mode which we can relate to a decreased electron–phonon coupling.