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 lambda = 800 +/- 200 meV(2) for La1.85Sr0.15CuO4 and lambda = 400 +/- 100 meV(2) for YBa2Cu3O6.5. These values suggest a possible fundamental role of the electron-phonon interaction in the superconducting pairing mechanism.