So far no consensus has been reached on the underlying physics for the unconventional superconductivity in iron-based compounds, including the FeSe- and FeAs-based, as well as FeSe-monolayer systems. Significantly, the driving force for the electron pairing remains an issue of hot debate.

Recently, Professor Jimin Zhao and his PhD student Qiong Wu, et al. investigated the quasiparticle ultrafast dynamics of FeSe-based high-T_c (Li_0.84Fe_0.16)OHFe_0.98Se single crystal by using ultrafast optical spectroscopy. They obtained the electron-phonon coupling (EPC) strength λ_A1g = 0.22 ± 0.04 directly from the experiment. Also obtained are the EPC strengths for two other iron-based superconductors with different SC transition temperature T_c, Fe_1.01Se_0.2Te_0.8 and Fe_1.05Se_0.2Te_0.8. Importantly, they found that almost all the optimally doped iron-based superconductors (including the FeSe-based, FeAs-based, and Monolayer FeSe systems) share a universal positive correlation between T_c and the EPC strength (λ_A1g or λ), shown in the figure. This result indicates that the EPC plays an important role in the electron pairing. The work entitled “Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in high-T_c iron-based superconductor (Li_0.84Fe_0.16)OHFe_0.98Se” is recently published in Chinese Physics Letters (Express Letter) 37, 097802 (2020). The results reveal the consonant physics with the isotope effect observed in FeAs-based superconductors by Professor Xianhui Chen's team in the University of Science and Technology of China [Nature 459, 64 (2009)].

This work is collaborated with Professor Zhongxian Zhao, Professor Xiaoli Dong, Dr. Huaxue Zhou, and Professor Fang Zhou, and is financially supported by the NNSFC, the MOST, and the CAS.

Link to the paper: http://cpl.iphy.ac.cn/article/10.1088/0256-307X/37/9/097802