Ultrafast dynamics in single-layer FeSe/SrTiO₃, (Li_0.84Fe_0.16)OHFe_0.98Se, and Sr₃Ir₂O₇: revealing coupling between different degrees of freedom

We have demonstrated ultrafast dynamics evidence of high-T_c superconductivity in single-layer FeSe/SrTiO₃, with clear identification of the SC T_c, SC gap, and electron-phonon coupling (EPC) constant λ. In addition, we also observe the quasiparticle dynamics and coherent phonons in the intercalated novel superconductor of (Li_0.84Fe_0.16)OHFe_0.98Se. Furthermore, the EPC strength is compared among the single-layer FeSe/SrTiO₃, bulk FeSe, KFe₂Se₂, (Li_0.84Fe_0.16)OHFe_0.98Se, Fe_1.01Se_0.2Te_0.8, Fe_1.05Se_0.2Te_0.8, and many other reported iron-based superconductors, we find that the EPC strength has a positive correlation with SC T_c among almost all types of iron-based superconductors, thus bridging the bulk and monolayer system, as well as FeAs- and FeSe-based superconductors. We also investigated the A_1g coherent phonon in strong correlated iridate Sr₃Ir₂O₇, finding the strongest pseudospin-lattice coupling (PLC) known so far. We discover that PLC is the dominating interaction in low temperature, which leads to phonon anomaly and negative thermal expansion.
Our results demonstrate promising venues for ultrafast optical investigation of the quantum properties of oxide interfaces, bulk high T_c superconductors, and transition metal oxides. It is non-contact, single-layer sensitive, and gap-perceptive, being capable of detecting ultrafast lifetime, EPC strength, and coherent bosons. Hence it provides important clues to understanding the SC mechanism and couplings between different degrees of freedom.
[1] Y. C. Tian, Jimin Zhao* et al., Ultrafast Dynamics Evidence of High Temperature Superconductivity in Single Unit Cell FeSe on SrTiO₃, PRL 116, 107001 (2016).
[2] Q. Wu, Jimin Zhao* et al., Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in (Li_0.84Fe_0.16)OHFe_0.98Se, arXiv:1910.09859 (2019).
[3] L. L. Hu, Jimin Zhao* et al., Strong Pseudospin-Lattice Coupling in Sr₃Ir₂O₇: Coherent Phonon Anomaly and Negative Thermal Expansion, PRB 89, 094307 (2019).