Electron-Phonon Coupling in High-Temperature Cuprate Superconductors Determined from Electron Relaxation Rates

We determined electronic relaxation times via pump-probe optical spectroscopy using sub-15 fs pulses for the normal state of two different cuprate superconductors. We show that the primary relaxation process is the electron-phonon interaction and extract a measure of its strength, the second moment of the Eliashberg function $\ensuremath{\lambda}⟨{\ensuremath{\omega}}^{2}⟩=800\ifmmode\pm\else\textpm\fi{}200\text{ }\text{ }{\mathrm{meV}}^{2}$ for ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}$ and $\ensuremath{\lambda}⟨{\ensuremath{\omega}}^{2}⟩=400\ifmmode\pm\else\textpm\fi{}100\text{ }\text{ }{\mathrm{meV}}^{2}$ for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$. These values suggest a possible fundamental role of the electron-phonon interaction in the superconducting pairing mechanism.

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