Electron-phonon interaction (EPI) is decisive for determining the functional properties of materials. It is a main scattering process in electron conduction and is vital for the formation of ordered electronic states such as charge-density waves. Its strength λ<ω~2> is most reliably and straightforwardly determined from femtosecond relaxation times τ_(e-ph) due to electron-phonon interaction, according to the expression λ<ω~2>=2πk_BT_1/3hτ_(e-p), with T_1 being the lattice temperature [1]. Here we study electron relaxation in two high- T_c cuprate superconductors, where EPI is expected to be strong. We use sub-20 fs pulses from two synchronized broad-band non-collinear optical parametric amplifiers (NOPAs) [2], in order to improve the time resolution compared to previous experiments. The obtained values for λ<ω~2> suggest that EPI has a crucial role in the high- T_c superconductivity mechanism, which is one of the great unsolved puzzles in condensed matter physics.
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