Strain-Induced Enhancement of the Electron Energy Relaxation in Strongly Correlated Superconductors

We use femtosecond optical spectroscopy to systematically measure the primary energy relaxation rate Gamma(1) of photoexcited carriers in cuprate and pnictide superconductors. We find that Gamma(1) increases monotonically with increased negative strain in the crystallographic a axis. Generally, the Bardeen-Shockley deformation potential theorem and, specifically, pressure-induced Raman shifts reported in the literature suggest that increased negative strain enhances electron-phonon coupling, which implies that the observed direct correspondence between a and Gamma(1) is consistent with the canonical assignment of Gamma(1) to the electron-phonon interaction. The well-known nonmonotonic dependence of the superconducting critical temperature T-c on the a-axis strain is also reflected in a systematic dependence T-c on Gamma(1), with a distinct maximum at intermediate values (similar to 16 ps(-1) at room temperature). The empirical nonmonotonic systematic variation of T-c with the strength of the electron-phonon interaction provides us with unique insight into the role of electron-phonon interaction in relation to the mechanism of high-T-c superconductivity as a crossover phenomenon.