Strain-induced enhancement of the electron energy relaxation in strongly correlated superconductors

TL;DR

This study demonstrates that applying negative strain to cuprate and pnictide superconductors increases electron-phonon coupling, which correlates with changes in energy relaxation rates and the superconducting critical temperature, offering insights into high-Tc mechanisms.

Contribution

It provides systematic experimental evidence linking strain-induced electron-phonon coupling enhancement to energy relaxation and Tc variation in high-Tc superconductors.

Findings

Negative strain increases electron-phonon coupling.

Energy relaxation rate k1 correlates with strain and Tc.

Tc exhibits a non-monotonic dependence on k1.

Abstract

We use femtosecond optical spectroscopy to systematically measure the primary energy relaxation rate k1 of photoexcited carriers in cuprate and pnictide superconductors. We find that k1 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 k1 is consistent with the canonical assignment of k1 to the electron-phonon interaction. The well-known non-monotonic dependence of the superconducting critical temperature Tc on the a-axis strain is also reflected in a systematic dependence Tc on k1, with a distinct maximum at intermediate values (~16 ps-1 at room temperature). The empirical non-monotonic systematic variation of Tc 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-Tc superconductivity as a crossover phenomenon.