Electron-Phonon Coupling Deduced from Phonon Line Shapes

TL;DR

This study analyzes the Fano line shape of a phonon mode in cuprate films to quantify electron-phonon coupling, revealing its dominance in phonon intensity and its variation with doping and disorder.

Contribution

It introduces an extended Fano model to extract bare phonon parameters and electron-phonon coupling strength from Raman spectra of cuprates.

Findings

Electron-phonon coupling strength lambda is approximately 6.8% near optimal doping.

Phonon intensity is mainly due to coupling with electronic response, not interband excitations.

Disorder at the Ba site reduces electron-phonon coupling and phonon intensity.

Abstract

We investigate the Fano-type line shape of the Ba mode of Y_{1-x}Ca_xBa_2Cu_3O_{6+y} films observed in Raman spectra with A_1g 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 lambda. We find lambda=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.